Chapter 14 THERE IS SOMETHING SPECIAL ABOUT

Coral Reefs tropical coral reefs. The warm, clear water,

spectacular colors, and multitude of living
things captivate almost everyone who sees a
reef. Coral reefs rival that other great tropical
community, the rain forest, in their beauty,
richness, and complexity. Tropical rain forests
and coral reefs are also similar in that the basic
physical structure of both communities is pro-
duced by organisms. Both reef-building corals
and the giant trees of a rain forest create a
three-dimensional framework that is home to
an incredible assortment of organisms. Coral
reefs are such massive structures, in fact, that
they must be considered not only biological
communities but geological structures, the
largest geological features built by organisms.
Oysters, polychaete worms, and red algae
can also form reefs, and a deep-water coral
(Lophelia pertusa) slowly builds mounds up
to 30 m (100 ft) high on the northeast At-
lantic sea floor. None of these features are as
widespread, as large, or as structurally com-
Coral reef plex as tropical coral reefs.

The Organisms
That Build Reefs
Coral reefs are made of vast amounts of cal-
cium carbonate (CaCO3), limestone, that is
deposited by living things. Of the thousands of
species in coral reef communities, only a frac-
tion produce the limestone that builds the reef.
The most important of these reef-building or-
ganisms, as you might guess, are corals.

Reef Corals
Coral is a general term for several different
groups of cnidarians, only some of which
help build reefs (Table 14.1). In reef-building,
or hermatypic, corals the polyps produce

Cnidarians Animals in the phylum

Cnidaria; they have radial symmetry, tissue-
level organization, and tentacles with
nematocysts, specialized stinging structures.
Chapter 7, p. 116
Life stages of cnidarians:
Polyp The sessile, sac-like
stage, with the mouth and
tentacles on top.
Medusa The swimming, bell-
An Indo-West Pacific like stage, with the mouth and
coral reef. tentacles underneath.
Chapter 7, p. 117
 286 Part 3 Structure and Function of Marine Ecosystems www.mhhe.com/marinebiology

Table 14.1
Major Groups of Corals and Related Cnidarians on Coral Reefs
IMPORTANCE AS
COMMON NAME REEF BUILDERS NOTES

ANTHOZOANS (GROUP THAT INCLUDES SEA ANEMONES; LACK MEDUSA STAGE AND LIVE ONLY AS POLYPS)

Scleractinian corals The main reef builders Produce a calcareous skeleton; nearly all have zooxanthellae.
Soft corals Not reef builders Common on coral reefs (Fig. 14.28); most have zooxanthellae but
do not produce a rigid calcareous skeleton.
Organ-pipe corals Minor Named for pipe-like structure of skeleton; have zooxanthellae and
(Tubipora) produce a calcareous skeleton.
Blue coral (Heliopora) Significant in some places Named for distinct blue color of skeleton; has zooxanthellae and
produces a calcareous skeleton.
Gorgonians Not reef builders Sea fans (Fig. 7.10) and sea whips; hard skeleton is made mostly of
protein and not calcareous; some have zooxanthellae.
Precious corals Gorgonians with a pink, red, or gold calcareous skeleton used for
jewelry; occur mainly in deep water rather than on coral reefs and
do not contribute to reef growth; lack zooxanthellae.
Black corals Not reef builders Used to make jewelry and sometimes called precious corals but
are a different group from gorgonian precious corals; hard skeleton
is made of protein and is not calcareous; commonly occur in deep
and cold surface waters as well as on coral reefs.

HYDROZOANS (GROUP THAT INCLUDES JELLYFISHES; HAVE BOTH MEDUSA AND POLYP STAGES BUT ARE SEEN ON REEF ONLY AS POLYPS)

Fire corals (Millepora) Significant in some places Named because touching them produces a burning sensation from
their powerful nematocysts; have zooxanthellae and produce a
calcareous skeleton.
Lace corals Insignificant Named for delicate, branched colonies; lack zooxanthellae but
produce a calcareous skeleton; commonly occur in deep and cold
surface waters as well as on coral reefs.

calcium carbonate skeletons. Billions of these their nematocyst-armed tentacles to capture

tiny skeletons build a massive reef. The most The primary architects of coral reefs are reef- food, especially zooplankton. The tentacles
important reef builders are a group known as building corals that produce calcium carbon- surround the mouth, the only opening to the
scleractinian corals, sometimes called the ate skeletons with the help of symbiotic zoo- sac-like gut.
stony or true corals. xanthellae. Most reef-building corals are colonies of
Nearly all reef-building corals contain many polyps, all connected by a thin sheet of
symbiotic zooxanthellae (Fig. 14.1) that help tissue. The colony starts when a planktonic
the corals make their calcium carbonate coral larva, called a planula, settles on a hard
skeletons. Corals can produce their skeletons The Coral Polyp You have to look surface. Coral larvae generally do not settle
without zooxanthellae but only very slowly, closely to see the little polyps that build coral on soft bottoms. Immediately after settling,
not nearly fast enough to build a reef. It is the reefs. Coral polyps are not only small but de- the larva transforms, or metamorphoses,
zooxanthellae as much as the corals them- ceptively simple in appearance. They look into a polyp. This single founder polyp, if it
selves that construct the reef framework, and much like little sea anemones, consisting of survives, divides over and over to form the
without zooxanthellae there would be no an upright cylinder of tissue with a ring of colony. The digestive systems of the polyps
reefs. Corals that do not help build reefs, or tentacles on top (Figs. 14.2 and 14.3b). Like usually remain connected, and they share a
ahermatypic corals, often lack zooxanthellae. anemones and other cnidarians, they use common nervous system (Fig. 14.4).
 Coral Reefs Chapter 14 287

(a) (b)

Figure 14.1 The microscopic, golden- (c)

brown zooxanthellae that pack these polyp
tentacles help corals build reefs. They also Figure 14.3 Coral polyps. (a) A few corals, like the mushroom coral (Fungia) from the Indo-
occur in giant clams, some sea anemones (see West Pacific, consist of a single polyp. Most are colonies of many individual polyps. (b) In some
Fig. 11.26), and other invertebrates. Cells in of these, like the large-cupped boulder coral (Montastrea cavernosa) from the Caribbean, each
the white, bulbous tips of the tentacles are polyp has its own individual cup, called a corallite. (c) In coral colonies like this brain coral
packed with nematocysts. (Diploria) from the Caribbean, meandering lines of polyps lie in a common corallite.

Tissue connections Tentacles Polyps

between polyps Plankton Primary producers
(phytoplankton) and consumers
(zooplankton) that drift with the currents.
Chapter 10, p. 222; Figure 10.21

Zooxanthellae Dinoflagellates (single-

celled, photosynthetic algae) that live
within animal tissues.
Mouth
Chapter 5, p. 95
Gut cavity
of polyp
Symbiosis The living together in close
Mesenterial
filaments
association of two different species, often
divided into parasitism, where one species
benefits at the expense of the other;
commensalism, where one species benefits
Bare without affecting the other; and mutualism,
skeleton where both species benefit.
of polyp
Chapter 10, p. 212

Figure 14.2 Cutaway view of one of the polyps in a coral colony and of the calcium
carbonate skeleton underneath. The polyps are interconnected by a very thin layer of tissue.
 288 Part 3 Structure and Function of Marine Ecosystems www.mhhe.com/marinebiology

Figure 14.4 The polyps in a coral colony

are interconnected. If you touch an extended Figure 14.5 This coral (Lobophyllia hemprichii) shows particularly well how corals build up
polyp, it will contract, and so will its neighbors, their skeleton. Each of the irregular rings (arrows) is a single polyp that has built up a column of
followed by their neighbors, and so on, so that calcium carbonate skeleton.
a wave of contraction passes over the colony.
In this colony of the coral Goniopora lobata the
wave of contraction is moving up from the Coral polyps catch zooplankton with their
bottom. tentacles or in sheets of mucus that they se-
crete on the colony surface. Tiny, hair-like cilia
gather the mucus into threads and pass them
A few reef-building corals consist of only a along to the mouth. Some corals hardly use
single polyp (Fig. 14.3a). their tentacles and rely on the mucus method.
Coral polyps lie in a cup-like skeleton of A few have even lost their tentacles altogether.
calcium carbonate that they make themselves. Corals have still other ways of feeding
The polyps continually lay down new layers themselves. There are a number of long,
of calcium carbonate, building up the skele- coiled tubes called mesenterial filaments at-
ton beneath them so that it grows upward and tached to the wall of the gut (Fig. 14.2). The
outward (Fig. 14.5). The skeleton forms nearly Figure 14.6 The calcium carbonate mesenterial filaments secrete digestive en-
all of the bulk of the colony (Fig. 14.6) and skeleton makes up most of a coral colony. A zymes. The polyp can extrude the filaments
can take many different shapes (Fig. 14.7). live colony (Pocillopora verrucosa) is shown on through the mouth or body wall to digest and
The actual living tissue is only a thin layer on the right. On the left is a colony with the living absorb food particles outside the body. Corals
the surface. It is the calcareous coral skeletons tissue removed. The main difference is the also use the mesenterial filaments to digest or-
that form the framework of the reef. color; the live part is only a thin layer of tissue ganic matter from the sediments. In addition,
on the surface. The corals sold in shell and
corals can absorb dissolved organic matter
aquarium shops are actually only the skeletons
Coral Nutrition Zooxanthellae nourish of live corals that were taken from the reef and
(DOM) (see The Trophic Pyramid, p. 216).
the host coral as well as help it deposit its bleached. Some reefs have been devastated
skeleton. They perform photosynthesis and by the collectors who supply these shops in Corals nourish themselves in a remarkable
pass some of the organic matter they make on order to feed their families. number of ways. Zooxanthellae are the most
to the coral. Thus, the zooxanthellae feed the important source of nutrition. Corals can also
coral from the inside. Many corals can sur-
capture zooplankton with tentacles or mucus
vive and grow without eating, as long as the zooplankton. The billions of coral polyps on
zooxanthellae have enough light. a reef, along with all the other hungry reef or- nets, digest organic material outside the body
Although corals get much of their nutri- ganisms, are very efficient at removing zoo- with mesenterial filaments, or absorb dis-
tion from their zooxanthellae, most eat when plankton brought in by currents. Indeed, solved organic matter (DOM) from the water.
they get the chance. They prey voraciously on the reef has been called a wall of mouths.
 Coral Reefs Chapter 14 289

Branching Many other organisms make calcium car-

bonate sediments and thus contribute to the
Massive growth of the reef. The shells of forams,
snails, clams, and other molluscs are very im-
Plate-like Foliaceous portant. Sea urchins, bryozoans, crustaceans,
(leaf-like) Columnar sponges, bacteria and a host of other organ-
isms add or help bind carbonate sediments.
Reef growth is truly a team effort.

