Four hundred coral species, both hard corals and soft corals inhabit the reef. A coral reef is an underwater ecosystem characterized by reef-building corals. These are most common in corals, sponges and sea anemones. The layered skeletons look similar to reef corals. Carbonates and calcium are essential for all growth of corals, coralline algae, clams, and invertebrates. The polyps resemble those of closely related anthozoans, such as sea anemones and corals. In the oceans were massive reefs built by corals and stromatoporoids. The limestone contains marine fossils including crinoids, brachiopods, gastropods, horn corals, and bryozoans. However, invertebrates such as muricids and ovulids feed on black corals and similar corals regularly. They feed on the corals, sponges, and bryozoans that litter the seabed. Cellular senescence is not observed in some organisms, including perennial plants, sponges, corals, and lobsters. They are large herbivores that graze on the algae that grows on hard dead corals. Corals, sea shells, and pearls are mostly made up of calcium carbonate. For most of their life corals are sessile animals of colonies of genetically identical polyps. The starfish show preferences between the hard corals on which they feed.