PORIFERA

Sponges are a conspicuous and colorful component of many seascapes. The attached,
often upright sponges are to coral reefs, sea grottoes, and floats what stalagmites
and stalactites are to terrestrial limestone caves, except that sponge colors are as
vivid and varied as those of van Gogh's flowers. When we look at sponges
underwater in tropical seas, it seems a stretch to admit these motionless organisms with
their irregular, often branched bodies to the pantheon of animals. Yet despite their
superficial similarity to plants, they are indeed animals, but, like plants, they capture
and concentrate dilute resources using their large surface area. Instead of relying on
leaves and roots to trap light, CO, and water for photosynthesis, sponges have
expanded their surfaces to catch the organic food particles suspended in seawater.
Other, higher metazoans also evolved the ability to sus- pension feed, but
sponges were surely the first to do so, and they continue to enjoy undiminished
success. This chapter ex- plores the functional design and diversity of
these strange but engaging animals.
Sponges evolved a multicellular body uniquely specialized for
filter feeding, the separation of suspended food particles from water by passing
them through a mesh that strains out the food. The body is unique because it
continuously remolds itself to fine-tune its filter-feeding system. This constant
rearrangement of tissues is brought about by the ameboid movements of cells that
wander throughout the sponge, adopt new positions, and change from one
differentiated form to another. Such dynamic tissues and totipotent cells suggest that sponges
are an intermediate form between protozoan colonies and other metazoans in which tissue
and cell specializations tend to be more permanent. Because of its intermediate
evolutionary status, Porifera generally is considered to be the sister taxon of the
remaining Metazoa (Eumetazoa).
As the name Porifera (= pore bearers) suggests, the sponge body is exceptionally porous. Water enters through the pores and flows
throughout the body in a system of flagellated canals. Food and other metabolites are removed
from the water flow for use by the sponge. Adult sponges are sessile and attached
organisms, although some are capable of limited movement of the body or its parts.
The connective tissue is well developed and typically forms a complex and often elegant
skeleton. Sponges range in size from a few millimeters to more than one meter in
diameter and height (as in, for example, loggerhead sponges). The body symmetry may
be radial (sphere, cone, cylinder), but asymmetry predominates. Indeterminate growth,
enlargement without a fixed upper size limit, is common. The growth forms may be
massive (thick), erect, branching, or encrusting, depending on the species and
environmental conditions (Fig. 5-1). Many species are brightly colored red,
calcareous rock require an exposed surface, but their elevated form
enables them to utilize water well above the substratum. The attachment area is a
relatively small part of their total body surface. B, The encrusting
sponges below the rock use much of their surface for attachment, but
their low encrusting form enables them to exploit the limited space of
crevices. C, The sponge on the cutaway vertical rock surface at the
left actually utilizes space in the substratum. Arrows indicate water
flow through the sponges.
purple, green, yellow, or orange, but some are brown or gray. The color results from
cell pigments or endosymbionts.
Approximately 8000 species of sponge have been described. Most
are marine, and they abound in seas wherever rocks, shells, submerged timbers, or corals
provide firm sites of attachment. Some species, however, anchor in sand or mud. Most prefer
relatively shallow water, but some taxa, including most glass sponges, live in deep
water. Some 150 species have colonized fresh water.
FORM
 The filter-feeding body of a sponge is built around one of three anatomical designs: asconoid,
syconoid, or leuconoid. The simplest of these, the asconoid design, is a hollow cylinder
attached by its base to the substratum (Fig. 5-2A, 5-3A). The body surface is covered by a
monolayer of flat cells called the pinacoderm (= platter skin). The hollow inte- rior, the atrium or
spongocoel, is lined with a monolayer of

flagellated collar cells called choanoderm (= collar skin). Many small pores, known
as ostia (sing., ostium) perforate the cylinder wall. A larger opening, the
osculum (pl., oscula), is situated at the upper, free end of the body. The
flagellated choanoderm creates a unidirectional water flow that enters the ostia, passes over
the choanoderm en route to the atrium, and exits through the osculum. This
circulatory system of choanoderm, pores, and chambers is called an aquiferous
system.
All asconoid sponges are small and have cylindrical, or tubular,
bodies, which typically do not exceed a diameter of 1 mm. Species of Leucosolenia
may be a single tube or a cluster of tubes joined at their bases, whereas Clathrina
species form a network of tubes (Fig. 5-3B). The asconoid design limits body size
because growth in diameter produces an unfavorable area-to-volume ratio. The amount
of water flow through the asconoid atrium (a volume) depends on the flagellated choan-
oderm (a surface). Because volume increases faster than
area with growth, the body volume soon exceeds the pump- ing capacity of the choanoderm (Fig. 5-2B; see Chapter 4 for a general
discussion of area and volume). Thus the attain- ment of larger body sizes in sponges
required a change of design.
One such innovation, the syconoid design (Fig. 5-2C, 5-3C), increased surface area and reduced atrial volume by forming alternating inpockets and
outpockets of the body wall. The arrangement can be visualized by interdigitating the
extended fingers of your two hands and imagining that your fingers are hollow. The
outpockets of the choanoderm are called choanocyte chambers (or radial canals). The
inpockets of the pinacoderm are incurrent canals. Incurrent canals dis- charge into the
choanocyte chambers via numerous small openings known as prosopyles (= front gates).
At the outer surface of the sponge, water may enter the incurrent canals directly or first pass
through a narrow ostium formed by a sec- ondary growth of tissue. Thus, water flow in
the syconoid
aquiferous system generally follows the route: ostia incurrent
canals prosopyles choanocyte chambers atrium → osculum. This new
design decreases the volume of the atrium and increases the area of flagellated
choano- derm. As a result, syconoid sponges generally are larger than asconoid
sponges. Their diameters are typically in the range of one to a few centimeters.
Syconoid sponges include species in familiar genera such as Grantia and Sycon
(previously called Scypha; Fig. 5-3B).
Sponges of leuconoid design achieve the largest body sizes,
ranging from a few centimeters to more than one meter (Fig. 5-3D, 5-4B-E). In
leuconoid sponges, the aquiferous system is a complex network of water vessels
that permeate a solid, spongy body. It consists of spherical choanocyte cham- bers
that lie at the intersection of incurrent and excurrent canals (Fig. 5-3D, 5-6). Small-diameter
excurrent canals and often multiple oscula replace the relatively voluminous
atrium and single osculum of asconoid and syconoid sponges. Water enters a
leuconoid sponge via surface ostia before flowing into incurrent canals (Fig. 5-
2D). From the incurrent canals, water passes through prosopyles into choanocyte
chambers. Water exits each choanocyte chamber through an apopyle (= back gate)
and then flows through the excurrent canals, which become progressively larger in
diameter as they join with other excurrent canals. Large excurrent canals discharge
exhaust water to the exterior via one or more oscula.

