CORAL REEFS: TYPES & ORIGINS

[Text reading – Thurman and Trujillo: Chp 3, pp 100-102; Chp 16, pp 473-479]

1. There are three types of shallow coral reefs; quite recently, deep-water coral reefs have also been found in the North Atlantic. Shallow tropical reef types are: a) fringing reefs that form at the edge of an island; they are closest to land in areas of low rainfall runoff; b) barrier reefs, which are separated from land by a lagoon; c) atolls, ring-shaped reefs that enclose (completely or mostly) a shallow lagoon.

2. Darwin first suggested that barrier reefs and atolls formed by the subsidence of the land around which they are formed. Carbon dating of cores from reefs has confirmed that the age of carbonate increases with depth, and that at the base of the reef is volcanic basalt.

3. The primary animal forming shallow coral reefs are scleractinian corals = hard corals (Phylum Cnidaria, Class Anthozoa, Order Scleractinia). These produce a massive skeleton of aragonite. The larvae of corals settle on the skeleton of dead corals, thus building a reef. Other corals (soft corals, gorgonians, black corals) and anemones also live on the reef, together with many sponges and other invertebrates.

4. Corals feed at night, by extruding their tentacles and using nematocysts to capture plankton. During the day, the tentacles are withdrawn into the skeleton to avoid strong sunlight and predators. a) Tropical reef-building corals are called hermatypic. b) Non-reef forming corals are called ‘ahermatypic’. Hermatypic corals harbor symbiotic algae called zooxanthellae in their outermost tissues (more than 16 species are known!). The algae carry out photosynthesis. Using sunlight, carbon from the ocean, and carbon dioxide and nitrogen from the coral host’s respiration, they make sugars and amino acids. They then ‘leak’ some of these to the host (glucose, glycerol, amino acids and oxygen), with which the host coral makes carbohydrates and proteins. Some corals farm the dinogflagellates and ‘harvest’ them for food.

5. Calcification: corals deposit calcium in the form of aragonite. Calcium is absorbed from the sea water and transferred by diffusion and by active pumping to the site of calcification. Carbon dioxide (from coral respiration) is converted into carbonic acid, and then into bicarbonate and carbonate ions.

a) Calcification occurs much more rapidly when zooxanthellae are present and calcification is enhanced in sunlight; thus photosynthesis enhances calcification. Photosynthesis may produce more oxygen and stimulate coral metabolism, and/or contribute to calcification by increasing the available energy (glucose etc.).

b) Also, fixation of carbon (photosynthesis) is enhanced at warmer temperatures (29ºC vs 23ºC), and in higher concentrations of calcium in the seawater; thus calcification enhances photosynthesis. See figure from Marshall and Clode paper.

6. Hermatypic corals need light and warm temperatures. a) light - 5-10 m depths & clear water; b) average or slightly elevated salinity; c) warm waters: temperatures of 25-30 °C (depends on the coral species). Thus hermatypic corals are restricted to warm tropical waters.