The accumulation of calcium carbonate sedi-

ments plays an important role in reef growth.
A calcareous green alga, Halimeda, and coral
rubble account for most of the sediment, but
many other organisms also contribute.

A great deal of the sediment on reefs

comes from the activities of organisms that
Encrusting Free-living do not deposit calcium carbonate themselves
but instead break down the solid reef struc-
Figure 14.7 Corals come in a multitude of shapes, or growth forms. ture. Many animals scrape or bite their food
off the reef with some sort of hard structure.
Parrotfishes, for example, are named for their
ments (Fig. 14.8). Sediment, especially fine fused teeth that form a parrot-like beak (see
Other Reef Builders Fig. 8.13d). Sea urchins scrape algae off the
Although they are the chief architects, corals sediment, damages corals when it settles di-
rectly on them, but the buildup of coarse reef with their Aristotles lantern. In the
cannot build a reef alone. Many other organ- process, these and many other grazers re-
isms help make a coral reef. The most impor- sediment in the reef framework is an essen-
tial part of reef growth. The structure of a move bits of calcium carbonate from the reef
tant of these are algae, which are essential to to form sediment. The erosion caused by
reef growth. In fact, some marine biologists reef is formed as much by the accumulation
think that coral reefs should be called algal of calcareous sediment as by the growth of
corals. Encrusting algae grow over the sedi-
reefs or, to be fair to both, biotic reefs. One Photosynthesis CO2 + H2O +
reason for this is that zooxanthellae, which ment as it builds up, cementing the sediment sun energy organic matter + O2
are algae, are essential to the growth of corals. in place. Thus, encrusting coralline algae are (glucose)
There are other algae, however, that also have the glue that holds the reef together. Some
invertebrates, notably sponges and bry- Chapter 4, p. 68
key roles in building the reef. Encrusting
coralline algae (Porolithon, Lithothamnion) ozoans, also help bind the sediments.
Coralline Algae Red algae that deposit
grow in rock-hard sheets over the surface of calcium carbonate (CaCO3) in their tissues.
the reef. They deposit considerable amounts Encrusting coralline algae help build the reef Chapter 6, p. 106
of calcium carbonate, sometimes more than by depositing calcium carbonate, by resisting
corals, and thus contribute to reef growth. Bryozoans Small, colonial, encrusting
wave erosion, and by cementing sediments.
Coralline algae are more important on Pacific animals that make delicate, often lace-like,
reefs than Atlantic ones. skeletons.
Encrusting coralline algae not only help Nearly all the sediment that accumulates to
build the reef but also help keep it from Chapter 7, p. 134; Figure 7.40
help form the reef comes from corals and the
washing away. The stony pavement formed shells or skeletons of other organisms. In other
Foraminiferans (Forams) Protozoans,
by these algae is tough enough to withstand words, nearly all the sediment is biogenous. often microscopic, with a calcium carbonate
waves that would smash even the most Coral rubble, fragments of broken coral, is shell.
rugged corals. The algae form a distinct ridge one important source of sediment on reefs.
on the outer edge of many reefs, especially in Another important sediment-forming organ- Chapter 5, p. 98; Figure 5.10
the Pacific. This algal ridge absorbs the force ism is a calcareous green alga called Halimeda
Aristotles Lantern A complicated set
of the waves and prevents erosion from de- (Fig. 14.9). Halimeda deposits calcium carbon-
of calcium carbonate (CaCo3) teeth and
stroying the reef. ate within its tissues to provide support and to associated muscles that is found in sea
Encrusting algae do yet another job that discourage grazersa mouthful of limestone is urchins.
is vital to reef growth. Coral skeletons and pretty unappetizing. The remnants of Hal-
fragments create an open network, full of imeda accumulate on reefs in huge amounts, Chapter 7, p. 138
spaces, that traps coarse carbonate sedi- to be bound together by encrusting organisms.
 290 Part 3 Structure and Function of Marine Ecosystems www.mhhe.com/marinebiology

Coral Reproduction
Corals are amazingly adaptable animals. They however, are broadcast spawners and re-
come in all shapes and sizes and have many lease eggs and sperm into the water.
ways to feed themselves. It should come as no The most spectacular form of sexual
surprise, then, that they also have more than reproduction in corals is mass spawning,
one way to reproduce. in which many different coral species on a
One form of reproduction in corals is es- reef all spawn at the same time. Mass
sentially the same as growth. As individual spawning was first reported from the Great
polyps reproduce vegetatively by dividing Barrier Reef, where it takes place for a few
into new polyps, the colony as a whole nights a year between October and early
grows. The process is taken a step further December just after a full moon. At a given
when a piece of a coral colony breaks off and place the time of the mass spawning event
grows into a new daughter colony. This re- can usually be predicted down to the night.
production by fragmentation seems to be Since its discovery on the Great Barrier Reef
quite important for some coral species, which mass spawning has been observed on many
may even be adapted to break easily to pro- other reefs around the world.
duce more fragments. The growth of broken Spawning corals release their eggs and
coral fragments into new colonies is also an sperm through the mouth. In some species
important part of the recovery of reefs from the gametes are packaged into little bundles,
storm damage and other disturbances, and which may contain both eggs and sperm or
transplanting fragments is one way that sci- only one or the other. The bundles float to
entists and conservation groups help restore the surface and break up, allowing the eggs
The release of sperm and egg
damaged reefs (see Restoration of Habitats, and sperm to mix.
bundles during the mass coral
p. 407). Nobody knows why the corals all spawn
spawning on the Great Barrier Reef.
Corals can also reproduce sexually. Like together. Maybe egg predators get so full
Photographs courtesy of the Great
other animals, they produce eggs and sperm, that most of the eggs go uneaten. Maybe it
Barrier Reef Marine Park Authority.
which fuse and develop into planula larvae, has something to do with the tides. There
the characteristic larvae of cnidarians. Some coral species have may be an explanation that no one has thought of. Another inter-
separate sexes, but about three-quarters are hermaphrodites and esting thing is that although mass spawning happens on some
make both eggs and sperm. The method of fertilization also reefs it does not occur on others. Is there something different
varies. In some corals, whether or not they are hermaphrodites, about these reefs? Finding answers to these questions is likely to
the egg is fertilized and develops inside the polyp. Most corals, occupy coral reef biologists for some time to come.

such organisms, or bioerosion, tends to wear ments that determine where reefs develop. water and do not need light, but these corals
the reef away, although some of the sediment Reefs are rare on soft bottoms, for example, do not contain zooxanthellae or build reefs.
they create does get reincorporated into the because coral larvae need to settle on a hard Corals also prefer clear waters, since water
reef structure. Many other organisms surface. clouded with sediment or plankton reduces
including sponges, clams, polychaete and light penetration.
other worms, and algaecause bioerosion by Light and Temperature Corals can Reef-building corals are limited to warm
burrowing into or through the reef limestone, grow only in shallow water, where light can water and can grow and reproduce only if
either scraping it away to form sediment or penetrate, because the zooxanthellae on the average water temperature is above about
dissolving it. A reef grows only if corals, which they depend need light. Calcareous 20C (68F). Most reefs grow in considerably
coralline algae, and sediment-forming and algae require sunlight as well. Particular warmer areas. Figure 14.10 illustrates the re-
-binding organisms accumulate limestone types of coral and algae have different depth lationship between coral reefs and water
faster than the bioeroders wear it away. limitssome can live deeper than others temperature.
but reefs rarely develop in water deeper than
Conditions for Reef Growth about 50 m (165 ft). Because of this, coral
Corals need light and warm temperatures, so
Other organisms may be important, but coral reefs are found only on the continental
reefs do not develop without reef-building shelves, around islands, or on top of reefs grow only in shallow, warm waters.
corals. Corals have very particular require- seamounts. Many types of coral live in deep
 Coral Reefs Chapter 14 291

Hard substrate
(a)
Figure 14.9 This calcareous green alga
(Halimeda) is one of the main sediment-
forming organisms on most reefs. About 95%
Loose sediment
and coral rubble
of the plants weight is calcium carbonate, and
there is only a thin layer of live tissue on the
outside. When the tissue dies the segments
separate, each leaving a piece of limestone.

new ones from the water. If the warm condi-

tions last too long or the temperature gets too
high, however, the coral dies.
Hard substrate Temperature limits vary among coral
(b) species and from place to place. Corals from
warm locations, for example, tolerate higher
temperatures than corals from cooler waters
(Fig. 14.11). Corals can also adapt to fluctuat-
Reef Consolidation ing conditions. For instance, reefs grow in
growth of sediment
parts of the Persian Gulf where the water
temperature ranges from 16 to 40C (60 to
104F). The symbiotic zooxanthellae, differ-
ent species and strains of which vary in their
temperature tolerances, may be partly re-
sponsible for this temperature adaptation.
Bleaching may occur simply because one
Hard substrate partner or the other is weakened or damaged
by stress, or be caused by viruses or other
(c) disease-causing organisms. Some biologists
have proposed, however, that corals bleach to
Figure 14.8 Reef growth involves several processes. (a) The framework is made when reef- expel zooxanthellae that are poorly adapted
building corals settle and grow on some hard surface, usually a preexisting reef. (b) The spaces in to the prevailing conditions, on the chance
this framework are partially filled in by coarse carbonate sediments. (c) When the sediments are that they will be able to acquire better-
glued together by encrusting organisms, new reef rock is formed and the reef has grown. On a adapted symbionts.
real reef all three steps go on at the same time. Reefs are not actually completely solid as depicted In any case, the upper temperature toler-
here but are porous, with many holes and crevices that serve as home to a multitude of organisms. ances of corals are usually not far above the
normal temperature range where they live.
Corals suffer when exposed to temperatures
Water that is too warm is also bad for golden-brown or greenish zooxanthellae give outside this normal range. This sometimes
corals. The upper temperature limit varies, the coral most of its color; without them, the happens during extreme low tides, when
but is usually around 30 to 35C (86 to coral is almost white. Corals also slough shallow pools on the reef may be cut off from
95F). The first outward sign of heat stress, off large amounts of slimy mucus when circulation. Heated by the sun, the water can
or stress of other kinds, is bleaching, in stressed. Corals often recover from bleaching, warm up to fatal temperatures. By discharg-
which the coral expels its zooxanthellae (see regaining their symbionts either from the re- ing heated water, power plants can also kill
Fig. 18.3). It is called bleaching because the growth of the few that remain or by acquiring corals.
 292 Part 3 Structure and Function of Marine Ecosystems www.mhhe.com/marinebiology