The choanocyte chambers of leuconoid sponges occur in high

densities. In Microciona prolifera, for example, there are approximately
10,000 chambers/mm3, each 20 to 39 μm in diameter and containing
approximately 57 flagellated cells (choanocytes). The leuconoid design vastly
increases the area of flagellated choanoderm while minimizing the water volume that must be
moved.
Most shallow-water marine sponges and all freshwater sponges are
built on the leuconoid design. Leuconoid sponges achieve large body size because
water pumping is decentralized: Any growth increment produces a sufficient number
of new choanocyte chambers to ventilate the new increment. Common leuconoid growth
forms include crusts, mounds, branches, fingers, plates, tubes, and vases (Fig. 5-3D,
5-4B-E).

BODY WALL
The body wall is thin in asconoid sponges, thick in most leu- conoid sponges, and
intermediate in thickness in syconoid species, but a clear-cut distinction occurs
between the glass sponges (Hexactinellida) and all others (Demospongiae and
Calcarea). In demosponges and calcareous sponges (among which are most sponges),
the body wall is cellular, but the glass-sponge body wall is a syncytium. A syncytium is
a large or extensive multinucleated cytoplasm enclosed by an external membrane but
not divided into cells by internal membranes.

Cellular

The bodies of demosponges and calcareous sponges (which together make up the
Cellularia) are composed of cells orga- nized into two types of tissue, epithelioid
and connective (Figures 5-5, 5-6). Epithelioid tissue resembles epithelium (see
Chapter 4), but it lacks epithelium's intercellular junc-
tions and hemidesmosomes and is not underlaid by a basal lamina (see Chapter 6). Sponge epithelioid tissues are the pinacoderm, which
covers the outer surface of the body (exopinacoderm) and lines the incurrent and
excurrent canals (endopinacoderm), and the flagellated choanoderm, which forms
the atrial lining (asconoid design) and the lin- ing of the choanocyte chambers
(syconoid and leuconoid. designs). The connective-tissue layer between the
pinaco- derm and the choanoderm is called the mesohyl (= middle wood) because it
forms a bushy, fibrous network that is espe- cially obvious in bath sponges.
The pinacoderm consists chiefly of two types of differenti- ated cells. By far the most common of these is the pinacocyte (= platter cell).
Pinacocytes are flattened (squamous) cells that abut each other edge-to-edge to
form a skinlike cellular pave- ment over the body surface and line the incurrent and
excurrent canals (Fig. 5-5, 5-6). Pinacocytes generally lack flagella, except for
species of Plakina and Oscarella that have a flagellated endopinacoderm lining their
canals. A less common but nevertheless important pinacoderm cell is the porocyte
(Fig. 5-5, 5-6). Porocytes form the ostia of all asconoid as well as many syconoid
and leuconoid sponges. They also constitute the prosopyles and apopyles of many
syconoid and leuconoid sponges, although in others the ostia are gaps between
adjacent pinacocytes and the prosopyles can be simple gaps in the choanoderm.
Each porocyte surrounds a pore, the diameter of which is regulated by contraction
of cytoplasmic filaments. Thus, porocytes are miniature sphincter valves.
The mesohyl is the only layer of the sponge body wall that typically is not bathed with environmental water. In this sense, the mesohyl
is the sole internal compartment of the body. As a connective tissue, the mesohyl is
composed of a pro- teinaceous, gel-like matrix that contains differentiated and
undifferentiated cells as well as skeletal elements (Fig. 5-5, 5-6). Among the many cells
present in the mesohyl are macrophage-like archeocytes (= progenitor cells), which
are large ameboid cells bearing a conspicuous nucleus and numerous large lysosomes.
Archeocytes are totipotent and can differentiate into any other type of sponge cell.
They are also phagocytic and play a role in digestion and internal transport.
 Lophocytes (= crest cells) are archeocyte-like ame- boid cells that secrete collagen
fibers from their trailing end as they move through the mesohyl. They produce and
main- tain the fine collagen fibers of the mesohyl. Spongocytes, which occur only
in the taxon Demospongiae, resemble archeocytes, but secrete collagen that
polymerizes into thick skeletal fibers known as spongin (see Skeleton, the next sec-
tion). Sclerocytes (= hard cells) secrete the mineralized skeletal spicules of many
sponges (see Skeleton). Myocytes (= muscle cells) are musclelike cells containing
actin and myosin that aggregate around the oscula of some demo- sponges. They
regulate the size of the oscular aperture and thus help to control water flow through
the sponge. Finally, oocytes and spermatocytes, which will be described later in
more detail, are reproductive cells that undergo game- togenesis in the mesohyl to form
sperm and eggs.
The choanoderm consists of flagellated collar cells, or choanocytes (= collar cells), that generate the water flow through the sponge.
Choanocytes have an apical collar of long microvilli around a single flagellum (Fig.
5-5). The collar is in

the form of a cylinder or an inverted cone. The basal part of the choanocyte
flagellum of many species (such as Microciona sp. and Grantia compressa), if not all,
bears a bilateral vane, as in the choanoflagellates (Fig. 4-13A).
Syncytial
The glass sponge's body wall lacks the sheetlike pinacoderm pavement that
covers the body and lines the aquiferous system of cellularian sponges. Instead,
the living tissue in hexactinellids is arranged in three-dimensional, cobweb-like
strands called a trabecular syncytium or network (Fig. 5-7). The membranes that
normally separate cells are absent and the cytoplasm is con- tinuous and
uninterrupted throughout the syncytium.
A cellular choanoderm also is absent; in its place is another syncytium, the choanosyncytium. Individual collar bodies, each with a collar and
flagellum but lacking a nucleus, arise from the surface of the choanosyncytium. Each
group of collar bodies occupies a syconoid-like pocket that is supported by the
trabecular network. The many collar bodies of each pocket arise developmentally as
outgrowths of a single nucle- ated stem cell, the choanoblast.
Each strand in the trabecular syncytium surrounds and encloses an axis of mesohyl. The mesohyl contains bundles of collagen fibers, spicules,
and cells-sclerocytes, archeocytes, and presumably germ cells. In
Rhabdocalyptus

