1. …and the organisms that build them.
Coral Reefs
…and the organisms that build them.

2. The Organisms that Build Reefs:
“coral” is a general term for several groups of cnidarians
Reef-building corals = hermatypic corals, the polyps produce calcium carbonate skeletons
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

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

4. Coral Nutrition:
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

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

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

7. Conditions for Reef Growth:
Limited to warm water and reproduce only if the average water temp is above 68oF
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

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

9. Kinds of Coral Reefs: 1. Fringing Reefs
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)

10. Kinds of Coral Reefs…

12. Kinds of Coral Reefs: 2. Barrier Reefs
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

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

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