60 90 120 150 180 150 120 90 60 30 0

60 60

Pacific
Persian Gulf Ocean Bermuda
30 30
Red
Sea Hawaiian West
Guam Marshall Islands Indies
Maldive Islands Belize
Islands Papua
New Guinea Galpagos Atlantic
0 Suvadiva n R. 0
Islands a zo Ocean
Indonesia Am Brazil
Indian
Fiji
Great French
Ocean Barrier New
Caledonia Polynesia
30 Reef 30

0 1000 2000 3000 Miles

0 1000 2000 3000 Kilometers

60 60

60 90 120 150 180 150 120 90 60 30 0

Coral reefs Reef-building corals Average 20 isotherm Warm current Cold current

Figure 14.10 The distribution of reef coral communities, including those that do not form structural reefs. Note that the distribution of reef
corals, like that of kelps, is related to temperature. Reef corals require warm water, however, while kelps need cold water. Compare the distribution
of reefs with that of kelps shown in Figure 13.21. Note that because of warm surface currents corals extend further north and south on the east
sides of continents than on the west sides.

El Nio (see The El NioSouthern Os- p. 394). On the other hand, bleaching is nat- Salinity, Sediments, and Pollution
cillation Phenomenon, p. 336), brings un- ural and past events probably often went unre- Most corals are quite sensitive to reduced
usually warm water to many parts of the ported because many reefs are in remote loca- salinity and do not do well near river mouths,
ocean. Widespread coral bleaching and mor- tions. Today scientists continually monitor sea for example, where there is a lot of freshwater
tality also occur during El Nio events. Dur- surface temperatures by satellite and can use input. This is not only because of the lowered
ing the unusually strong El Nio of 199798, the Internet to immediately report bleaching salinity but also because rivers bring in a lot
severe bleaching occurred on many reefs when it occurs. El Nio is also a natural event, of silty sediment, which is generally unfavor-
around the world, probably as a direct result and the unusually strong recent El Nios that able to corals. It clouds the water, cutting
of the unusually warm water. In some places, have brought such widespread coral bleaching down light for the zooxanthellae. Sediment
including the Caribbean and parts of the are to some extent probably just normal, ran- on the colony surface can smother the coral
Great Barrier Reef, bleaching did not kill dom fluctuation. Temperature records stored or cause disease, though the coral can clean
many corals, and reefs quickly recovered after in the oxygen isotopes of fossil corals, in fact, itself to some extent by sloughing off mucus
the water cooled. In other areas, such as the show that the most intense El Nio events of that carries the sediment away.
Indian Ocean, Southeast Asia, and the far the last millennium took place in the mid-17th Some corals tolerate high levels of sedi-
western Pacific, many corals died after century, long before the smokestacks of the in- ment and build reefs in silty environments,
bleaching and some reefs were severely dam- dustrial revolution began belching huge quan- where some even feed on the organic-rich
aged. Many of these reefs have recovered tities of carbon dioxide (CO 2 ) and other sediment particles. Most reefs, however, de-
slowly or not at all. greenhouse gases. Nonetheless, the sea has velop in clear, low-sediment waters and are
High water temperatures have led to other steadily warmed in many reef areas over at vulnerable to high levels of sediment unless
major, though more localized, bleaching least the last 50 to 100 years, and climate sci- there is enough wave or current action to
events since the 199798 El Nio. Reef scien- entists predict that with global warming this wash the sediment away (Fig. 14.12). Many
tists are increasingly concerned that bleaching trend will continue. Combined with other reefs around the world have been damaged
is becoming more frequent and more intense human-induced stresses (see Coral Reefs, by human activities like mining, logging, con-
as a result of global climate change (see Living p. 391), this warming threatens coral reefs struction, and dredging that greatly increase
in a Greenhouse: Our Warming Earth, around the world. the flow of sediment onto the reef.
 Coral Reefs Chapter 14 293

The Kaneohe Bay Story One of the

Alaska
best-known examples of the harmful effects
of nutrient enrichment occurred in a partially
enclosed bay in the Hawaiian Islands.
North
North Kaneohe Bay, located on the northeast shore
Japan America
Pacific of the island of Oahu, once had some of the
Ocean most luxuriant reefs in Hawaii. Until the
1930s the area around the bay was sparsely
Hawaiian populated. In the years leading up to World
Islands War II, with the military buildup of Oahu,
Philippines
the population began to increase. This in-
Marshall crease continued after the war as the shores
Islands
of the bay were developed for residential use.
The sewage from this expanding popula-
Equator
tion was dumped right into the bay. By 1978
about 20,000 m3 (over 5 million gallons) of
sewage were being dumped into the bay
Temperature every day. Long before then, by the mid-
1960s in fact, marine biologists began to no-
F C
tice disturbing changes in the middle part of
95 35 the bay. Loaded with nutrients, the sewage
Lethal limits acted as a fertilizer for seaweeds. A green
for corals alga, the bubble alga (Dictyosphaeria caver-
nosa), found the conditions particularly to its
liking and grew at a tremendous rate, literally
86 30
covering the bottom in many parts of the bay.
Mean high
water Bubble algae began to overgrow and smother
temperature the corals. Phytoplankton also multiplied
with the increase in nutrients, clouding the
77 25 water. Kaneohe Bays reefs began to die. Such
accelerated algal growth due to nutrient input
Marshall Hawaiian is called eutrophication (see Eutrophica-
Islands Islands
tion, p. 392).
Figure 14.11 The upper temperature limit of corals is related to the temperature at their For a time it appeared that the story had a
home sites. For example, the water in the Marshall Islands is warmer on average than that in happy ending. As the once beautiful reefs
Hawaii. During the hottest months of the year the average high temperature is several degrees smothered, scientists and the general public
higher in the Marshalls. Corals from the Marshalls can tolerate correspondingly higher began to cry out. It took a while, but in 1978
temperatures. Why do you think the highest temperature that the corals can tolerate would be public pressure finally managed to greatly re-
higher than the average high temperature at their location? duce the discharge of sewage into the bay,
and the sewage was diverted offshore. The re-
sult was dramatic. Bubble algae died back in
Corals are also sensitive to pollution of poor water, seaweeds do not grow very ra- much of the bay, and the bays corals began
many kinds. Even low concentrations of pidly and are kept under control by grazers. to recover much faster than anyone expected.
chemicals like pesticides and industrial This allows corals to compete successfully for By the early 1980s, bubble algae were fairly
wastes can harm them. The larvae are espe- space and light. When nutrients are added, scarce and corals had started to grow again.
cially sensitive. In high concentrations, nutri- seaweeds may grow much faster and shade The reefs were not what they once were, but
ents too can be harmful to reef growth. Hu- and choke out the slow-growing corals. This they seemed to be on track to recover.
mans release tremendous amounts of is a particular problem when, as is often the Then the ghost of pollution past reared its
nutrients in sewage and in fertilizers that are case, fishing has reduced populations of graz- ugly head. In November 1982, Hurricane Iwa
washed from farmland and carried to the sea. ing fishes and other organisms. struck Kaneohe Bay. During the years of pol-
The nutrients may harm the corals directly by lution a layer of the coral skeleton had weak-
interfering with the formation of their skele- Corals are very sensitive to fresh water, fine ened, becoming fragile and crumbly. When
tons. More importantly, increased nutrients the hurricane hit, this weak layer collapsed
sediment, and pollution, including pollution by
can alter the ecological balance of the com- and many reefs were severely damaged. For-
munity. Most coral reefs grow in water that is high nutrient levels. tunately, the corals were already beginning to
naturally low in nutrients. In such nutrient- recover, and the broken pieces were able to
 294 Part 3 Structure and Function of Marine Ecosystems www.mhhe.com/marinebiology

grow back. If the hurricane had hit during

the years of pollution, the reefs of Kaneohe
Bayand their benefits to fishing, tourism,
and recreationmight have disappeared
forever.
The rapid recovery of Kaneohe Bays reefs
that was seen during the early 1980s did not
continue. By 1990 the recovery seemed to
have leveled off. Some areas even began to
decline again, with bubble algae once more
becoming abundant. There are a number of
possible explanations for this. Even though
most sewage is now discharged outside the
bay, some nutrients continue to enter from
boats, the septic tanks and cesspools of pri-
vate homes, and other sources. Not only
that, nutrients from the old sewage outfalls
were stored in the sediments, and are still
being released even after 30 years. There is
also evidence that fishing has reduced popu-
lations of fishes that graze on bubble algae.
Furthermore, the grazing fishes that remain
prefer to eat other species of seaweed that
have been introduced from outside Hawaii, Figure 14.12 Corals often flourish where there is plenty of wave action. The water motion
and have shifted away from eating bubble keeps sediment from settling on the corals and brings in food, oxygen, and nutrients.
algae. Thus, bubble algae may be abundant
because they are not being eaten as fast as
they once were. Another introduced sea-
weed, which like bubble algae is not a pre-
ferred food of herbivorous fishes, has also
started to proliferate and smother corals in
the bay (Fig. 14.13). It will be much harder
to restore the coral gardens of Kaneohe Bay
than it was to destroy them.
The Kaneohe Bay story is far from unique.
Most of the worlds tropical coasts are under-
going increasing development and population
growth. As a result, more and more nutrients
are ending up in the waters that support coral
reefs, and there are many reports of reefs
being threatened by eutrophication. New re-
search, however, is showing that the effects of
added nutrients on coral reefs are more com-
plicated than we thought. Experiments indi-
cate that algal growth is not nutrient-limited
on at least some reefs. There are even sugges-
tions that added nutrients may sometimes be
good for the zooxanthellae and help corals
grow faster. The bulk of the evidence,
Figure 14.13 The red alga Kappaphycus striatum, introduced to Kaneohe Bay from the
though, is that eutrophication is damaging,
Philippines, has gotten out of control. The arrow shows one of many clumps of K. striata in this
especially when populations of algal grazers picture that are growing over and smothering reef corals.
are reduced. Many reef biologists consider it
one of the most serious threats to the worlds
coral reefs.
 Coral Reefs Chapter 14 295