WATER PUMPING
The volume of water pumped by a sponge is impressive. In general, a sponge
pumps a volume of water equal to its body volume once every 5 seconds. Because
water is incompressible, the volume entering must be equal to the volume exiting the
sponge at any moment. The flow velocity is fastest through
Cross-sectional
area
 The water current is produced by the activity of the choanocyte flagella (Fig. 5-5). The undulatory beat of each flagellum is in a single
plane. The flagellar vane, which is restricted to the collar region of the flagellum, may
help to "pump" water from the collar. In at least one species (Trochospongilla
pennsylvanicus), the beating plane of the choanocyte flagellum shifts every few seconds,
eventually rotating around 360°. The choanocyte flagella and collars are oriented away
from the ostia (in the asconoid design) or prosopyles (in the syconoid and leuconoid
designs), and each flagellum beats from base to tip, driving water toward the excurrent
canals and osculum.
The oscula of many sponges are situated on chimneys well above the main body and ostia (Fig. 5-1A). That elevated position exposes the
oscula to environmental water currents faster than those that occur near the base of the
sponge. The higher-velocity flow over the chimneys lowers the pressure at the oscula in
relation to the ostia and induces a flow from the high-pressure to low-pressure (ostium-to-
osculum) end of the system. Because most sponges are exposed to significant
ambient water currents, such induced flows undoubtedly sup- plement flagellar pumping and
conserve metabolic energy.

SKELETON
Whatever their growth form, most sponges live in moving water and support themselves
with a well-developed skeleton. The skeleton is chiefly a mesohylar endoskeleton, but an

Spicules stiffen the mesohyl to varying degrees depending on their

density, arrangement, and the extent to which they fuse or interlock. In the extreme, a
spicular skeleton can be a rigid, brittle, three-dimensional lattice or framework, as in the
glass sponge Euplectella aspergillum (Fig. 5-4A, 5-9A) or the calcareous sponge
Minchinella sp. The relict sphinctozoan sponge, Vaceletia crypta, has a chambered
calcareous exoskele- ton. The calcifying demosponges ("sclerosponges") secrete a
massive basal exoskeleton of CaCO3 on which the body rests. These sclerosponges also secrete
siliceous spicules in the mesohyl (Fig. 5-10).
Some sponges lack spicules, but secrete organic spongin (Fig. 5-
11A,B). Such sponges, for example the bath sponges (Spongia, Hippospongia), are
often compressible, elastic, and "spongy." A high density of spongin produces a
skeleton that is firm, tough, and rubbery, as in the tropical chicken-liver sponge,
Chondrilla nucula (Fig. 5-4E).
Spongin and spicules occur together in most species. of sponges. In
some species of Haliclona, spongin welds together the tips of spicules to form a
skeletal network (Fig. 5-11C). In other parts of the skeleton, spicules are embedded
into the spongin fibers themselves (Fig. 5-11D). Sometimes,
 foreign material, such as sand grains, is incorporated into the skeleton as a substitute for the
spicules, as happens in the tropical ethereal-blue sponge Dysidea etheria and other
species (Fig. 5-11E). In Dysidea janiae, the sponge produces no spicular skeleton of its own,
but instead uses the calcare- ous skeleton of its symbiotic red alga (Jania). The result of
these and other combinations of spicules and spongin is a wide variety of skeletal properties,
from soft and spongy to hard and brittle..
Spicules are siliceous (SiO2) or calcareous (CaCO3) ele- ments whose composition, size, and shape are used at all lev- els in the
classification of sponges (Fig. 5-9). As a result, an extensive nomenclature describes the forms and
sizes of spicules. At the most general level, spicules are separated into two size classes, large
megascleres and small micro- scleres. Megascleres typically form the principal
skeletal framework, whereas the considerably smaller microscleres may support the
pinacodermal lining of the canal system or, in high density, toughen the body wall
(Fig. 5-9A,C,E). Megasclere names are based on the spicule's number of axes or number
of rays or points. The suffix -axon refers to the number of axes; -actine indicates the number
of points. A monaxon spicule, for example, has one axis and is shaped. like a needle or rod,
although it may be straight or curved, with pointed, knobbed, or hooked ends (Fig. 5-9D1-6).
Triax- ons have either three rays (triactines; Fig. 5-9F) or six (hexa- ctines; Fig. 5-9B).
Spicules are secreted extracellularly by sclerocytes in calcareous sponges, intracellularly in sclerocytes in demo- sponges, and
intrasyncytially in glass sponges. From one to several sclerocytes typically secrete a
single spicule in the cal- careous sponges. A three-rayed spicule, for example, origi- nates
between three sclerocytes derived from a single stem cell (scleroblast; Fig. 5-11G). Each
member of the trio then divides and one ray of the spicule is secreted between each
pair of daughter cells. The three rays fuse at their bases. Each of the three pairs of
sclerocytes now moves outward along a ray, one cell lengthening its end while the other cell
thickens its base (Fig. 5-11G). The secretion of a siliceous monaxon spicule is initiated
around an organic filament in an intracellular vesicle (Fig. 5-11F). As the spicule crystallizes and
grows, the cell first elongates and then divides into two cells, each of which adds
additional silica to a growing tip of the spicule.

