Corals
- Corals are a subset of cnidaria
- Sessile reef fauna
o Sponges
o Hard corals and soft corals
- Ecologically important
o Only inhabiting 0.2% of ocean floor but 1/3-1/4 of all marine
species
o Most species-rich marine community
o • 800 coral spp.
o • At least 4,000 fish spp.
o • 168,000 invert spp.
o • Estimated 1 -10 million spp.
o • Less diverse in species than rainforests,
o but considerably richer in phyla
o • 30 of 34 animal phyla & all algal
o Divisions
- Scleractinia
o Robust and Complex clades The robust coral clade branches
off from within the complex corals
Robust and complex clades
- Calcification:
o Complex corals are nominally less calcified/lower calcification
density than robust corals.
- Mitochondrial genome composition:
o Robust corals have a lower (G + C) content in their
mitochondrial genomes than complex corals.
- Embryogenetic morphological characteristics:
o Robust coral embryos pass through two periods of
invagination, separated by a return to a spherical shape.
Complex clade corals have no or little blastocoel, the robust
clade corals have an apparent blastocoel
- Stress-response:
o Robust corals have uneven expansions in genes associated
with innate immunity and stress response; Robust corals have
a unique histidine biosynthesis pathway that is absent from
complex corals. This pathway affects coral nutrition and
symbiosis. Robust corals have more HSP20 loci than more
stress-sensitive corals
The coral holobiont
- coral polyps, made of a stomach, mouth and tentacles
- tissue joining colony
- each polyp lays down its corralite structures, caco3 structure
The coral microbiome
- includes symbionts, bacteria, viruses, fungi and other
microorganisms
- The host coral and its microbiome together make up the coral
holobiont, with these microorganisms existing throughout different
tissues of the coral animal
- Each of these microbial groups impacts the coral in various ways
that influence the health of the coral and aid in resistance to disease
and environmental stress.
- Bacteria play a pivotal role in the coral microbiome and coral health
– in acquisition and cycling of nutrients, and microbes in the surface
mucus of corals act as a first line of defense against pathogens
Coral – Zooxanthellae symbiotic relationship
- The coral provides the algae with a protected environment and
compounds they need for photosynthesis.
- The algae produce oxygen and help the coral to remove wastes.
- Most importantly they supply the coral with glucose, glycerol, and
amino acids - the products of photosynthesis. The coral uses these
products to make proteins, fats, and carbohydrates, and produce
calcium carbonate
- The relationship facilitates a tight recycling of nutrients in nutrient-
poor tropical waters. ~ 90% of the organic material
photosynthetically produced by the zooxanthellae is transferred to
the host coral tissue
Single celled algae – zooxanthenellae
- Zooxanthellae are adapted dinoflagellates, in the genus
Symbiodinium
- Many different clades (A-H) and > 200 sub-clades
- Give coral their colour
- Different clades have different thermal tolerances
Thermal tolerances of zooxanthellae
- Identify and diversity of zooxanthellae from 3 Palauan reefs exposed
to different temperature ranges
- Higher temperature more clade D and less diversity in symbionts
Light enhanced calcification
- Coral needs carbonate ions, byproduct of respiration
- Significant positive relationship between calcification rate and light
intensity
Coral growth
- The skeletons of stony corals are secreted by the lower portion of
the polyp. The floor of the coralite is called the basal plate.
Periodically, a polyp will lift off its base and secrete a new basal
plate above the old one, creating partition in the skeleton. With
Growth rate of Monastrea faveolata, Tobago each added partition,
the polyp is elevated and colonial coral structure grows.
- Massive corals growth more slowly (few mm per year), branching
corals more quickly (>15 cms per year) – consider surface area to
volume ratio, competition for space.
