Off Enewetak, grey reef sharks exhibit different social behaviors on different parts of the reef. Sharks tend to be solitary on shallower reefs and pinnacles. Near reef drop-offs, loose aggregations of five to 20 sharks form in the morning and grow in number throughout the day before dispersing at night. In level areas, sharks form polarized schools (all swimming in the same direction) of around 30 individuals near the sea bottom, arranging themselves parallel to each other or slowly swimming in circles. Most individuals within polarized schools are females, and the formation of these schools has been theorized to relate to mating or pupping.
^ Garla, R.C.; Chapman, D.D.; Shivji, M.S.; Wetherbee, B.M.; Amorim, A.F. (2006). "Habitat of juvenile Caribbean reef sharks, Carcharhinus perezi, at two oceanic insular marine protected areas in the southwestern Atlantic Ocean: Fernando de Noronha Archipelago and Atol das Rocas, Brazil". Fisheries Research. 81 (2–3): 236–241. doi:10.1016/j.fishres.2006.07.003.
Living in warm shallow waters often near coral reefs in the Western Atlantic, from Florida to Brazil, the Caribbean reef shark (Carcharhinus perezi) is the most abundant shark in the Caribbean. It feeds mostly on bony fishes and rarely attacks humans. Despite the shark's abundance in some regions, it has a high mortality rate from bycatch and is sought by commercial fisheries for its fins and meat. It is illegal to catch Caribbean reef sharks in U.S. waters. The International Union for the Conservation of Nature (IUCN) lists the species' status as "Near Threatened."
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Cyanobacteria do not have skeletons and individuals are microscopic. Cyanobacteria can encourage the precipitation or accumulation of calcium carbonate to produce distinct sediment bodies in composition that have relief on the seafloor. Cyanobacterial mounds were most abundant before the evolution of shelly macroscopic organisms, but they still exist today (stromatolites are microbial mounds with a laminated internal structure). Bryozoans and crinoids, common contributors to marine sediments during the Mississippian (for example), produced a very different kind of mound. Bryozoans are small and the skeletons of crinoids disintegrate. However, bryozoan and crinoid meadows can persist over time and produce compositionally distinct bodies of sediment with depositional relief.