LOCOMOTION AND DYNAMIC TISSUES

Although sponges are basically sessile animals, some species have a limited capacity
for locomotion. Both freshwater (Ephy- datia) and marine (Chondrilla, Hymeniacidon,
Tethya) species can move over a substratum at rates of 1 to 4 mm/day (Fig. 5-12A). The
movement apparently results from the collective ameboid movements of pinacodermal and other
cells. Other sponge movements include whole-body con- traction (Clathrina coriacea) and, in
many species, constriction of oscula by myocytes. These movements probably arrest or
limit flow through the aquiferous system in response to an

A hallmark of sponges is the dynamism of their tis- sues. Mesohyl

cells, all of which are ameboid, are more or less in constant motion. Similarly,
endopinacocytes and choanocytes can move about to remodel the aquiferous system
(Fig. 5-12B). This remodeling, which involves the addition or fusion of flagellated
chambers and the merging and branching of canals, may "fine-tune" the
system to optimize water flow as the sponge grows or as it encounters changes in
environmental water currents. The absence of intercellular junctions, basal lamina,
and hemidesmosomes in sponge tissues allows these independent and frequent cell
movements.
PHYSIOLOGICAL
COMPARTMENTALIZATION
The physiological importance of water flow through the aquiferous system of a sponge
cannot be overstated. This single system accomplishes the tasks of gas
exchange, food acquisition, waste disposal, and the release of sperm and larvae.
The functions associated with, for example, the mam- malian trachea and lungs,
alimentary canal, circulatory ves- sels, kidneys, and gonoducts are, in sponges,
combined in this one multifunctional system. Such a low level of phys- iological
compartmentalization in sponges has two implica- tions. First, because there is little
segregation of function, integrating systems, such as the nervous or endocrine sys- tems,
are not well developed or necessary. In fact, sponges lack nervous tissue (discussed
later in this chapter). Second, because of functional overlapping in tissues and cells,
the efficiency of any individual function is likely to be low in comparison to the
efficiency in animals such as ourselves, which have a specialized compartment for
each function. This minimal efficiency manifests itself as a low level of activity. Few
animals are less mobile or more plantlike than sponges.
We should not think that, because of their low level of
compartmentalization and integration, sponges are somehow
at a disadvantage. On the contrary, their low level of organiza- tion allows them to adaptively remold their bodies, to regener- ate readily after
damage, and to clone themselves.

NUTRITION
Sponges filter food particles from water flowing through their bodies. Generally, the filtered particles range in size from 50 μm to 1 pm or less.
This range includes unicellular plankton, such as dinoflagellates and bacteria, viruses,
small organic debris, and perhaps even dissolved organic material. In tropical seas,
where sponges are abundant, the smallest fractions of food are approximately seven times
more available than the larger size classes. All sponge cells can ingest particles by
phagocytosis.
The food-trapping filters are the incurrent canals, which progressively decrease in diameter as they penetrate inward, and choanocytes
(Fig. 5-13). Food and other particles are filtered as they lodge in different parts of the system,
depending on their diameter. The largest particles, exceed- ing about 50 μm in
diameter, are too large to enter an ostium and can be phagocytosed by cells of the
exopina- coderm. Particles in the size range of 5 to 50 pμm lodge in an incurrent canal and
may be phagocytosed by endopina- cocytes or by archeocytes that have entered the canal
(either between pinacocytes or through porocytes in the canal lining). The smallest,
bacteria-size particles enter the choanocyte chambers and are removed by
phagocytosis or pinocytosis on the choanocyte surface. The choanocyte col- lar of
microvilli, and its extracellular matrix, may be the mesh that traps the finest material.
Both choanocytes and archeocytes engulf and digest parti- cles in vesicles, but the choanocyte often transfers particles to the archeocyte for digestion.
The archeocytes probably also store nutrients such as glycogen or lipids.
Carnivorous species occur in the demosponge family Cladorhizidae. These sponges trap crustaceans and other small animals on sticky cellular
threads that extend out from the surface of the sponge. Once an animal is trapped, the
threads shorten, drawing the prey onto the body surface, which slowly overgrows the
prey and consumes it, presumably with archeocytes. These strange sponges lack
choanocytes and an aquiferous system.
The two sources of particulate wastes in sponges are indigestible
products of intracellular digestion and inorganic mineral particles that enter the sponge
in the water stream. Mineral particles must be removed from the incurrent canals,
which they would otherwise block and inactivate. An inorganic
particle that lodges in an
incurrent canal is phagocytosed by an archeocyte, which transports it to the
downstream side of the canal system and then exocytoses it into an excurrent canal
 (Fig. 5-13). In those species that incorporate foreign material into their skeletons, the
archeo- cyte may transport the intercepted particle to a site of skeleton secretion.
Many sponges, both marine and freshwater, harbor photosynthetic
endosymbionts in their tissues and derive a nutritional benefit from the photosynthate.
Freshwater sponges typically harbor green algae (zoochlorellae) in archeocytes and
other cells. Marine sponges-both calcare- ous and demosponges-may host
dinoflagellates (zooxanthel- lae) or, more commonly, Cyanobacteria. One species,
Mycale laxissima in Belize, incorporates both green and red algae in the spongin
fibers of its skeleton. The cyanobacterial symbionts of some sponges, including
Verongia, may constitute up to one-third of the sponge's biomass. Such sponges live
in shallow, well-lit habitats and may have symbiotic bacteria restricted to the outer
layers of the body. Excess photosynthate in the form of glycerol and a
phosphorylated compound are utilized by the sponge. Some sponges studied on
Australia's Great Barrier Reef obtain from 48 to 80% of their energy requirements from their
Cyanobacteria. Sponges frequently
Cyanobacteria and other symbionts.
contain intra- and extracellular bacteria in addition to the
The significance of such bacteria, however, is unknown.

INTERNAL TRANSPORT, GAS EXCHANGE, AND

EXCRETION
Because the aquiferous system ventilates the entire body to within 1 mm of all cells,
simple diffusion accounts for the transport of gases and metabolic wastes (largely
ammonia) between the body and environmental water in the aquiferous system.
Nutrients, too, probably diffuse from the widespread. sites of intracellular digestion,
although archeocytes, by ame- boid movement, deliver nutrients to developing
gametes and tissues throughout the body. At least one species of Aplysina (also
known as Verongia) has specialized internal fibers that serve as tracks for the movement of
nutrient-laden archeocytes.
Internal transport of food in glass sponges is intrasyncytial. Once ingested by the collar bodies, food-containing vesicles are transported by
dynein motor molecules on bundles of microtubules that extend throughout the syncytium of
the sponge. This mode of transport is identical to vesicular trans- port in radiolarian
and foram pseudopodia (Chapter 3), as well as in the nerve-cell axons of higher
animals.
The near absence of intercellular junctions in pinacoderm and choanoderm suggests that these layers constitute a poor regulatory barrier
between the mesohyl and water of the exter- nal environment. Physiologically
speaking, sponges are said to be "leaky" animals. Accordingly, the composition of
their interstitial fluid (the fluid between cells) is likely to be similar
to that of the environmental water, even among freshwater species. Most
cells of freshwater sponges contain contractile vacuoles, but those vacuoles are
osmoregulating for individual cells and not for the sponge body as a whole.