Kinds of Coral Reefs Mangroves

Coral reefs are usually divided into three main
categories: fringing reefs, barrier reefs, and Rocky shore
atolls. Many reefs do not fit neatly into any Reef crest
particular category or fall between two cate- Beach
gories. Still, the division of reefs into the three
major types works well for the most part. Reef
flat

The three main types of reefs are fringing Reef

reefs, barrier reefs, and atolls. slope

Fringing Reefs
Sediment and
Fringing reefs are the simplest and most com- rubble
mon kind of reef. They develop near shore
throughout the tropics, wherever there is
some kind of hard surface for the settlement Figure 14.14 Typical structure of a fringing reef. Fringing reefs, like this one in the Bismarck
of coral larvae. Rocky shorelines provide the Archipelago in the southwest Pacific (photo), can grow right up to the shore.
best conditions for fringing reefs. Fringing
reefs also grow on soft bottoms if there is
even a small hard patch that lets the corals
get a foothold. Once they get started, the
corals create their own hard bottom and the
reef slowly expands.
As their name implies, fringing reefs grow
in a narrow band or fringe along the shore
(Fig. 14.14). Occurring close to land, they are
especially vulnerable to sediment, freshwater
runoff, and human disturbance. Under the
right conditions, however, fringing reefs can
be impressive. In fact, the longest reef in the
world (though not the one with the largest
coral area) is not the famous Great Barrier
Reef in Australia but a fringing reef that runs
some 4,000 km (2,500 mi) along the coast of
the Red Sea. Part of the reason this reef is so
well-developed is that the climate is dry and
there are no streams to bring in sediment and
fresh water.
The typical structure of a fringing reef is
shown in Figure 14.14. Depending on the Figure 14.15 The upward growth of most reefs is limited by the tides. When extreme low
place, the shore may be steep and rocky or tides occur, shallow areas like this reef flat on the Great Barrier Reef are exposed. If the corals are
have mangroves or a beach. The reef itself only exposed for a short time, they can survive, but they will die if exposed for too long. It is this
consists of an inner reef flat and an outer occasional exposure at extreme low tides that keeps reef flats flat because all the corals above a
reef slope. The reef flat is the widest part of certain depth are killed. Photograph courtesy of the Great Barrier Reef Marine Park Authority.
the reef. It is shallow, sometimes exposed at
low tide (Fig. 14.15), and slopes very gently colonies nor as many different species as on the densest cover (Fig. 14.16) and the most
toward the sea. Being closest to land, it is the the reef slope. Seaweeds, seagrasses, and soft species of coral, because the slope is away
part of the reef most strongly affected by sedi- corals may also occupy the reef flat, some- from shore and therefore away from the ef-
ments and freshwater runoff. The bottom is times in dense beds. fects of sediment and fresh water. Also, the
primarily sand, mud, or coral rubble. There The reef slope can be quite steep, nearly waves that bathe the slope provide good cir-
are some living corals, but neither as many vertical in fact. It is the part of the reef with culation, bring in nutrients and zooplankton,
 296 Part 3 Structure and Function of Marine Ecosystems www.mhhe.com/marinebiology

and wash away fine sediments. The reef crest

is the shallow upper edge of the reef slope.
The crest usually has even more luxuriant
coral growth than the rest of the reef slope. If
there is intense wave action, however, the
crest may consist of an algal ridge, with the
richest coral growth just below. Because there
is less light in deep water, the deepest part of
the reef slope usually has less live coral and
fewer coral species.

Fringing reefs grow close to shore and consist

of an inner reef flat and an outer reef slope.

Large amounts of sediment and coral rub-

ble tumble down the reef slope and settle at
the base. As this material builds up, reef or-
ganisms may begin to grow on it, depending
on the water depth and other factors. Thus,
the reef can grow outward as well as upward.
Beyond the base of the slope, the bottom is
usually fairly flat and composed of sand or Figure 14.16 The elkhorn coral (Acropora palmata) is a dominant coral on the reef slopes of
mud. On many Caribbean reefs, turtle grass fringing reefs in the Caribbean and Florida. Its broad branches rise parallel to the surface to collect
(Thalassia testudinum) dominates the bottom light. This colony suffers from a disease known as white-band disease (see Coral Reefs, p. 391).
beyond the slope.
Patch reefs
Barrier Reefs Spur and groove
The distinction between barrier reefs and Sand cay
fringing reefs is sometimes unclear because
the two types grade into one another. Like Lagoon
fringing reefs, barrier reefs lie along the coast,
but barrier reefs occur considerably farther Back-reef Reef
from shore, occasionally as far as 100 km slope flat Reef
crest
(60 mi) or more. Barrier reefs are separated
from the shorewhich may also have a Fore-reef
slope
fringing reefby a relatively deep lagoon
(Fig. 14.17). Largely protected from waves and
currents, the lagoon usually has a soft sediment
bottom. Seagrass beds often grow in shallow
parts of the lagoon. Scattered coral formations
variously known as patch reefs, coral knolls, Figure 14.17 Typical structure of a barrier reef.
or pinnacles depending on their size and
shape may grow up nearly to the surface. especially gentle ones, have luxuriant coral with coral growth most luxuriant just below
The barrier reef consists of a back-reef growth (Fig. 14.18). the crest. Exposed fore-reef areas often have a
slope, a reef flat, and a fore-reef slope, which The reef flat, like that on fringing reefs, is series of finger-like projections alternating with
corresponds to the reef slope of a fringing reef a shallow, nearly flat platform. Sand and coral sand channels (Figs. 14.17 and 14.19). There is
and has a reef crest (Fig. 14.17). The back-reef rubble patches are interspersed with seagrass still debate about what causes these formations,
slope may be gentle or as steep as the fore-reef or seaweed beds, soft corals, and patches of known as spur-and-groove formations or but-
slope. It is protected from waves by the rest of dense coral cover. Waves and currents may tresses. The wind, waves, or both are defi-
the reef, but waves wash large amounts of pile up sand to form small sand islands called nitely involved, because spur-and-groove for-
sediment from the reef down the slope. As a sand cays or, in the United States, keys. mations develop primarily on reef slopes that
result, coral growth is often not as vigorous on The richest coral growth is usually at the are exposed to consistent strong winds. These
the back-reef slope as on the fore-reef slope. outer reef crest. There may be a well-developed formations are found on atolls and some fring-
This is not always true; some back-reef slopes, algal ridge if the reef is exposed to wave action, ing reefs as well as barrier reefs.
 Coral Reefs Chapter 14 297

Fore-reef slopes vary from relatively gentle

to nearly vertical. The steepness depends on
the action of wind and waves, the amount of
sediment flowing down the slope, the depth
and nature of the bottom at the reef base, and
other factors. As with other types of reefs, the
abundance and diversity of corals on the fore-
reef slope generally decreases with depth. The
growth form of the corals also changes down
the slope. At the crest, under the pounding of
the waves, the corals are mostly stout and
compact; many are massive (Fig. 14.7). Below
the crest there is great variety in form.
Whether they form branches, columns, or
whorls, corals in this zone often grow verti-
cally upward. This may be an adaptation for
competition. Corals that grow upward like
skyscrapers rather than outward need less
space to attach. They are also less likely to be
shaded, and if they spread out at the top, can
shade out other corals. Deeper on the reef
slope, corals tend to grow in flat sheets,
which probably helps them collect light (see
Fig. 10.1).
The largest and most famous barrier reef is
the Great Barrier Reef. It runs more than
2,000 km (1,200 mi) along the northeastern
Figure 14.18 A rich back-reef slope on a Pacific barrier reef. coast of Australia, varying in width between
about 15 and 350 km (10 to 200 mi) and
covering an area of over 225,000 km 2
(80,000 mi2). Though not the longest reef in
the world, it covers such a large area and is so
complex and well developed that it is gener-
ally regarded as the largest reef structure. Ac-
tually, the Great Barrier Reef is not a single
reef but a system of more than 2,500 smaller
reefs, lagoons, channels, islands, and sand
cays (Fig. 14.20).
The largest barrier reef in the Caribbean
lies off the coast of Belize, Central America.
Other major barrier reefs include the Florida
Reef Tract and barrier reefs associated with
the islands of New Caledonia, New Guinea,
and Fiji in the Pacific. There are many other
smaller barrier reefs, especially in the Pacific.
Like the Great Barrier Reef, these usually are
not single reefs but complex systems of
smaller reefs.

Atolls
An atoll is a ring of reef, and often islands or
Figure 14.19 Spur-and-groove formations at Kure atoll in the northwestern Hawaiian sand cays, surrounding a central lagoon
Islands. The dark elevated ridges are the coral spurs; the light-colored grooves (arrows) channel (Figs. 14.21 and 14.22). The vast majority of
sand down the reef slope. The northwestern Hawaiian Islands include 70% of the coral reef in atolls occur in the Indo-West Pacific region,
United States waters and are one of the largest relatively pristine reef areas in the world. that is, the tropical Indian and western Pacific
 298 Part 3 Structure and Function of Marine Ecosystems www.mhhe.com/marinebiology

two largest atolls are Suvadiva, in the Maldive

Islands of the Indian Ocean, and Kwajalein,
one of the Marshall Islands in the central Pa-
cific. These atolls cover areas of more than
1,200 km2 (700 mi2). Atolls may include a
dozen or more islands and be home to thou-
sands of people.
An atolls reef flat is much like the reef flat
on a fringing or barrier reef: a flat, shallow
area. The fore-reef and back-reef slopes can
now be thought of as outer and inner slopes,
respectively, since they extend all the way
around the ring-shaped atoll.
Atoll reef crests are strongly influenced by
wind and waves. Because most atolls lie in the
zone of the trade winds, the wind usually
comes from a consistent direction. Conse-
quently, the wind affects various parts of the
atoll in different ways. Encrusting coralline
algae, which can endure the constant pound-
Figure 14.20 The Great Barrier Reef is a complicated system of thousands of small reefs, sand ing of the waves, build a distinct algal ridge
cays, and lagoons. Shown here are four reefs that are part of the Great Barrier Reef Marine Park. on the reef crest of the windward side of the
Photograph courtesy of the Great Barrier Reef Marine Park Authority. atoll, the side that faces the prevailing wind.
On the few Caribbean atolls, the coralline
algae may be replaced by especially wave-
resistant corals. The algal ridge is less promi-
nent or absent on the leeward, or sheltered,
side of the atoll. Spur-and-groove formations,
too, are better developed on the windward side.
The fore-reef, or outer, slope is nearly ver-
tical, though there is usually a series of ledges
and overhangs. The rocky wall of the reef ex-
tends down to great depths, far beyond the
limit of living corals themselves. The water
may be hundreds, even thousands, of meters
deep just a stones throw from the reef.
The lagoon, on the other hand, is rela-
tively shallow, usually about 60 m (195 ft)
deep. The bottom of a lagoon is very uneven,
with many depressions and coral pinnacles.
Some pinnacles rise almost to the surface,
where they may form mini-atolls, small
rings of coral within the lagoon.