- Many different measures: Linear extension (calcification), biomass,
density, volume
- Many different techniques: direct measurement, photography,
SHALLOW DEEP x-ray and banding (seasonal density differences),
radioactive isotopes
- Banding (analogous to tree growth rings) occurs because density of
skeleton depends on rates of deposition which are temperature and
light dependent
- Therefore seasonal variations in growth rate, especially in shallow
water
Coral morphology
- Wide variety of morphologies
Autotrophic and heterotrophic
- Heterotrophic corals ingest organisms ranging from bacteria to
zooplankton
- Heterotrophy accounts for between 0 and 66% of the fixed carbon
incorporated into coral skeletons and can meet from 15 to 35% of
daily metabolic requirements in healthy corals
- Feeding important for some nutrient acquirement e.g. nitrogen,
phosphorus
- Goniastra retiformis, long tentacles for heterotrophic feeding
- Porites cylindrica, much smaller tentacles and polyps
Changes in tissue biomass under light/shade and low/high suspended
particulate matter (SPM)
Reproduction, dispersal and connectivity
- Asexual reproduction
o called ‘budding’
o Cannot start new colonies, only help colonies grow
o Fragmentation
- Sexual reproduction
o Hermaphroditic or gonochoristic
o Self- or cross-fertilization
o Brooders or broadcast spawners
How do coral larvae know where to settle?
- Bacteria and their chemical “scents” play critical roles by producing
chemical cues that tell corals where to settle
- The bacteria produce a specific chemical called tetrabromopyrole
(TBP) that the larvae can detect as they swim near
- Once they reach a crustose coralline alga covered with the right
bacteria, they settle there
Dispersal of coral larvae
- Broadcasters, lasted longer in the water column (2 weeks) than the
brooders
Global Connectivity & Dispersal
- 50% of broadcast larvae settle within 50–100 km BARRIERS TO
LARVAL DISPERSAL
- Rarer dispersal between regions linking entire oceans in a ‘stepping
stone’ fashion.
- Central Pacific was an almost complete barrier to dispersal, only
rarely breached westward from the Galapagos. Areas showing
strong isolation also included Hawaii, Easter Island, the Red Sea and
the eastern Atlantic.
- The Indo-West Pacific and Great Barrier Reef showed the highest
levels of connectivity, with secondary peaks in the western Indian
Ocean.
- The central Indo-Pacific diversity hotspot was overall a greater
source than sink for dispersal
Ocean currents vs pelagic larvae duration
-
Threats, Resilience & Management
- Worth tens of billions in ecosystem services
- Synergies of human threats
o Such as dynamite fishing
o Starfish killer, due to nutrients from pollution
o Plastic pollution
- Coral bleaching – coral gets stressed, zooxanthellae gets expelled or
eaten
o The fish leave the reefs
o They can recover depending on duration
o Phase shift, algae dominated state
- Coral life strategies
o Four life-history strategies that appear globally consistent
across 143 species of reef corals: competitive, weedy, stress
tolerant and generalist taxa.
Competitive: grow quickly, broadcast spawners,
branching, however susceptible to bleaching, die quickly
but grow back quickly
Stress tolerant: more tolerant in sea surface temp
changes, higher lipid content, can survive longer without
bacteria, domed morphology, slower growth
o Most important traits: colony morphology, growth rate, and
reproductive mode.
- Global Mass Coral Bleaching Events
o 4 global mass coral bleaching events to date: 1998, 2005,
2014 17, 2023-current.
o The great majority of coral reefs (>80%) are expected to
experience Annual Severe Bleaching (ASB) this century even if
global temperature rises remain under 2°C
o Reef areas projected to experience ASB much later than other
reef areas could be temporary refugia that have lower climate
vulnerability
- Factors influencing recovery
o Loss of > 90% of corals due to bleaching in 1998
o Around 50% of the reefs recovered and 40% of the reefs
experienced regime shifts to macro-algae dominated
compositions.
o Assessment of factors influencing the probability of recovery,
identified five major factors:
density of juvenile corals
initial structural complexity
water depth, deeper the water the less warm it wil be
biomass of herbivorous fishes, fish eat the algae?
nutrient conditions on the reef. More nutrients = more
algae growth, which outcompetes coral
o Resilience was seen most in coral reef systems that were
structurally complex and in deeper water
o Resilience is the ability of a system to absorb or recover from
disturbance and change, while maintaining its functions and
services.
- Coral reef management strategies
- Reducing local stressors:
o Marine protected areas:
Large no-take MPAs
Locally management MPAs
o Sustainable fisheries management
o Reducing land based impacts
o Restoration techniques
o Assisted - evolution (genetic engineering techniques)
o Integrated approaches
