<P> Coral reefs form some of the world's most productive ecosystems, providing complex and varied marine habitats that support a wide range of other organisms. Fringing reefs just below low tide level have a mutually beneficial relationship with mangrove forests at high tide level and sea grass meadows in between: the reefs protect the mangroves and seagrass from strong currents and waves that would damage them or erode the sediments in which they are rooted, while the mangroves and sea grass protect the coral from large influxes of silt, fresh water and pollutants . This level of variety in the environment benefits many coral reef animals, which, for example, may feed in the sea grass and use the reefs for protection or breeding . </P> <P> Reefs are home to a large variety of animals, including fish, seabirds, sponges, cnidarians (which includes some types of corals and jellyfish), worms, crustaceans (including shrimp, cleaner shrimp, spiny lobsters and crabs), mollusks (including cephalopods), echinoderms (including starfish, sea urchins and sea cucumbers), sea squirts, sea turtles and sea snakes . Aside from humans, mammals are rare on coral reefs, with visiting cetaceans such as dolphins being the main exception . A few of these varied species feed directly on corals, while others graze on algae on the reef . Reef biomass is positively related to species diversity . </P> <P> The same hideouts in a reef may be regularly inhabited by different species at different times of day . Nighttime predators such as cardinalfish and squirrelfish hide during the day, while damselfish, surgeonfish, triggerfish, wrasses and parrotfish hide from eels and sharks . </P> <P> Reefs are chronically at risk of algal encroachment . Overfishing and excess nutrient supply from onshore can enable algae to outcompete and kill the coral . Increased nutrient levels can be a result of sewage or chemical fertilizer runoff from nearby coastal developments . Runoff can carry nitrogen and phosphorus which promote excess algae growth . Algae can sometimes out - compete the coral for space . The algae can then smother the coral by decreasing the oxygen supply available to the reef . Decreased oxygen levels can slow down coral's calcification rates weakening the coral and leaving it more susceptible to disease and degradation . In surveys done around largely uninhabited US Pacific islands, algae inhabit a large percentage of surveyed coral locations . The algal population consists of turf algae, coralline algae, and macro algae . Some sea urchins (such as Diadema antillarum, ...) eat these algae and could thus decrease the risk of algal encroachment . </P>

Most of the world's coral reefs are in tropical water