Atolls are rings of reef, with steep outer

slopes, that enclose a shallow lagoon.
Figure 14.21 Fulanga atoll, in the South Pacific nation of Fiji.

oceans. Atolls are rare in the Caribbean and in pure blue ocean water, atolls display spec- How Atolls Form When atolls were dis-
the rest of the tropical Atlantic Ocean. Un- tacular coral growth and breathtaking water covered, scientists were at a loss to explain
like fringing and barrier reefs, atolls can be clarity. They are a divers dream. them. It was known that corals can grow only
found far from land, rising up from depths of in shallow water, yet atolls grow right in the
thousands of meters or more. With practi- Atoll Structure Atolls range in size from middle of the ocean, out of very deep water.
cally no land around, there is no river-borne small rings less than a mile across to systems Therefore, the atoll could not have grown up
silt and very little freshwater runoff. Bathed well over 30 km (20 mi) in diameter. The from the ocean floor. If the reefs grew on
 Coral Reefs Chapter 14 299

Sand cay

Reef flat

Lagoon
Fore-reef Wind
(outer) Back-reef
slope (inner) slope
Spur and
groove
Back-reef
(inner) slope
Algal
Pinnacle ridge

Fore-reef
(outer)
slope

Figure 14.22 Typical structure of an atoll.

some kind of shallow structure that was al- hypotheses, none of which held up. Finally The Ecology of Coral Reefs
ready there, like a seamount, why is there no scientists found conclusive evidence that Dar-
sign of it? The islands on atolls are simple win was right. Unlike other hypotheses for Coral reefs may be impressive to geologists,
sand cays that have been built by the accu- atoll formation, Darwins hypothesis predicted but to the biologist they are simply awesome.
mulation of reef sediments and would not that, below the thick calcium carbonate cap They are easily the richest and most complex
exist without the reef. They are products of formed by the reef, there should be volcanic of all marine ecosystems. Literally thousands
the reef and could not provide the original rockthe original island. In the 1950s the of species may live on a reef. How do all
foundation for reef growth. Finally, why do United States Geological Survey drilled several these different species live? How do they af-
atolls always form rings? deep holes on Enewetak atoll in the Marshall fect each other? What is their role in the reef
The puzzle of atoll formation was solved Islands. These cores revealed exactly what ecosystem? These and countless other ques-
by Charles Darwin in the mid-nineteenth cen- Darwin predicted: volcanic rock far beneath tions fascinate coral reef biologists.
tury. Darwin is most famous, of course, for the calcium carbonate of the reef. The thick- Our ability to answer the questions, how-
proposing the theory of evolution by natural ness of the carbonate cap is impressive. The ever, is surprisingly limited. This is partly be-
selection, but his theory of atoll formation was volcanic island underlying Enewetak is cov- cause reefs are so complicated. Just keeping
another important contribution to science. ered by more than 1,400 m (4,600 ft) of cal- track of all the different organisms is hard
Darwin reasoned that atolls could be ex- cium carbonate!
plained by reef growth on a subsiding island. Scientists now believe almost unanimously
The atoll gets its start when a deep-sea vol- that Darwins hypothesis of atoll formation is
The trade winds, the steadiest winds on
cano erupts to build a volcanic island. Corals correct. There are, of course, a few details to earth, blow from latitudes of about 30
soon colonize the shores of the new island, be added to the picture, in particular the ef- toward the Equator.
and a fringing reef develops (Fig. 14.23a). As fects of changes in sea level (see Climate and
with most fringing reefs, coral growth is most Changes in Sea Level, p. 32). When sea level Chapter 3, p. 53; Figure 3.20
vigorous at the outer edge of the reef. The is low, atolls may be left above the surface. The
Subsidence The slow sinking into the
inner reef is strongly affected by sediment corals die and the reef is eroded by the wind
mantle of a part of the earths crust that
and runoff from the island. and rain. If sea level rises rapidly, the atoll may contains a landmass.
Surprisingly, Darwins explanation of atoll be drowned, unable to grow in deep water. In
formation was pretty much ignored for a cen- either case, corals recolonize the atoll when Chapter 11, p. 226
tury while scientists proposed various other the sea level returns to normal.
 300 Part 3 Structure and Function of Marine Ecosystems www.mhhe.com/marinebiology

Island
Fringing reef

Barrier reef
Lagoon

(a)
Atoll

(b)

Figure 14.23 (a) An atoll begins as a

fringing reef around a volcanic island. (b) As
the island slowly sinks, the reef flat gets wider Calcium carbonate
and deeper and eventually becomes a lagoon. Original island
At this stage the fringing reef has become a
barrier reef. (c) Eventually the island sinks (c)
altogether, leaving only a ring of living
growing reefan atoll.

enough; the task of figuring out what they all

do is mind-boggling. Furthermore, until the Sunlight
last half century or so most marine biologists
lived and worked in the temperate regions of CO2 H2O
the Northern Hemisphere, far from the near- Diffusion Seawater
est reef. As a result, relatively few biologists
studied reefs. Tremendous progress has been Coral polyp
made in recent years, but there is still much
to learn. The rest of this chapter summarizes Coral
CO2 + H2O Organic polyp
what is known about the ecology of coral + nutrients Photosynthesis matter
+ O2
reefs and points out some of the important
questions that remain. Zooxanthellae
Diffusion
Diffusion
The Trophic Structure
of Coral Reefs
CO2 + H2O Organic
The tropical waters where coral reefs are matter + O2
Respiration
+ nutrients
found are usually poor in nutrients (see Pat-
terns of Production, p. 329) and therefore
have very little phytoplankton or primary Figure 14.24 Carbon dioxide and nutrients are continually recycled between a coral polyp
and its zooxanthellae.
production. In these barren waters, coral
reefs are oases of abundant life. How can
such rich communities grow when the sur- ate skeleton. In return the zooxanthellae get by the zooxanthellae. Using sunlight, the
rounding sea is so unproductive? not only a protected place to live, but also a zooxanthellae incorporate the nutrients into
A good part of the answer lies in the mu- steady supply of carbon dioxide and nutrients organic compounds, which are passed on to
tualistic relationship between corals and their such as nitrogen and phosphorus. Most of the coral. When the coral breaks down the
zooxanthellae. We have already learned what the corals nitrogen and phosphorus waste organic matter, the nutrients are released and
the zooxanthellae do for the coral: They pro- products are not released into the water. In- the whole process begins again (Fig. 14.24).
vide food and help make the calcium carbon- stead, they are taken up and used as nutrients The nutrients are recycled, used over and
 Coral Reefs Chapter 14 301

over, so that far fewer nutrients are needed Wrasses Damselfishes

than would otherwise be the case. (eggs, larvae) (eggs, larvae)
Nutrient recycling occurs not just between Triggerfishes Zooplankton
corals and their zooxanthellae, but among all (coral) (mucus)
the members of the coral reef community.
Sponges, sea squirts, giant clams, and other
reef invertebrates have symbiotic algae or Parrotfishes
(coral) Flatworms Butterflyfishes
bacteria and recycle nutrients just as corals (coral) (coral, eggs,
do. Recycling also takes place outside the Shrimps larvae)
bodies of reef organisms. When fishes graze (mucus)
on seaweeds, for example, they excrete nitro-
gen, phosphorus, and other nutrients as
waste. These nutrients are quickly taken up
by other algae. Many corals provide shelter to Snails
(coral)
schools of small fish (see Fig. 8.20). The fish
leave the coral at night to feed and return Sea urchins
(coral)
during the day. The waste products of the Crabs (mucus) Sea stars
Sea fans (mucus) Bacteria (mucus) (coral)
fish can be an important source of nutrients
and help the coral grow faster. In this way
nutrients are cycled from whatever the fish Figure 14.25 A number of animals eat coral directly, and many others feed on the mucus
feed on to the coral. Nutrients pass through that corals produce or on coral eggs and larvae. The primary production of coral zooxanthellae is
the community again and again in this cycle thus passed on to coral feeders, and then to the animals that eat them.
of feeding and excretion.
Coral reef communities use nutrients very
efficiently as a result of recycling. The recy- ents in the zooplankton are passed on to the tissue on a coral colony. It was therefore be-
cling is not perfect, however, and some nutri- reef community. In fact, many biologists lieved that, even though zooxanthellae pro-
ents are lost, carried away by the currents. think that corals eat zooplankton not so duce a lot of organic matter, most of it was
Thus, the reef still needs a supply of new nu- much to feed themselves as to get nutrients consumed by the coral and not much passed
trients. Recycling alone is not enough to ac- for their zooxanthellae. Animals like seabirds, on to the rest of the community. As biolo-
count for the high productivity of reefs. dolphins, and large fishes that spend part of gists looked closer, however, they found
The reef is able to provide some of its own their time around reefs but venture out into more and more animals that eat corals or
nutrients. Coral reefs have among the highest the open ocean to feed also transfer nutrients their products (Fig. 14.25). Primary pro-
rates of nitrogen fixation of any natural from oceanic waters to the reef community. duction by coral zooxanthellae therefore may
community. The main nitrogen fixers are be important not only to corals but to the
cyanobacteria, especially a free-living one community at large. Exactly how much pro-
called Calothrix and another group that lives Coral reefs are very productive even though
duction corals and their zooxanthellae con-
symbiotically in sponges. There is evidence the surrounding ocean water lacks nutrients
tribute is still unknown.
that corals, too, have symbionts that fix nitro- because nutrients are recycled extensively, ni- Seaweeds are also important primary pro-
gen, providing nutrients for the zooxanthel- trogen is fixed on the reef, and the zooplank- ducers on the reef (Fig. 14.26), especially the
lae. Just what the symbionts are is not ton and nutrients that occur in the water are small, fleshy or filamentous types that are
known. Because of this nitrogen fixation, ni- called turf algae because they often grow in
used efficiently.
trogen probably does not limit most coral reef
communities, though not all reef biologists
agree about this. The production and efficient use of nutri-
Primary Production The conversion of
Ocean currents bring in additional nitro- ents by coral reef communities result in high
carbon from an inorganic form, carbon
gen and, more importantly, phosphorus and primary productivity. This is reflected in the dioxide, into organic matter by autotrophs,
other nutrients that are not produced on the overall richness of the community. Scientists that is, the production of food.
reef. Corals, bacteria, algae, and other organ- arent sure, however, just how much primary
isms absorb nutrients directly from the water. production there is on coral reefs, or which Chapter 4, p. 70
Even though the water is nutrient-poor, if particular organisms are the most important
Nitrogen Fixation Conversion of
enough water washes over the reef the nutri- producers. There is no doubt that zooxan-
nitrogen gas (N2) into nitrogen compounds
ents add up. In business terms, this is a low- thellae are very important, but because they that can be used by primary producers as
margin, high-volume proposition. More im- live inside corals, it is hard to measure ex- nutrients.
portantly, the water carries zooplankton, a actly how much organic matter they pro-
rich nutrient source. When zooplankton are duce. For a time it was thought that very few Chapter 5, p. 90; Table 5.1
captured by the wall of mouths, the nutri- animals eat coral, since there is so little live
 302 Part 3 Structure and Function of Marine Ecosystems www.mhhe.com/marinebiology

