Coral Reefs and the organisms that build them

Coral Reefs …and the organisms that build them.

The Organisms that Build Reefs: n “coral” is a general term for several groups of cnidarians – Reef-building corals = hermatypic corals, the polyps produce calcium carbonate skeletons n Ahermatypic corals lack zooxanthellae (photosynthetic algae) and do not help build reefs – Reef-building corals are colonies of polyps, all connected by a thin sheet of tissue

Succession of a Coral Reef: 1. 2. 3. Planula (planktonic coral larvae) settle on a hard surface Larvae metamorphosize (transform) into a polyp Polyp divides over and over to form the colony n Digestive systems usually remain connected and they share a common nervous system

Coral Nutrition: n n n Zooxanthellae perform photosynthesis and pass some of the organic material on to the coral (feed the coral from the inside) Most corals eat when they get the chance, they eat zooplankton by catching them in their tentacles or in sheets of mucus they secrete on their surface Can also absorb dissolved organic matter (DOM) from the water

Bioerosion n Organisms that tend to wear the reef away – Examples: grazers like urchins and parrot fish, sponges, clams, polychaetes and other worms, and algae n A reef can only grow if the coral accumulates limestone faster than the bioeroders can wear it down

Conditions for Reef Growth: 1. 2. Hard surface Shallow water, where light can penetrate (zooxanthellae need light) n 3. Only found on the continental shelves, around islands, or on top of seamounts Prefer clear waters n Cloudy water reduces light penetration

Conditions for Reef Growth: 4. Limited to warm water and reproduce only if the average water temp is above 68 o. F n n n There is an upper temp limit. Bleaching – first outward sign of heat stress (or stress of other kinds) in which the coral expels its zooxanthellae…w/o it the coral looks white. If the warm conditions last too long the corals die

Conditions for Reef Growth: 5. 6. Salinity – do not do well near river mouths Sensitive to pollution n The larvae are extremely sensititve.

Kinds of Coral Reefs: 1. Fringing Reefs n n n Simplest and most common Develop near shore, rocky shorelines provide the best conditions Grow in a narrow band or “fringe” along the shore Longest reef in the world is a fringe reef along the coast of the Red Sea (2, 500 mi) Upward growth is limited by the tides (corals will die if exposed for too long)

Kinds of Coral Reefs…

Kinds of Coral Reefs: 2. Barrier Reefs n n Like fringing reefs, they lie along the coast, but occur considerably farther from shore Separated from shore (which may also have a fringing reef) by a relatively deep lagoon Waves and currents may pile up sand to form small islands called sand cays or “keys” Largest barrier reef = Great Barrier Reef along the northeastern coast of Australia (1, 200 mi) – Actually a system of about 2, 500 smaller reefs, lagoons, channels, islands, and sand cays

Kinds of Coral Reefs: 3. Atolls An atoll is a ring of reef, and often islands or sand cays, surrounding a central lagoon n Majority of atolls occur in the Indo. West Pacific region (tropical Indian and Western Pacific oceans) n Can be found far from land n

The Ecology of Coral Reefs n The richest and most complex of all marine ecosystems and 1, 000’s of species may live on a reef.