Ecology of tropical coral reefs
This is part 4 of the essay about coral reefs.
Competition
The coral reefs are generally no open space at all. Coral or algae cover every single area. This results, of course, in fierce competition. Generally speaking there are two types of competition, exploitative competition and interference competition.
Exploitative competition is in general for fast growing corals such as branching corals. By growing faster and higher than the competitors, they are shutting off the light from underlying individuals which in the long run will kill them, freeing up space for the exploitative competitor.
Slow-growing massive and encrusting forms have developed another form of competition, interference competition. When the branching coral is closing in, the digestive filaments from the gastrovascular cavities are extended, killing the competing coral species. There are also other methods used, for example water-borne toxic chemical.
Competition is very complex and it is difficult to predict the winner in a coral battle. It is also reduced by the presence of grazing fish and invertebrates.
Predation
Crown of Thorns - Acanthaster Planci
There are several groups of predators on a coral reef. The first group, which consists of invertebrates such as nudibranches, polychaete crabs and molluscs, seems only to be able to cause minor effect on coral colonies. The two other groups however, the crown-of-thorns starfish Acanthaster planci and various fishes can severely modify the reef structure.
Acanthaster is only abundant in the Indo-Pacific where it feeds solely on living coral. When the starfish population is on a normal level, this is no major threat but large populations can devastate entire reefs. The corals can protect themselves by allying themselves with particular shrimps and in some cases by using their own nematocysts.
The Acanthaster itself is predated by the puffer and triggerfish, the shrimp Hymenocera and the polychaete Pherecardia.
The third group of fish can be divided in fish that consume the coral by biting off and ingesting portions of the coral skeleton (puffers (Tetradontidae), file fish (Monacanthidae), triggerfish (Balistidae) and butterfly fish (Chaetodontidae)) and a group of mulitvores such as surgeonfish (Acanthuridae) and parrotfish (Scaridae) that remove the coral polyps. These fish can, under certain conditions, largely modify the reef structure.
[...] The next section is about the ecology of tropical coral reefs. [...]