Predators
fishes, squids, snails Herbivory
Predation
Detritus feeding or export
Contribution to detritus pool

Consumers Grazers Detritus feeders Coral and coral mucus feeders Plankton feeders
fishes, urchins, snails, sea cucumbers, worms, fishes, sea stars, crabs fishes, sea fans, feather stars
chitons amphipods, soft corals

From other
communities Plankton
(estuaries, Detritus
subtidal, especially
mangroves and
seagrass beds)

Seaweeds, coralline algae, Corals / Zooxanthellae

photosynthetic bacteria
Producers

Figure 14.26 A generalized coral reef food web. Coral reefs are extremely diverse, and most components include many organisms in addition
to those listed here.

a short, thick turf on the reef flat. A great is fascinating; what remains to be learned will neighbors off from the light. Other corals
many fishes, sea urchins, snails, and other be even more so. take a more direct approach and actually at-
animals graze on these seaweeds. The tack their neighbors (Fig. 14.27). Some use
turf algae may perform more photosynthesis Competition Space is at a premium on their mesenterial filaments for this. When
on the reef than the zooxanthellae, but biolo- coral reefs, as it is in the rocky intertidal (see they contact another coral, they extrude the
gists are not sure. The Battle for Space, p. 232). Corals, sea- filaments and digest away the tissue of the
weeds, and many others need a hard place on other coral. Still other corals develop special
Zooxanthellae and turf algae are probably the which to anchor themselves. Corals and sea- long tentacles, called sweeper tentacles, that
weeds need not just space but space in the are loaded with nematocysts and sting neigh-
most important primary producers on coral
sunlight. The reef is crowded, and most of boring colonies. Corals differ in their aggres-
reefs. sive abilities. The most aggressive corals tend
the available space is taken. As a result the
sessile organisms, those that stay in one to be slow-growing, massive types, whereas
Cyanobacteria, some other bacteria, and place, must compete for space. the less aggressive forms are usually fast-
coralline algae are also primary producers on growing, upright, and branching. Both strate-
coral reefs. They probably account for less gies have their advantages, and both kinds of
primary production than do zooxanthellae Sessile coral reef organisms must compete for corals thrive on the reef.
and turf algae. space. Corals and seaweeds compete for light
as well.
The two main ways in which corals compete
Coral Reef Communities
for space are by overgrowing their neighbors
With so many species on coral reefs, the in- Corals compete for space in different
teractions among them are exceedingly com- ways. The fast-growing ones tend to grow up- and by directly attacking them.
plex. What is known about these interactions ward and then branch out, cutting their
 Coral Reefs Chapter 14 303

Figure 14.27 When different species of

coral come in contact, they attack each other.
The pink band separating the brown (Porites
lutea) and blue (Mycedium elephantotus) corals
is a dead zone where the blue coral has killed
the brown one in the process of overgrowing
it. The width of the pink band corresponds to
the length of the blue corals tentacles. To the
upper left of the brown coral is a soft coral Figure 14.28 Soft corals can form dense patches on coral reefs, as on this reef in Papua
(Sarcophyton), which may be attacking the New Guinea.
brown coral by releasing poison. The brown
coral seems to be stuck between a rock and a
a calcium carbonate skeleton and are able to also more easily torn away by storm waves.
soft place! grow faster than hard corals. Some soft corals They have symbiotic zooxanthellae like reef-
contain sharp little calcium carbonate nee- building corals, but are much less efficient
dles, or spicules, that discourage predation. photosynthesizers. They also seem to depend
Corals compete for space and light not Many of them also contain chemicals that are on very favorable physical conditions. It is
only with each other, but also with seaweeds toxic or taste bad to predators. Because of not known how these factors interact to de-
and sessile invertebrates. Like corals, encrust- these defense mechanisms, only a few spe- termine when and where soft corals compete
ing algae have to produce a calcium carbon- cialized predators eat soft corals. The defen- successfully for space on the reef.
ate skeleton and therefore grow relatively sive chemicals can also be released into the Like soft corals, sponges often have
slowly. They tend to be found in places water, where they kill hard corals that come spicules and nasty chemicals that protect
where corals dont do well because of sedi- too close (Fig. 14.27). Another competitive them from predators. They can be important
mentation, wave action, or predation. advantage enjoyed by some soft corals is that users of space on reefs, much more so in the
Under the right conditions seaweeds, ex- they are not completely sessile. Though they Caribbean than in the Pacific and Indian
cept for encrusting forms, can grow much stay in one place most of the time, they can oceans. Part of the reason for this seems to be
faster than either corals or encrusting algae. move about slowly. This helps them invade that there are fewer species of coral in the
Even with the nitrogen fixation and nutrient and occupy available space on the reef. Caribbean than in the Indo-West Pacific. This
cycling that occur on reefs, seaweeds are is a result of geological history. During the
probably somewhat nutrient-limited most of Soft corals are important competitors for space most recent series of ice ages, the sea surface
the time. Hence they grow fairly slowly. The was cooler. Corals survived in the heart of the
on reefs. They grow rapidly, are resistant to
reef also has an abundance of hungry grazers Indo-West Pacific region, around Indonesia
that eat the seaweeds. The combination of predators, and can occasionally move about. and New Guinea, but many coral species be-
nutrient limitation and grazing keeps the sea- came extinct in other parts of the ocean.
weeds in check. If the nutrient levels increase With all these competitive weapons at When the ice age ended, corals spread out
or the grazers are removed, seaweeds may ra- their disposal, why dont soft corals take again across the Pacific, recolonizing areas
pidly take over, overgrowing and choking out over? Not much is known about how soft where they had died out. The Caribbean,
corals and other organisms. corals compete with reef-building corals and however, was not recolonized because the
Soft corals are also important competitors other reef organisms or what determines the Isthmus of Panam blocked their dispersal. It
for space on reefs (Fig. 14.28), and in some winner. Many soft corals appear to have is thought that the Caribbean contains only
places they make up almost half the living tis- shorter lives than reef-building corals al- those coral species that managed to survive
sue. Like most seaweeds, soft corals lack though some may live for decades. They are the ice ages there.
 304 Part 3 Structure and Function of Marine Ecosystems www.mhhe.com/marinebiology

There is much debate about how competi-

tion affects coral reef fishes. One hypothesis
is that competition is relatively unimportant,
and that the abundances of individual species
are determined mainly by how many of their
larvae settle out from the plankton. With fa-
vorable currents and other conditions, many
larvae will be available to settle out and the
species becomes abundant. If the supply of
larvae is low, for example if currents carry the
larvae away from the reef, the species be-
comes rare. This is called a pre-settlement
hypothesis because it holds that the nature of
reef fish communities is determined by the
availability of different species larvae before
the larvae settle.
An alternative, post-settlement, hypothe-
sis is that for most species there are plenty of
larvae available to settle out on the reef. The
species that survive and become abundant are
those that compete successfully for space,
Figure 14.29 Diving on a coral reef is like taking a swim in a tropical fish aquariumbrightly food, and other resources after the larvae set-
colored fishes seem to be everywhere. The competitive relationships among the various species tle, that is, as juveniles and adults. This hy-
of fishes are poorly understood. pothesis holds that so many different fish
species can live on the reef because each does
something a little different from other species,
thereby avoiding competition and the risk of
competitive exclusion. In other words, each
species has its own particular ecological
niche. According to this view, the structure of
the fish community on a reef is determined by
the range of resources available on that reef,
and the fish communities of reefs vary be-
cause different reefs offer different resources.
It is still not clear whether the supply of
larvae for settlement or post-settlement com-
petition is more important in structuring reef
fish communities. Their relative importance
probably varies from species to species and
from reef to reef, and other factors such as
predation and natural disturbances are also
significant.

There are two competing schools of thought

about what controls the structure of reef fish
communities. One holds that reef fish abun-
Figure 14.30 The chevron butterflyfish (Chaetodon trifascialis) is one of many reef animals dances are determined by how many larvae
that feed on corals without killing the entire coral colony. Its mouth is adapted for nipping off
are available to settle out from the plankton.
individual coral polyps (also see Fig. 8.28).
The other asserts that there is an ample supply
of larvae for most species, and that reef fish
Coral reef fishes are another group in similar diets; for example, many species eat communities are structured by competition
which competition may be important. Along coral, many graze on algae, and many are car-
among juveniles and adults after the larvae
with corals, fishes are probably the most con- nivores. Different species of fishes of the same
spicuous and abundant animals on the reef feeding type at least potentially compete with settle out.
(Fig. 14.29). Many of these fishes share each other.
 Coral Reefs Chapter 14 305

Figure 14.31 The crown-of-thorns sea star

(Acanthaster planci) is an important and
controversial coral predator.

