<P> Sponges are essential for the functioning of the coral reef's ecosystem . Algae and corals in coral reefs produce organic material . This is filtered through sponges which convert this organic material into small particles which in turn are absorbed by algae and corals . </P> <P> Over 4,000 species of fish inhabit coral reefs . The reasons for this diversity remain unclear . Hypotheses include the "lottery", in which the first (lucky winner) recruit to a territory is typically able to defend it against latecomers, "competition", in which adults compete for territory, and less - competitive species must be able to survive in poorer habitat, and "predation", in which population size is a function of postsettlement piscivore mortality . Healthy reefs can produce up to 35 tons of fish per square kilometer each year, but damaged reefs produce much less . </P> <P> Sea urchins, Dotidae and sea slugs eat seaweed . Some species of sea urchins, such as Diadema antillarum, can play a pivotal part in preventing algae from overrunning reefs . In Hawaii, researchers are even investigating the use of native collector urchins, Tripneustes gratilla, for their potential as biocontrol agents to mitigate the spread of invasive algae species on coral reefs . Nudibranchia and sea anemones eat sponges . </P> <P> A number of invertebrates, collectively called "cryptofauna," inhabit the coral skeletal substrate itself, either boring into the skeletons (through the process of bioerosion) or living in pre-existing voids and crevices . Those animals boring into the rock include sponges, bivalve mollusks, and sipunculans . Those settling on the reef include many other species, particularly crustaceans and polychaete worms . </P>

Where are coral reefs found in the ocean