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 (CaCO 3 ) 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/m 2 /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 CaCO 3 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 coral Acropora cervicornis Acropora palmata

Plastic and hybrid Acroporids Acropora cervicornis Acropora palmata

Agaricia agaricites Among the most common caribbean corals in shallow water

Pocilloporids in a patch reef in Guam

Montastrea sibling species complex Reef-building Corals of the Caribbean 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 planci Tridacna 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 autotrophic as 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 Bio. Bull. 194:

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; CO 2, NH 3, PO 4, 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 heads to demonstrate light-dependence. Without exception, shaded corals die rapidly

Photosynthates of zooxanthellae are translocated to coral host Labeling experiments with 14 C (taken up by symbionts as a carbon source for photosynthesis) 14 C 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 MEPS 161:

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 MEPS 161:

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 J. Phycol. 23: ; Trench, R In: Biology of Dinoflagellates, Blackwell Scientific, Oxford, pgs ; Rowan, R. & D. Powers Science 251: ; _____ MEPS 71: 65-73) The coral-algal endosymbiosis is more flexible evolutionarily than previously thought

Different symbionts zoned with depth within Montastrea species Rowan, R. & N. Knowlton PNAS 92:

Different parts of a colony may have different symbionts Rowan, R. et al Nature 388:

Symbionts show zonation within a coral head Rowan, R. et al Nature 388:

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 Nature 388: 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 of corals in tropical waters? The nutrients remain tied up in the biomass; little chance of phytoplankton in the water column using them for production, but in the benthos zooxanthellae inside corals get them from host and are very productive