Predation on Corals As in other commu-

nities, predation and grazing are important in
structuring coral reef communities. A variety
of animals eat corals, but instead of killing
the coral and eating it entirely, most coral
predators eat individual polyps or bite off
Figure 14.32 An outbreak of crown-of-thorns sea stars (Acanthaster planci) in Australia. The
pieces here and there (Fig. 14.30). The coral white branches of this colony of staghorn coral (Acropora) show the bare calcium carbonate
colony as a whole survives and can grow skeleton left behind after being fed upon by several sea stars. Photograph courtesy of the Great
back the portion that was eaten. In this re- Barrier Reef Marine Park Authority.
spect, coral predation is similar to grazing by
herbivores.
Coral predation affects both the number 1950s, people began to notice large numbers, The debate over what causes these out-
and type of corals that live on a reef, as well sometimes thousands, of the sea star on reefs breaks has been controversial and sometimes
as how fast the reef as a whole grows. In scattered across the Pacific (Fig. 14.32). The emotional. At first it seemed obvious that the
Kaneohe Bay, for example, a butterflyfish sea stars in these large aggregations move in a outbreaks must be unnatural, with humans to
(Chaetodon unimaculatus) slows the growth mass across the reef, consuming almost every blame. After all, the plagues had never oc-
of a particular coral (Montipora verrucosa) coral in their path. Reefs recover in 10 to curred before. Or had they? People had only
that it likes to eat. When the coral is pro- 15 years, though it may take longer for slow- begun using scuba a short time before the sea
tected from the fish by a cage, it grows much growing coral species to grow back. star plagues were noticed. Even if the plagues
faster. If it were not for the fish, this fast- The first response to the problem was had been occurring for a long time, there were
growing coral would probably dominate panic. Coral reefs are valuable resources:
other corals in the bay. Coral-eating snails They support fisheries, tourism, and recre-
(Coralliophila, Drupella) have similar effects ation and protect coastlines from erosion.
on some reefs. With the crown-of-thorns apparently threat- Competitive Exclusion The
elimination of one species by another as a
ening reefs, people decided to take action
result of competition.
The Crown-of-Thorns Sea Star Another and control the sea star. The first attempt
example of the effect of coral predators is backfired. With limited knowledge of the Chapter 10, p. 210
the crown-of-thorns sea star (Acanthaster animals biology, some people cut the sea
planci; Fig. 14.31). The crown-of-thorns stars into pieces and dumped them back in Ecological Niche The combination of
what a species eats, where it lives, how it
feeds by pushing its stomach out through the the sea. Because sea stars can regenerate, the
behaves, and all the other aspects of its
mouth, covering all or part of the coral pieces grew into new sea stars! More sophis-
lifestyle.
colony with the stomach, and digesting away ticated methods, such as poisoning the sea
the live coral tissue. The crown-of-thorns has stars, were tried, but these methods were Chapter 10, p. 211
distinct preferences for certain types of coral time consuming and expensive, did not
and avoids otherssome corals must taste work that well, and sometimes did more Tube Feet Water-filled tubes possessed
only by echinoderms, many of which end in
bad. Other corals harbor symbiotic crabs (see harm than good. Fortunately, the outbreaks
a sucker and can be extended and
the photo on p. 113), shrimps, and fishes mysteriously went away by themselves. Did
contracted to grip things and to move
that discourage the sea stars by pinching and the sea stars starve? Did they move away? around.
biting their tube feet. No one knows. Crown-of-thorns outbreaks
The crown-of-thorns has had a major im- are still appearing, and disappearing, with- Chapter 7, p. 136
pact on some reefs. Beginning in the late out explanation.
 306 Part 3 Structure and Function of Marine Ecosystems www.mhhe.com/marinebiology

no scientists around to see them. Geologists for example, a popular dive spot or a scien-
found fossil evidence of crown-of-thorns out- tific research site. Otherwise, they allow the
breaks dating back thousands of years, and in outbreak to run its course.
some places there are old stories and other
historical evidence for past outbreaks. The The crown-of-thorns sea star has undergone
outbreaks, then, may be a natural part of the
population explosions on many Pacific reefs.
reef ecosystem. A leading hypothesis is that in
unusually wet years river runoff naturally There is still debate about what causes the
brings more nutrients into the sea than nor- outbreaks and what should be done about
mal. According to this hypothesis, the extra them.
nutrients increase the growth of phytoplank-
ton that are food for sea star larvae. It may Figure 14.33 The long-spined black sea
also be that periodic population explosions Grazing Grazing on algae by herbivores is urchin (Diadema antillarum) is one of the most
are a natural part of the sea stars biology. at least as important in coral reef ecosystems important grazers on Caribbean reefs. Closely
The evidence for past outbreaks of the as is predation on corals. Many fishes, espe- related species are found on reefs in other
crown-of-thorns, however, is hotly disputed. cially surgeonfishes (Acanthurus), parrot- parts of the world.
Some scientists are convinced that the fishes (Scarus, Sparisoma), and damselfishes
plagues result from some human activity that (Pomacentrus, Dascyllus) graze intensively on
has altered the ecological balance of reefs. reefs. Among the invertebrates, sea urchins slack for the fishes, and seaweed populations
Even if outbreaks did occur in the past, they (Diadema, Echinometra) are especially impor- remained more or less stable. In 1983, how-
seem to be happening more often. If occa- tant. Many microherbivores, small inverte- ever, a disease wiped out populations of the
sional peaks in nutrient inputs cause natural brates like snails, chitons, crustaceans, and urchin over much of the Caribbean. Sea-
outbreaks, nutrients from fertilizers, sewage, polychaete worms, also eat algae. weeds, released from grazing pressure, be-
and other human sources may be making Many seaweeds grow rapidly and have the came much more abundant on many reefs, at
them worse. Another hypothesis is that fish- potential to outcompete and overgrow corals. the expense of corals. In Jamaica they took
ers have caught too many fishes that eat Under natural conditions they are kept in over many reefs almost completely, and the
young sea stars, allowing more to survive to check by grazers, and to some extent by nu- formerly rich coral reefs are now more sea-
adulthood. Some biologists have suggested trient limitation. Caging experiments (see weed bed than coral reef.
that plagues occur because shell collectors Transplantation, Removal, and Caging Ex- Many reef scientists fear that such
have removed the triton shell, a large snail periments, p. 236) have been used to changes are becoming more and more com-
that preys on adult crown-of-thorns. Others demonstrate the importance of grazers. For mon as fishing pressure on reefs escalates be-
argue that even without shell collectors the example, seaweeds are abundant on sand flats cause of increasing populations and demand
triton shell has always been naturally rare and next to many Caribbean reefs but relatively for tasty reef fish. There is often a double
unlikely to control sea star populations. They scarce on the reef itself. To test the hypothe- whammy. Coastal development often leads
also point out that crown-of-thorns outbreaks sis that reef fishes were responsible for this, not only to more intensive fishing, but also
continue to occur in areas where collecting biologists transplanted seaweeds from the to greater release of nutrients from sewage
triton shells has been banned for years. sand flat to the reef. If left unprotected, they and agricultural fertilizer. Thus, seaweed
The mystery of what causes crown-of- were soon eaten by fishes. When protected growth may be artificially enhanced by eu-
thorns plagues remains unsolved. The ques- by cages, they grew even faster than on the trophication at the same time that the grazers
tion has practical implications for the man- sand flat! The seaweeds are perfectly capable that keep the seaweeds under control are re-
agement and protection of reefs. If the of living on the reef, therefore, but are rare moved. This is one of the main global threats
plagues are caused by humans and threaten there because they get eaten. Caging experi- to coral reefs.
reefs, then we should probably try to do ments on the Great Barrier Reef have had
something to stop them. On the other hand, similar results. Grazers help prevent fast-growing seaweeds
the plagues may be a natural, potentially If grazers are removed, seaweeds can flour-
from overgrowing other sessile organisms on
important, part of the ecosystem. We might ish and take over space from corals and other
do more harm than good by interfering in a organisms. In many parts of the Caribbean, the reef.
system we dont understand. The managers of for example, grazing reef fishes have become
the Great Barrier Reef have taken a middle less common because of fishing. When this In addition to controlling how much algae
road. They have developed an effective happened another important grazer, a sea there is, grazers affect which particular types
method to kill sea stars by injecting them urchin (Diadema antillarum; Fig. 14.33), be- of seaweed live on the reef and where.
with a poison. They use the poison, however, came more common. The urchin apparently Coralline algae, for example, are abundant
only if a crown-of-thorns outbreak is threat- benefited from the reduced competition. For because the calcium carbonate in their tissues
ening a particularly valuable part of the reef, years, the urchin seems to have picked up the discourages grazers. Other seaweeds produce
 Coral Reefs Chapter 14 307

