Measuring up to 3 m (9.8 ft) long, the Caribbean reef shark is one of the largest apex predators in the reef ecosystem, feeding on a variety of fishes and cephalopods. They have been documented resting motionless on the sea bottom or inside caves, unusual behavior for an active-swimming shark. If threatened, it may perform a threat display in which it frequently changes direction and dips its pectoral fins. Like other requiem sharks, it is viviparous with females giving birth to 4–6 young every other year. Caribbean reef sharks are of some importance to fisheries as a source of meat, leather, liver oil, and fishmeal, but recently they have become more valuable as an ecotourist attraction. In the Bahamas and elsewhere, bait is used to attract them to groups of divers in controversial "shark feedings". This species is responsible for a small number of attacks on humans. The shark attacks usually happen in spring and summer.
Grey reef sharks are active at all times of the day, with activity levels peaking at night. At Rangiroa, groups of around 30 sharks spend the day together in a small part of their collective home range, dispersing at night into shallower water to forage for food. Their home range is about 0.8 km2 (0.31 sq mi). At Enewetak in the Marshall Islands, grey reef sharks from different parts of the reef exhibit different social and ranging behaviors. Sharks on the outer ocean reefs tend to be nomadic, swimming long distances along the reef, while those around lagoon reefs and underwater pinnacles stay within defined daytime and night-time home ranges. Where there are strong tidal currents, grey reef sharks move against the water: towards the shore with the ebbing tide and back out to sea with the rising tide. This may allow them to better detect the scent of their prey, or afford them the cover of turbid water in which to hunt.
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Most observed displays by grey reef sharks have been in response to a diver (or submersible) approaching and following it from a few meters behind and above. They also perform the display towards moray eels, and in one instance towards a much larger great hammerhead (which subsequently withdrew). However, they have never been seen performing threat displays towards each other. This suggests the display is primarily a response to potential threats (i.e. predators) rather than competitors. As grey reef sharks are not territorial, they are speculated to be defending a critical volume of "personal space" around themselves. Compared to sharks from French Polynesia or Micronesia, grey reef sharks from the Indian Ocean and western Pacific are not as aggressive and less given to displaying.
This sturdy shark is abundant in the Caribbean, and because of its average features, is often confused with other requiem sharks. Usually growing 6.5 to 10 feet long, these are the apex predator of their food web. They have been found ‘sleeping’ in caves and on the ocean floor, behavior that is still unexplained. There has been concern over eating these sharks because of the build-up of toxins in their flesh, but now they are valued for tourism more than food, which brings its own safety issues.
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.