INTEGRATION
Sponges lack nerve cells and nervous tissue, though some are capable of limited
impulse conduction. In most cases, this con- duction is a slow "epithelial" spread of
electrical activity over a few millimeters that results in a local myocyte contraction in
response to a local stimulus. Such impulse conduction is slow because specialized
intercellular junctions (gap junctions), which promote epithelial conduction, are absent.
Thus, the membranes between cells tend to isolate rather than conduct the wave of depolarization.
An exception to this generalization, however, occurs in the syncytial tissues of
glass sponges. In Rhabdocalyptus dawsoni, electrical impulses (action
potentials) are propagated rapidly along the syncytial strands from a point of
stimulation to all parts of the sponge. This activity arrests flagellar beating and shuts
down water pumping by the sponge.

BIOACTIVE METABOLITES AND BIOLOGICAL

ASSOCIATIONS
Many sponges produce metabolites that may prevent settle- ment of other organisms
on their surfaces or deter grazing predators. Nine out of 16 Antarctic sponges and 27
of 36 Caribbean species were found to be toxic to fish. The fish toxins, however, did not
necessarily discourage nonfish graz- ers, and some fish, such as angelfishes, filefishes,
and the moorish idol, are specialized spongivores. Turtles, especially
the Hawksbill turtle, commonly feed on sponges and up to 95% of their feces may
consist of siliceous spicules. A taxon of sea slugs (dorid nudibranchs) specializes on
sponge species in a manner similar to certain caterpillars on their host plants. Some
sponges use metabolites to compete for space with other organisms. For example, the
Caribbean chicken-liver sponge (Chondrilla nucula) releases compounds that kill adja-
cent stony corals, allowing the sponge to overgrow their skele- tons. Some species
have distinctive odors, such as the garlic sponge, Lissodendoryx isodictyalis. A few,
such as the Caribbean fire sponge, Tedania ignis, cause a severe rash when handled.
Various sponge biochemicals are being investigated to deter- mine their potential medical
and commercial benefits.
Many sponges harbor endosymbionts that occupy space in the aquiferous system and take advantage of the water flow and protection
afforded by their host. Some large leu- conoid sponges are veritable apartment houses for
shrimps, amphipods, and brittle stars. One investigator collected over 16,000 snapping
shrimps from within the water canals of one large loggerhead sponge. Certain worms
(spionid polychaetes) infest, eat, and thereby adopt the color of their sponge host.
Decorator crabs attach sponges, algae, and other sessile organ- isms to their backs to form a
microcommunity. The community grows on this mobile substratum, providing the crab with
an effective camouflage. Certain other crabs (Dromiidae) cut out a cap of sponge and affix
it to their back, or attach a sponge fragment that then grows, covers, and camouflages the crab.

REPRODUCTION

Clonal Reproduction
Sponges reproduce clonally (asexually) by fragmentation, bud- ding, and by the
formation of overwintering propagules called gemmules. Fragmentation primarily
results from current or wave damage, and perhaps from damage done by grazing
carnivores. The dislodged fragments rely on their remodeling capacity for
regeneration. The fragment soon attaches to the substratum and reorganizes itself into a
functional sponge. An extreme form of fragmentation-dissociation of a sponge into
individual cells or clumps of cells-can be accomplished in the laboratory by squeezing a
piece of sponge through finely woven cloth. This experiment was first conducted by
the zoologist H. V. Wilson early in the 20th century. Since that time, it has been repeated
frequently to demonstrate species recognition at the cellular level, to model
morphogenesis, and to study the mechanisms by which cells recognize and adhere to
each other.
Budding is uncommon but does occur in a few sponges. In
Clathrina, for example, the free ends of the asconoid tubes are said to swell into buds,
break free, and then attach and grow into another sponge. Some species of Tethya produce
stalked buds. Species of Oscarella and Aplysilla are reported to produce
papillae that self-amputate and grow into new sponges.
Many freshwater sponges and a few marine species produce
hundreds to thousands of sporelike gemmules, typically in the fall of the year (Fig. 5-
15A). The autumn gemmules of fresh- water species may enter diapause, a state of near
metabolic arrest, and then require a period of very cold temperature before they are
activated, germinate, and differentiate into a new sponge, usually in the spring. While
the gemmule is in diapause, it is resistant to environmental extremes of tempera- ture,
salinity, and desiccation. A standard practice of sponge biologists, in fact, is to keep
a humidified jar of gemmules in a refrigerator. When sponges are needed for
observation or experimentation, the gemmules are germinated by "seeding" some into a
container of pond water.
Gemmules are produced in the mesohyl of a dying sponge around a
cluster of nutrient-laden archeocytes. Spongocytes secrete a spongin shell around the
cellular mass. The shell may also contain spicules secreted by sclerocytes. The shell
completely encloses the cell mass except at one pole where an opening, the micropyle,
remains. The completed gemmule consists of a shell and its enclosed archeocytes, each of
which
A

soon becomes spherical, resembles an embryonic cell, and then is called a thesocyte
(Fig. 5-15A).
During the spring gemmule "hatch," the peripheral theso- cytes differentiate into a pinacoderm that balloons out, like a bubblegum bubble,
through the micropyle (Fig. 5-15B). This pinacoderm bubble makes contact with
and attaches to the substratum. Next, the deeper thesocytes issue from the micropyle into the
bubble and establish, after differentiation, the interior of the juvenile sponge. An
interesting variation on this theme, which challenges the notion of individuality, is
that thesocytes from gemmules of the same or different parentage (but of the
same species) can intermingle during germination to form one "individual" sponge.