Must Have Been Something I Ate

Ahhhhh! Youre relaxing on a warm tropical night after a delicious places, but poisonous in particular spots. To make matters even
fish dinner. The palm trees sway, a warm tropical breeze blows more confusing, ciguatera may disappear from one area and pop
and suddenly your bowels begin to churn. Before long your lips up in another. All of which makes things difficult for lovers of fresh
burn and tingle, and your hands and feet feel like pins and nee- reef fish.
dles. You go into the kitchen for a drink, but the cool water feels Ciguatera is caused by toxic dinoflagellates that live on the
warm. The cool floor and the cold compress you put on your fore- reef. Herbivorous fishes eat the dinoflagellates, and the poison is
head are also warm to the touch: The sensations of hot and cold passed on to the predatory fishes that eat them. As blooms of the
are reversed. As you stagger to the bathroom, your arms and legs dinoflagellates arise and die out, ciguatera comes and goes in the
are heavy and weak. Now feeling really sick, you mumble, Must area. Predatory fishes range over larger areas and eat many fish,
have been something I ate. which may all contain small amounts of the toxin. Thus, they are
Youre right. What you have is a case of ciguateratropical most likely to cause ciguatera. Herbivorous fishes probably carry
fish poisoning. Maybe it will comfort you to know that youre not the toxin only when they feed in an area with a bloom.
alone. Tens, perhaps hundreds of thousands, of people get The big problem is trying to tell when a fish carries the poi-
ciguatera every year. And relax, you probably wont die, though son. You could just avoid reef fish altogether, but if you are in the
you might be a little sick (if youre unlucky, very sick) for months or tropics that means missing out on a lot of tasty meals. One way to
even years. Dont count on a cure, though there are various folk tell if a fish is carrying ciguatera is to feed a bit to a cat: Cats are
remedies that might help a little. The intravenous administration highly sensitive to ciguatera. If any mongooses are handy, theyre
of an inexpensive sugar called mannitol also helps sometimes. good tasters too. Of course, this isnt exactly kind to the animals,
So what is ciguatera, anyway? The disease has been known who usually die if the fish carries ciguatera. Chemical tests for
for hundreds of years. The name comes from the Spanish word ciguatera poison are available but are not yet in widespread use.
for a Caribbean snail: You can also get ciguatera from eating mol- Furthermore, there are actually several different dinoflagellate
luscs, and perhaps sea urchins, though fish dinners are the most toxins that can cause ciguatera and it is difficult to test for them
common cause. Ciguatera is most common in the top predatory all. We arent even sure we know what they all are. For now at
fishes on the reef, like jacks, barracudas, and groupers, but it can least, the best strategy is to avoid eating fish species known to
also be caused by eating herbivorous fishes like parrotfishes and carry a high risk of ciguatera. Otherwise, its go hungry or take
surgeonfishes. A given kind of fish may be perfectly safe in most your chances.

noxious chemicals that are poisonous or taste geonfishes gobble up the algae, clearing space
bad; these seaweeds also tend to be abun- for other organisms. Thus, the community in- Symbiotic relationships are very important in
dant. Seaweeds that lack such defenses are side the territory is very different from that coral reef communities. Coral reefs probably
most heavily grazed and thus tend to be rare. outside. One interesting point is that have more examples of symbiosis than any
Even so, they generally grow rapidly and are cyanobacteria, which are nitrogen fixers, are
other biological community.
an important food source. much more common inside damselfish terri-
Damselfishes provide some interesting ex- tories than outside. Thus, damselfishes may
amples of the effects of grazing on reefs. indirectly have an important role in the nutri- We have already seen how mutualism be-
Many damselfishes graze on seaweeds inside ent balance of the reef. tween corals and their zooxanthellae is the es-
territories that they vigorously defend, chas- sential feature of reef formation. Many other
ing away other fishes that happen to venture
inside. Many such damselfishes actually Living Together Among the vast number
farm their territories. They weed out un- of species that live on coral reefs, many have
Chitons Molluscs whose
palatable algae, pulling them up and carrying evolved special symbiotic relationships. There shells consist of eight
them outside the territory. What is left in the are far too many cases of symbiosis on the reef overlapping plates on their
territory is a dense mat of tasty seaweeds, to describe here. In fact, coral reefs probably upper, or dorsal, surface.
usually fine, filamentous types. Protected by have more different symbiotic relationships
the damselfish, these algae grow very rapidly than any habitat on earth. The few examples Chapter 7, p. 129
and outcompete corals and coralline algae. discussed here will give you some idea of how
Outside the territory, parrotfishes and sur- fascinating these relationships are.
 308 Part 3 Structure and Function of Marine Ecosystems www.mhhe.com/marinebiology

kinds of sea anemones. The anemones in-

habited by anemonefishes have a powerful
sting and are capable of killing the fish. The
fish have a protective mucus, however, that
keeps the anemone from stinging. It is not
known whether the mucus is produced by
the fish themselves, by the anemone, or by
both. When anemonefishes are newly intro-
duced to an anemone, they typically rub
against and nip the anemones tentacles. They
may be coating themselves with the
anemones mucus. To understand why this is
an advantage, you have to remember that
anemones have no eyes and no brain, but lots
of tentacles. If the anemone simply stung
everything it touched, it would end up sting-
ing itself every time the tentacles bumped
into each other. The anemone recognizes it-
self by the taste of its own mucus. When
one tentacle touches another, the anemone
detects the mucus coating on the tentacles
and refrains from stinging. Anemonefishes
Figure 14.34 The giant clam (Tridacna Figure 14.35 An orange-fin anemonefish may be taking advantage of this. On the other
gigas) does not deserve its reputation as a hand, there is evidence that the fishes own
(Amphiprion chrysopterus) and its sea anemone
killer: Stories of people getting trapped in
host. mucus protects them against the anemone,
them and drowning are myths. Symbiotic
and in some cases the fish can enter an
zooxanthellae give the upper surface its blue
and green colors, and provide food that anemone unharmed without having ever con-
allows the clam to get so big. The clam also tacted it before.
symbiotic organisms that eventually became
filter feeds. The hole visible on the clams Anemonefishes are protected by the
the chloroplasts of plants (see From Snack to
fleshy upper surface is one of the siphons anemones stinging tentacles and brood their
Servant: How Complex Cells Arose, p. 75).
through which it pumps water to feed and eggs under the anemone. In at least some
Another important example of mutualism
obtain oxygen. Giant clams do not occur in cases the relationship is mutually beneficial
the Caribbean. on the reef is the relationship between corals
because the anemonefish drive away other
and the crabs, shrimps, and fishes that help
fishes that eat anemones. Anemonefishes
protect them from the crown-of-thorns sea
kept in aquaria will sometimes feed their
organisms also have photosynthetic sym- star and other predators. Most corals host a
host, but this behavior does not seem to be
bionts. Sea anemones, snails, and giant clams number of facultative and obligate sym-
important in nature. It is also possible that
(Tridacna) all harbor zooxanthellae. The bionts, especially crustaceans. Some of the
the fishes benefit their host in ways that are
deal between the two partners is the same obligate symbionts are parasites and harm the
not yet understood.
as in corals: The zooxanthellae get nutrients coral, some are commensals and apparently
and a safe place to live, and the host gets have no effect on the coral, and some are mu-
food. Giant clams grow so large (Fig. 14.34) tualists that benefit the coral. It is often hard
because their zooxanthellae provide a con- to tell which is which, because for most sym-
stant food supply. bionts the nature of the relationship between
There are primary producers other than coral and symbiont is not well understood. Facultative Symbiont A symbiont that
zooxanthellae that live inside reef animals. As Those who study such organisms frequently is not completely dependent on its partner
and can live outside the symbiosis.
mentioned previously, some sponges have revise their ideas as more information is ob-
Obligate Symbiont One that depends
cyanobacteria that fix nitrogen in addition to tained. The crabs that protect their coral from
on its partner and does not occur outside
performing photosynthesis. Certain sea predators, for instance, were once thought to the symbiosis.
squirts house a photosynthetic bacterium be parasites.
called Prochloron. Prochloron is especially in- Anemonefishes (Amphiprion; Fig. 14.35) Chapter 10, p. 212
teresting because it may be similar to the have an interesting relationship with several
Interactive Exploration
Check out the Online Learning Center at www.mhhe.com/marinebiology and
click on the cover of Marine Biology for interactive versions of the following activities.

Do-It-Yourself Summary pp. 3057. A fabulous tour of the between latitudes. Oceanography and
The Online Learning Center provides a fill- earths largest reef area. Marine Biology: An Annual Review,
in-the-blank summary that allows you to vol. 36, pp. 6569.
Maragos, J. and D. Gulko (Editors),
review and check your understanding of
2002. Coral reef ecosystems of the Munday, P. L. and G. P. Jones, 1998. The
this chapters subject material.
northwestern Hawaiian Islands: Interim implications of small body size among
results emphasizing the 2000 surveys. coral-reef fishes. Oceanography and
Key Terms U.S. Fish and Wildlife Service and the Marine Biology: An Annual Review,
All key terms from this chapter can be Hawaii Department of Land and vol. 36, pp. 373411.
viewed by term, or by definition, when Natural Resources, Honolulu, Hawaii,
studied as flashcards in the Online Spalding, M. A., C. Ravilious and E. P.
46 pp.
Learning Center. Green, 2001. World Atlas of Coral Reefs.
Marshall, J., 1998. Why are reef fish so University of California Press, Berkeley,
colorful? Scientific American Presents, 424 pp.
Critical Thinking
vol. 9, no. 3, Fall 1998, pp. 5457. The
Wilkinson, C. (Editor), 2002. Status of
1. What factors might account for the fact often gaudy colors of reef fish serve to
Coral Reefs of the World: 2002. Australian
that the vast majority of atolls occur in attract mates, fend off rivals, and deter
Institute of Marine Science, Townsville,
the Indian and Pacific oceans and that predators.
378 pp.
atolls are rare in the Atlantic?
Pain, S., 1997. Swimming for dear life. New
2. Scientists predict that the ocean will Scientist, vol 155, no. 2099, 13 See It in Motion
get warmer and the sea level will rise September, pp. 2832. The tiny larvae Within this chapter, this icon
as a result of an intensified greenhouse of coral reef fishes are champion represents an underwater videoclip that
effect (see Living in a Greenhouse: swimmers. They have to be. helps illustrate a specific topic being
Our Warming Earth, p. 394). How discussed. These videoclips can be
might this affect coral reefs? Ross, J. F., 1998. The miracle of the viewed within the Online Learning Center
reef. Smithsonian, vol. 28, no. 11, for this chapter.
3. There are only a few reefs off the February, pp. 8696. Scientists study
northeast coast of Brazil (see map in mass coral spawning on reefs in Florida.
Fig. 14.10), even though it lies in the Marine Biology on the Net
tropics. How would you explain this? To further investigate the material
IN DEPTH discussed in this chapter, visit the Online
Baker, A. C., 2003. Flexibility and Learning Center and explore selected
For Further Reading
specificity in coral-algal symbiosis: web links to related topics.
Some of the recommended readings
listed here may be available online. These Diversity, ecology, and biogeography Coral reefs
are indicated by this symbol and will of Symbiodinium. Annual Review of
Class Anthozoa
contain live links when you visit this page Ecology and Systematics, vol. 34,
in the Online Learning Center. pp. 661689. Color bleaching
Hedley, J. D. and P. F. Sale, 2002. Are Mangroves
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Trends in Ecology and Evolution, vol. 17,
Benchley, P., 2002. Cuba reefs. National Competition
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Parasitism, predation, and herbivory
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