Coral reefs are the largest, durable bioconstruction

projects on earth

• In the context of space, the scale of reefs can be >2000 km in length (Great Barrier Reef)

• In the context of time, over millions of years reefs have accumulated 2 to 3 km thick piles of limestone (CaCO3)

• Modern reefs are quite young: < 10,000 years old, because of the rapid post-glacial sea level rise that drowned old reefs

• The most diverse of all marine ecosystems, and as diverse as tropical rain forests (~100,000 described species, maybe as many as a million)

• Coral reef ecosystems have high annual production (>1000 gC/m2/yr) because of the efficient use of resources by a large mass of organisms

Accumulated piles of CaCO3 rock (limestone)

built by a thin veneer of organisms living on top.

Biogeographic distribution of coral reef ecosystems

Two distinct realms: Indo-Pacific and Atlantic

Coral reefs are confined to the tropics largely because warm temperatures are necessary for calcification to occur. In cold water,calcium carbonate is much more soluble.

Corals are foundation species• Provide habitat & food

for all other species in the system

• They affect adjacent environments & ecosystems as well– Reefs are the protective barrier against the open sea for sea grass beds and mangroves

• Reef growth a balance of cementation and erosion– The former done by growth of many organisms & accumulation of debris that is held together in a frame

– The latter done by storms, grazing & excavation by other species

The Major Players-Anthozoans & their symbionts

• Coral colonies consist of many polyps living together in a skeleton of CaCO3 that they secrete beneath them

• Each polyp sits in a protective cup in the skeleton that has radiating fins projecting from the base– When polyps contract, tissue is crammed over & between fins

– Difficult for polyps to be preyed on

Branching elkhorn coralAcropora cervicornis

Acropora palmata

Plastic and hybrid Acroporids

Acropora cervicornis

Acropora palmata

Agaricia agaricitesAmong the most common caribbean corals in shallow water

Pocilloporids in a patch reef in Guam

Montastrea sibling species complex

Reef-buildingCorals of theCaribbean Sea

Montastrea sibling species complex

A sample of the diversity of common nekton on a reef

Wrasse

Shy hamlet

Parrotfish (a voracious coral predator)

Clownfish

French angelfish

A sample of reef diversity

Sea turtle hatchling

Sea turtle

Serpulid worm

crinoidNudibranch & eggs

Gorgonian

Diadema antillarum

Acanthaster planciTridacna spp.

A sample of reef diversity

Polyp & colony anatomy•Polyps or reef-building corals can be heterotrophic

-They capture particles from the plankton with their tentacles and transport it to the mouth for ingestion

•Food is digested in the gastric cavity•Food is transported throughout colony by the gastrovascular system -This system is the collection of vascular canals that connect other polyps in the colony and extends to the growing margin

Unlike other cnidarians,reef corals are autotrophicas well!

Hexacoral polyp(stony corals)

Octocoral polyp(soft corals)

A symbiotic association with dinoflagellates

• Zooxanthellae are single-celled algae that live inside the endodermal cells (those lining the gastrovascular cavity)of the coral host

• Corals derive much of their nutrition from photosynthesis carried out by zooxanthellae

The symbiotic dinoflagellates are called zooxanthellae

Gateno et al. 1998. Bio. Bull.194: 178-186

Benefits for each member of the mutualism

• Aid in the removal of excretory waste products

• Manufacture carbohydrates for coral nutrition

• Enhance coral calcification

• Aid in lipid synthesis

• Being fixed within the photic zone

• Being protected from zooplankton grazing

• Acquiring nutrients from the host corals respiration and excretion for use in photosynthesis– Includes; CO2, NH3, PO4, free amino acids, lipids, carbohydrates, dissolved organic matter

Reef corals depend on symbionts for growth; zooxanthellae:Zooxanthellae benefit by:

Connell (1973) did shading experiments:Put either clear or black domes over coral headsto demonstrate light-dependence. Without exception, shaded corals die rapidly

Photosynthates of zooxanthellae are translocated

to coral host• Labeling experiments

with 14C (taken up by symbionts as a carbon source for photosynthesis)

• 14C ends up in both coral tissue and coral skeleton– e.g., In Pocillopora

damicornis, 50% of carbon fixed by zooxanthellae is transferred to the coral

– Range of total nutrition provided by the alga ranges between 50-95% depending on coral species

– Highly branched forms tend to be more autotrophic, more massive, encrusting forms tend towards heterotrophy

Oren, U. et al. 1997.MEPS 161: 117-122

Translocation directed towards the biggest sinks (metabolic demands)

•Black dots show translocation of 14C through coral

•Most photosynthate transported to sites of large lesions where metabolic demand to replace wounded tissue & regrow greatest

Oren, U. et al. 1997.MEPS 161: 117-122

There are many types of zooxanthellae

• Previously thought there was 1 species, Symbiodinium microadriatecum, in a close co-evolutionary relationship between endosymbiont and coral host– Different strains thought to to be

specific to different coral species

• This is now known to be untrue

• There are at least 4 species of Symbiodinium (Trench, R. and Blank. 1987. J. Phycol. 23: 469-481; Trench, R. 1987. In: Biology of Dinoflagellates, Blackwell Scientific, Oxford, pgs. 530-570; Rowan, R. & D. Powers. 1991. Science 251: 1348-1351; _____. 1991 MEPS 71: 65-73)

• The coral-algal endosymbiosis is more flexible evolutionarily than previously thought

Different symbionts zoned with depth within Montastrea

speciesRowan, R. & N. Knowlton. 1995. PNAS 92: 2850-2853

Different parts of a colony may have different symbionts

Rowan, R. et al. 1997. Nature 388: 265-269

Symbionts show zonation within a coral head

Rowan, R. et al. 1997. Nature 388: 265-269

Single coral colonies can have all the types of symbionts

• A coral host represents an ecological landscape of different environmental conditions (e.g., light level) over which the different symbionts may be distributed

• In Montastrea annularis, corals acquire symbionts from the sea (eggs & sperm lack zooxanthellae) each generation– Can host all 4 types in one colony

• Symbionts show a dynamic pattern of zonation with respect to light intensity– In shallow water on offshore reefs, two types, A & B,

tolerant of high light prevail– In deeper water, a shade-loving form (type C) prevails– Nearshore in shallow water, a 4th type (E), (rare

offshore), is abundant with A & B– At intermediate depths, light-tolerant symbionts occur

on tops of colonies, and shade-loving symbionts on the sides or other low-light microenvironments

Rowan, R. et al. 1997. Nature 388: 265-269

Within and between colony variation in light-tolerant symbionts may explain the poorly predictable variability in coral bleaching

Nutritional symbiosis between coral & zooxanthellae drives formation of

coral reef ecosystems• Enables reef growth in unproductive tropical waters where concentrations of inorganic nutrients, like N & P, are low

• The key is the tight coupling between symbiont and host that enables them to recycle nutrients with extreme efficiency

A paradox of high production ofcorals in tropical waters?

The nutrients remain tied up in the biomass; little chance of phytoplankton in the watercolumn using them for production, but in thebenthos zooxanthellae inside corals get themfrom host and are very productive