Sexual Reproduction and Development

Sponges, with few exceptions, are hermaphrodites. At the appropriate time, sperm are
spawned from one sponge and transported by water currents to another, in which fertilization
occurs internally. A few species (such as Cliona) are oviparous and release zygotes
into the water, where they complete their development. Most sponges are viviparous,
retaining the zygotes in the parent's body and releasing larvae (sometimes called
larviparity). Embryos and larvae are lecithotrophic.
Sponges are said to lack genital organs (gonads), and germ cells occur in either simple clusters (sperm) or individually

its flagella on the surface of the larva. During metamorphosis, the external flagellated cells, after losing their flagella, return to the interior of the
body and differentiate into choanocytes. The metamorphosed juvenile of a calcareous sponge is
 called an olynthus. In C, Haliclona (Demospongiae), a differentiated parenchymella is
released from the sponge into the plankton. After settlement, it undergoes a complex metamorphosis to
form a juvenile, or rhagon (see text).

(eggs) throughout the mesohyl. Sperm arise from choanocytes or entire choanocyte
chambers that sink into the mesohyl and become enclosed in a thin cellular wall to
form a sper- matic cyst. Eggs arise from archeocytes or dedifferentiated choanocytes (in
some calcareous sponges). Each egg generally accumulates its yolk by phagocytosis of
adjacent nurse cells. The egg and nurse cells together may be enclosed in a follicle of
ensheathing cells. Because the aquiferous system supplies all parts of the body equally,
the germ cells also are widely distrib- uted throughout the mesohyl of the body, but
always within diffusion distance of a canal or chamber.
During spawning, sperm rupture the wall of the spermatic cyst,
enter the excurrent canals (or atrium), and are released from the oscula. Certain
tropical species, known to scuba divers as smoking sponges, suddenly spew sperm in
milky clouds from their oscula. Such sudden sperm release may be typical for most sponges.
When the spawned sperm drift into contact with another sponge,
they are swept into its aquiferous system by the incurrent

By definition, an organ is composed of two or more tissues. If these cyst or follicle cells are
shown to have a different tissue origin than the germ cells, then the spermatic cyst and egg follicle
would be organs (gonads).
water flow. Once in the aquiferous system, sperm are transported to the choanoderm or
choanocyte chamber and are phagocy- tosed, but not digested, by a choanocyte. The
choanocyte then loses its flagellum and collar, becomes ameboid, and transports the
sperm head (nucleus) to the egg. The transformed ameboid choanocyte is called a carrier cell. After
the carrier cell reaches an egg in the nearby mesohyl, it either transfers the sperm nucleus to
the egg or the carrier cell and sperm nucleus together are phagocytosed by the egg. In
either case, fertilization occurs internally in the "ovary" of the sponge.
The sperm of most sponges lack an acrosome, the structure responsible for penetrating the egg-cell membrane during fertilization in most
other animals. An acrosome probably is unnecessary because the sperm nucleus enters the egg
by phagocytosis. Acrosomal sperm do occur in Oscarella lobularis, suggesting that it has
a conventional means of egg fertilization, but the reproductive details of this species are
unknown.
The zygote cleaves holoblastically into equal-size blasto- meres. The pattern of cells that results from cleavage, however, varies among
species of sponges. The larvae that develop from embryos are also diverse and are described
under the names coeloblastula, amphiblastula, parenchymella, and trichimella
larvae.

A coeloblastula larva is produced by calcareous sponges, such as species of Clathrina (Calcinea; Fig. 5-16A). This larva is
a hollow sphere composed of a single layer of flagellated cells. While in the
plankton, some of the surface cells lose their flagella, become ameboid, and enter the
blastocoel, eventually obliterating it. This converts a hollow coeloblastula into a solid
stereoblastula.

An amphiblastula larva occurs in other calcareous sponges, for

example, Grantia, Sycon, and Leucosolenia (Cal- caronea, Fig. 5-16B). An
amphiblastula larva develops as a hollow ball composed of two types of cells, anterior
flagel- lated cells and posterior nonflagellated granular cells. Ini- tially, within the
mesohyl of the parent, the flagella are directed into the blastocoel, but a break soon
develops in the granule-cell surface of the larva and it turns itself inside-out (inverts) through
that opening. After inversion, the flagella project outward from the surface of the
larva, enabling it to swim. It is released from the parent at that stage. Inversion is correlated
with eggs that arise from choanocytes: After fertil- ization, the cells divide as if to form new
choanocyte cham- bers, with the flagella directed toward the interior of the chamber. The
demosponge genera Oscarella and Plakina also produce amphiblastula larvae, but these
form secondarily, after passing through a parenchymella stage.
A parenchymella larva is characteristic of most demosponges (Fig.
5-16C). In this case, the embryo develops directly into a solid mass of cells, forming a
stereoblastula. The outer layer is composed of widespread flagellated cells interspersed
with occasional vesicle-containing cells that lack flagella. The larval interior houses many
types of differentiated cells-sclerocytes, collencytes, pinacocytes, even choanocyte
chambers-and archeocytes. To a certain degree, then, parenchymella larvae are
prefabricated juveniles specialized for swimming.
Trichimella larvae typify the glass sponges. These are stereo- blastulae that
bear a band of flagellated cells around the equator of the larval body. The interior is
occupied by yolk-bearing cells, sclerocytes (spicules), other cells, and choanocyte
chambers.
All sponge larvae are lecithotrophic and therefore relatively short-lived.
Typically, they are released at dawn in response to a light cue. After a period of a few
hours to a few days, the larvae settle and creep over the bottom in search of a suitable site for
attachment. Once a site is found, the larva metamorphoses into a juvenile sponge, which
differs some- what for each of the larval types (Fig. 5-16). Because the
metamorphosis involves a rearrangement of cells into more or less definite layers, it is
frequently compared with gastrula- tion in other metazoans, but the ingression of cells
that results in the so-called stereoblastulae of many sponges might also be regarded
as a form of gastrulation (see Chapter 4).
Immediately prior to metamorphosis, the cells of the coeloblastula, now a
stereoblastula, dedifferentiate into a mass of totipotent cells (Fig. 5-16A). Once attached, this
mass spreads over the substratum, the surface cells become pinacoderm, and the deeper
cells differentiate into other typical sponge cells. Gaps that form between the
interior cells merge together to form the atrium as the interior cells undergo rearrangement.
The amphiblastula larva settles and attaches on its flagel- lated end (Fig. 5-
17B). Those flagellated cells, now attached to the substratum, lose their flagella,
migrate internally, and form the sponge interior. The granular cells become the
pinacoderm. When the juvenile sponge becomes functional,
begins to feed, and is a miniature asconoid in design, it is called an olynthus (Fig. 5-16B).
Metamorphosis of parenchymella larvae differs among species. In general, following larval attachment, the interior cells differentiate and
rearrange themselves to build most, if not all, of the sponge body. The question is, What, if any,
con- tribution to the juvenile body is made by the larval flagellated cells? In one
species, Mycale contarenii, the flagellated cells contribute to the formation of
choanocytes, as might be expected (Fig. 5-16C). But in other species (such as some
freshwater sponges and Microciona prolifera), the flagellated cells are phagocytosed
by archeocytes and do not contribute directly to the juvenile body. In any case, the
metamorphosed juvenile sponge often initially has an asconoid or syconoid design,
but with thick walls, before transforming into a leuconoid sponge. This early juvenile
stage is called a rhagon (Fig. 5-16C).
Sponges in temperate zones may live for from 1 to a few years, but some tropical species and perhaps many in the deep sea can be long-lived,
up to 200 or more years. Some sponges do not reproduce sexually until they are several
years old, whereas others begin to reproduce when they are only 2 or 3 weeks old. Some of
the calcified demosponges grow very slowly, at a rate of only 0.2 mm/year. If that
growth rate is constant, these reef sponges, which can reach 1 m in size, may be
5000 years old.
 DIVERSITY OF PORIFERA

