Biology of hermatypic corals
This is part 3 of the essay about coral reefs.
Nutrition
Coral reefs require lots of nourishment, which means that the surrounding ocean is notoriously poor in plankton. Some corals use their ciliary mucous mechanism for capturing plankton but the most common is to snare them directly with the tentacles.
Plankton can, however, not support an entire reef. It has been estimated that only 5-10% of the total food requirement comes from plankton and the obvious question is: where does the rest come from? There is in fact only one possible solution, the zooxanthellae in the coral tissues.
Corals in Thailand
Experiments have shown that coral can be kept in the dark for weeks, surviving by capturing zooplankton. It has also been shown that corals kept in light, but not being fed, still gained weight. This is considered to be an evidence for the importance of zooxanthellae and that corals are actually phototrophic (Porter (1976), Wellington (1982b). Species with large tentacles are supposed to be less dependant on zooxanthellae.
Sexual maturity and reproduction
Corals can reproduce both sexually and asexually. As has been mentioned, asexual reproduction is generally accomplished by a new individual budding off the parent. This is generally used for expanding an existing colony. It appears that corals reach sexual maturity at seven to ten years of age. They are also primarily hermaphroditic.
Sexual production is mainly performed by broadcast spawning, releasing eggs and sperm into the seawater. Some, however, retain fertilized eggs in the gastrovascular cavity until the planula larva is developed. Spawning is regulated by the lunar cycle and this can be quite spectacular in some regions where this event is performed synchronous. This is particularly true for the Great Barrier Reef where it occurs in the late spring to early summer.
The planulae are believed to swim in the open water for period of a few days up to a week or more. Since the adult corals are fixed in place, the planula is the only way for the coral to significantly disperse geographically.
The next section is about the ecology of tropical coral reefs.
[...] The next section describes the biology of hermatypic corals. [...]