Coral reefs may only span less than 0.01% of the ocean floor, but they house nearly a quarter of all known marine species. They are built using limestone produced by coral polyps in the warm shallow oceans near the tropics and are very complex and unique structures. Over the course of 1 year, a massive coral can expect to grow 2 cm, making the creation of such habitats incredibly slow. Smaller species of marine life use the densely populated corals as homes while predators such as cuttlefish and octopus use specialized techniques to draw them out.
One particularly interesting mutualism is the clownfish and sea anemones. Found near the reefs, anemones are half-plant half animals that sting their prey such as fish and mussels. However, they seem to allow clownfish to inhabit the same space. It happens that clownfish clean the anemone of dirt and debris so keeping these kinds of fish around is actually very beneficial. Likewise, clownfish gain a home and a place to lay their eggs. The anemone provides protection to clownfish natural predators while clownfish do the same for anemones. Clownfish are normally eaten by larger fish, eels, and sharks, but having an anemone shelter can provide a secluded hiding spot. Anemones are eaten by starfish, eels, and other fish, which clownfish have the ability to ward off. They not only protect their home, but also the anemone.