Symplasmas (Hexactinellida)
Glass sponges; have syncytial tissues; spicules are siliceous triaxonal hexactines that form intracellularly (sclerocytes are cellular, not syncytial). Many
species have elongate bundle ("root") of monaxons that anchor the sponge in mud bottoms;
trichimella larva resembles modified parenchymella. Marine; approximately 400 extant
species. Euplectella, Dactylo calyx, Hyalonema, Monoraphis, Rhabdocalyptus.

CellulariasP
Porifera with cellular tissues.

DEMOSPONGIAEC

Cellularia of leuconoid design; 80 to 90% of all described species. Skeleton of siliceous spicules, spongin, spicules and spongin, or mesohyl only;
fused calcareous basal exoskele ton in some relict species. Megascleres: monaxons, triaxons,
tetraxons; all spicules secreted intracellularly; mesohyl well developed; choanocytes
typically smaller than pinacocytes and archeocytes. Marine and fresh water.
(Currently, there is lack of consensus over the classification of subtaxa of Demospongiae.)
Homoscleromorphas: Demospongiae lacking distinction
between mega- and microscleres. Spicule types not local- ized in body; siliceous spicules are di-, tri-, and tetractines; spongin mostly absent.
Larviparous with coeloblastula larva. Octavella and Oscarella have only mesohylar
skeleton, lack spongin and spicules. Plakina (syconoid). Tetractinomorphas:
Demospongiae with tetraxons, aster- ose microscleres, and mostly without spongin.
Oviparous.
 Choanoflagellata

N.N.
Acanthochaetes, Ceratoporella, Merlia, all "sclerosponges" with a basal
calcareous exoskeleton as well as siliceous spicules; Chondrilla nucula (chicken-liver
sponge of West Indies); Cliona spp. (boring sponges); Geodia; Suberites ficus (fig
sponge); Tethya actinia (tangerine sponge); Tetilla. Ceractinomorphas:
Demospongiae with distinct mega- and mi- croscleres, if microscleres are present;
spongin is often well developed and several taxa ("keratosa") have spongin only; spicule
types localized to specific tissues or regions. Larviparous with parenchymella
larva. Aplysilla longispina (sulfur sponge); Asbestopluma (Cladorhizidae),
carnivorous sponges; Callyspongia vaginalis (tube sponge); Dysidea etherea
(ethereal blue sponge); Ephydatia fluviatilis, Spongilla lacustris, Trochospongilla
pennsylvanicus, freshwater sponges; Halichondria bowerbanki (bread sponge);
Haliclona; Halisarca lacks spicular and spongin skeleton; Hymeniacidon helio-
phila (sun sponge); Hippospongia, Spongia (bath sponges); Niphates digitalis
(vase sponge); Lissodendoryx isodictyalis (garlic sponge); Microciona prolifera
(red-beard sponge); Mycale; Ophlitaspongia; Neofibularia nolitangere
(touch-me-not sponge); Spheciospongia vesparia (loggerhead sponge);
Tedania ignis (fire sponge); Vaceletia crypta, a relict sphinctozoan with a
chambered calcareous exoskeleton; and Verongia (also called Aplysina).

CALCAREAC
Cellularia including species of asconoid, syconoid, and leu- conoid design; spicules
are calcite, mostly unfused triaxons,
tetraxons and monaxons; each spicule formed extracellu- larly by more than one sclerocyte. Mesohyl is thin;
choanocytes relatively large, same size as pinacocytes and archeocytes. Larva a
hollow blastula. Marine; 500 extant species.
Calcineas: Calcarea with a choanocytic flagellum not in close association with the
nucleus; nucleus in base of cell; triax- ons have equiangular rays of equal length.
Coeloblastula larva. Clathrina (asconoid) forms tubular network; Murray- ona has a
reticulate rigid skeleton of fused calcareous bod- ies (sclerodermites). Calcaroneas:
Calcarea with a choanocyte flagellum close to the nucleus; nucleus is apical in cell;
triaxons are inequian- gular, one ray longer than other two. Amphiblastula larva.
Grantia; Leucandra (leuconoid); Leucosolenia (asconoid), single tubes or tube cluster from a
stolon, among algae; Minchinella, rigid skeleton of fused spicules and cement;
Petrobiona, skeleton is a solid calcareous mass; Sycon or Scypha (syconoid), cylindrical to
nearly spherical body, com- mon under rocks.

