They are also found in mangrove areas, moving in and out with the tide and even in fresh water near the sea. They occur singly or in small groups. Adults often aggregate in reef channels at low tide. This is one of the three most common reef sharks in the Indo-Pacific, the two others are the grey reef shark, Carcharhinus amblyrhynchos and whitetip reef shark, Triaenodon obesus.
Are there so few reef sharks because of human activities such as fishing and finning, or were there never very many to start with? To answer this question, a team of marine biologists (which did not include Friedlander) decided to count reef sharks at coral reefs close and far to human settlements to better understand how humans impact their populations.
The grey reef shark is native to the Indian and Pacific Oceans. In the Indian Ocean, it occurs from South Africa to India, including Madagascar and nearby islands, the Red Sea, and the Maldives. In the Pacific Ocean, it is found from southern China to northern Australia and New Zealand, including the Gulf of Thailand, the Philippines, and Indonesia.[4][9] This species has also been reported from numerous Pacific islands, including American Samoa, the Chagos Archipelago, Easter Island, Christmas Island, the Cook Islands, the Marquesas Islands, the Tuamotu Archipelago, Guam, Kiribati, the Marshall Islands, Micronesia, Nauru, New Caledonia, the Marianas Islands, Palau, the Pitcairn Islands, Samoa, the Solomon Islands, Tuvalu, the Hawaiian Islands and Vanuatu.[1]
One useful definition distinguishes reefs from mounds as follows: Both are considered to be varieties of organosedimentary buildups – sedimentary features, built by the interaction of organisms and their environment, that have synoptic relief and whose biotic composition differs from that found on and beneath the surrounding sea floor. Reefs are held up by a macroscopic skeletal framework. Coral reefs are an excellent example of this kind. Corals and calcareous algae grow on top of one another and form a three-dimensional framework that is modified in various ways by other organisms and inorganic processes. By contrast, mounds lack a macroscopic skeletal framework (see stromatolite). Mounds are built by microorganisms or by organisms that don't grow a skeletal framework. A microbial mound might be built exclusively or primarily by cyanobacteria. Excellent examples of biostromes formed by cyanobacteria occur in the Great Salt Lake in Utah, and in Shark Bay on the coast of Western Australia.
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Grey reef sharks are active at all times of the day, with activity levels peaking at night.[4] 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).[25] 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.[26] 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.[25]
Despite sharks being portrayed as notorious aggressive animals, very few incidents have involved blacktip reef sharks, none being fatal. Still the importance of an apex predator is vital to a balanced and healthy ecosystem. Unfortunately, this species is very susceptible to reef gill netting. And sharks all around continue to be threatened by fishing pressure resulting in a decrease in many shark populations.
The Caribbean reef shark is a viviparous species, meaning its developing embryos are nourished via a placental connection. The litters average four to six pups. Although this shark’s reproduction has not been studied in the northern hemisphere, but to the south, parturition occurs during the Amazon summer of November to December. Pregnant females are often found to have biting scars from males on the sides of their bodies, due to the aggressive behaviors of males during mating. Gestation is believed to take approximately one year. A pregnant female with biting scars and wounds on the sides of her body, taken off the coast of north-northeastern Brazil, carried four near-term embryos. One was a 27.5 in. (700 mm) long male and three were females measuring 27.0 in. (685 mm), 27.4 in. (697 mm), and 27.7 in. (704 mm) in length. Because she was carrying near-term embryos, it is postulated that this area may be a pupping ground. Although such captures have shed light on the topic, relatively little is known about the reproduction of the Caribbean reef shark. Much information has been obtained from a pregnant female carrying four near-term embryos off the coast of northeastern Brazil. This female had scars and wounds on her side. Because the shark carried near-term embryos, it is postulated that this area may be a pupping ground.
Another danger posed to humans by the Caribbean reef shark involves the accumulation of toxins in the flesh of the shark. Since sharks are apex marine predators, they may contain toxic levels of mercury and other heavy metals due to bioaccumulation (increasing concentrations at higher levels in the food web). It was found that methylmercury levels (MeHg) in sharks off the coast of Florida were higher than the FDA guidelines.
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One useful definition distinguishes reefs from mounds as follows: Both are considered to be varieties of organosedimentary buildups – sedimentary features, built by the interaction of organisms and their environment, that have synoptic relief and whose biotic composition differs from that found on and beneath the surrounding sea floor. Reefs are held up by a macroscopic skeletal framework. Coral reefs are an excellent example of this kind. Corals and calcareous algae grow on top of one another and form a three-dimensional framework that is modified in various ways by other organisms and inorganic processes. By contrast, mounds lack a macroscopic skeletal framework (see stromatolite). Mounds are built by microorganisms or by organisms that don't grow a skeletal framework. A microbial mound might be built exclusively or primarily by cyanobacteria. Excellent examples of biostromes formed by cyanobacteria occur in the Great Salt Lake in Utah, and in Shark Bay on the coast of Western Australia.
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