PALEONTOLOGY AND PHYLOGENY OF PORIFERA

The fossil record of the three major taxa of extant sponges- Hexactinellida,
Demospongiae, and Calcarea-is rooted in the Cambrian or Ordovician periods. Two
extinct taxa of Choanoflagellata

2BPHYLOGENETIC
HIERARCHY OF
PORIFERA
Porifera

Symplasma (Hexactinellida)
 Cellularia

Demospongiae Calcarea

organisms often considered to be sponges are Archaeocyatha (Cambrian) and

Stromatoporata (Ordovician to Cretaceous). The archeocyathan body consisted of a double-
walled, porous, calcareous skeleton in the form of an inverted hollow cone with radially arranged
septa. Stromatoporates resembled present-day calcifying demosponges (sclerosponges)
in that they had a massive basal calcareous skeleton with internal tubes, but unlike
sclerosponges, they lacked siliceous spicules. Another taxon, Sphinctozoa (primarily
Ordovician to Trias- sic), had a porous calcareous skeleton that was annulated like a string of
pearls. The extant relict sphinctozoan Vaceletia crypta indicates that the skeleton
was external to the soft tissue. Similarities in its soft tissues with those of demosponges sug-
gest that Vaceletia, and perhaps some other sphinctozoans, should be classified in
Demospongiae.
Archaeocyathans, sphinctozoans, stromatoporates, and
demosponges were important reef builders in Cambrian and Mesozoic seas. Some
sponge biologists, accordingly, have suggested that an evolutionary trend in sponges
has been a reduction of the massive reef-building skeleton (Fig. 5-10) in favor of a
spicular skeleton. The slow growth rate of extant sponges with massive skeletons, as
compared with stony corals, may be a reason for the decline of reef-building sponges.
On the other hand, the choanoflagellates, which share a common ancestor with
sponges (Fig. 5-17), produce a siliceous spicular skeleton.
Sponge classification is controversial, even at the highest levels.
Most of the recent phylogenetic discussions have included the sclerosponges
(formerly Sclerospongiae) in Demospongiae, and that change is adopted here.
Another recent suggestion is the establishment of two subtaxa, Sym- plasma
(Hexactinellida) and Cellularia, which formally recognize the distinction between
hexactinellid syncytial organization and the cellular bodies of Calcarea and Demo-
spongiae. Current systematic discussions center on the phy- logenetic relationships
of the three extant taxa (Fig. 5-17), the position of the sclerosponge species, and the
systematic relationships among Archaeocyatha, Stromatoporata, and Sphinctozoa.

PLACOZOAP

In 1883, a minute metazoan superficially resembling a large ameba (Fig. 5-18) was
discovered in an Austrian seawater aquarium and named Trichoplax adhaerens. It has
since been collected in the sea in various parts of the world and cultured numerous times.
The flattened body, which reaches 2 to 3 mm in diameter but only 25 μm in thickness, is enclosed in a layer of cells, one cell thick, that resembles an
epithelium, particularly because typical intercellular junctions join the adjacent cells
(Fig. 5-18B). The epithelioid layer, however, lacks a basal lam- ina, which is a
typical epithelial characteristic (see Chapter 6). The cells on the upper surface of the
body differ from those on the lower surface. The upper cells are flat and monocili- ated, and
each usually contains a large, spherical lipid droplet. The lower surface is a
creepsole composed of gland and monociliated cells with microvilli. Because these cells are tall
and slender, the individual cilia are close together, producing a densely ciliated surface for
locomotion. Between the upper and lower cell layers is a connective tissue of watery extracellu-
lar matrix and a syncytial network, the fiber syncytium, the fourth type of "cell" in
the placozoan body. The multiple nuclei of the fiber syncytium are separated from
each other by intracellular septa, which are not membranes. Such septa are common in the
syncytial network of hexactinellid sponges and in fungi. The fiber syncytium, which
 is thought to be contractile, contains actin (and presumably myosin) and microtubules.
Trichoplax resembles a large macroscopic ameba in form and locomotion (Fig. 5-18A). The animals change shape more or less constantly
as they glide slowly over the substratum. Apart from having differentiated upper and
lower surfaces, Trichoplax is not polarized. As a result, it can move in any direction
without turning. Sometimes it moves in two direc tions simultaneously and may pull itself
apart in the process.
Trichoplax feeds on algae and other material on the substratum. It digests its food extracellularly and extracorpore ally (outside of its body)
between its ventral surface and the substratum or it can arch upward to produce a pocket
in which food is digested. The lower cell layer absorbs the prod ucts of digestion.
The predominant mode of reproduction is clonal by fragmentation, as mentioned earlier, and by budding. The buds, which are more or less
spherical bodies, appear to emerge from the upper surface of the body, but they
contain cells from the upper and lower surfaces as well as connective tissue. The
flagellated buds are released from the surface and swim away. Sexual reproduction has
not been observed with certainty. Eggs have been described in laboratory individuals whose
bodies were swollen, spherical, and detached from the substratum. Apparently, the eggs
arise from cells of the ventral surface that dedifferentiate and ingress into the connective-
tissue space. Definite sperm have not been observed. If eggs are confirmed and
sperm discovered, then the number of specialized cells in placozoans rises to
six. The DNA content of Trichoplax is smaller than that determined for any other
animal.

The taxon Placozoa was created for Trichoplax adhaerens, which, like sponges, is probably an early evolutionary line among Metazoa
(although the fiber syncytium and extracor poreal digestion are reminiscent of fungi).
Placozoans, being composed of only four types of cells, are indeed simple meta-
zoans. Their small, flat bodies enable them to rely on simple diffusion for transport,
thus avoiding the complexity asso- ciated with a circulatory system. In some
respects, placo- zoans are intermediate between sponges and the remaining
metazoans. They resemble the hypothetical protometazoan (Fig. 4-12D) that has
adopted a benthic crawling existence and has differentiated its upper and lower
surfaces accord- ingly. The monociliated cells resemble collar cells in which the
collars have been reduced to low microvilli, perhaps corre- lated with locomotion over a
substratum and the abandon- ment of filter feeding. The outer epithelioid layer is one step
closer than that of sponges to a true epithelium, which appears fully formed in
Cnidaria (Chapter 7). The lower cell layer is reminiscent of the digestive epithelium
of the guts of other animals.