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.
Like many sharks, the Caribbean reef shark mainly eats bony fishes. The shark uses six keen senses to locate its prey: olfactory, visual, tactile (including water vibration sensitivity through a lateralis canal system), auditory, gustatory, and electric reception. The Caribbean reef shark is especially adapted to detecting low frequency sounds (indicative of a struggling fish nearby).
Grey reef sharks were the first shark species known to perform a threat display, a stereotypical behavior warning that it is prepared to attack. The display involves a "hunched" posture with characteristically dropped pectoral fins, and an exaggerated, side-to-side swimming motion. Grey reef sharks often do so if they are followed or cornered by divers to indicate they perceive a threat. This species has been responsible for a number of attacks on humans, so should be treated with caution, especially if they begin to display. They are caught in many fisheries and are susceptible to local population depletion due to their low reproduction rate and limited dispersal. As a result, the International Union for Conservation of Nature has assessed this species as Near Threatened.
The Caribbean reef shark infrequently attacks humans. In general, a shark attack on a human is behaviorally similar to an attack upon natural prey. A human is more susceptible to being attacked if the shark is cornered and feels that there is no escape route. In situations like these, the shark may rake the victim during the attack resulting in lacerations.
During mating, the male grey reef shark will bite at the female's body or fins to hold onto her for copulation. Like other requiem sharks, it is viviparous: once the developing embryos exhaust their supply of yolk, the yolk sac develops into a placental connection that sustains them to term. Each female has a single functional ovary (on the right side) and two functional uteruses. One to four pups (six in Hawaii) are born every other year; the number of young increases with female size. Estimates of the gestation period range from 9 to 14 months. Parturition is thought to take place from July to August in the Southern Hemisphere and from March to July in the Northern Hemisphere. However, females with "full-term embryos" have also been reported in the fall off Enewetak. The newborns measure 45–60 cm (18–24 in) long. Sexual maturation occurs at around seven years of age, when the males are 1.3–1.5 m (4.3–4.9 ft) long and females are 1.2–1.4 m (3.9–4.6 ft) long. Females on the Great Barrier Reef mature at 11 years of age, later than at other locations, and at a slightly larger size. The lifespan is at least 25 years.
But another potential cause is that these sharks are skittish around people. So when too many people move into the area, the reef sharks flee to other coral reefs. Indeed, the researchers found far more sharks at small, isolated reefs than they expected. But this in itself is a danger to the reef sharks. With so many sharks concentrated in a small area, “if you really wanted to, you could fish out a few hundred sharks very easily,” said Friedlander.
The Caribbean reef shark feeds on a wide variety of reef-dwelling bony fishes and cephalopods, as well as some elasmobranchs such as eagle rays (Aetobatus narinari) and yellow stingrays (Urobatis jamaicensis). It is attracted to low-frequency sounds, which are indicative of struggling fish. In one observation of a 2 m (6.6 ft) long male Caribbean reef shark hunting a yellowtail snapper (Lutjanus crysurus), the shark languidly circled and made several seemingly "half-hearted" turns towards its prey, before suddenly accelerating and swinging its head sideways to capture the snapper at the corner of its jaws. Young sharks feed on small fishes, shrimps, and crabs. Caribbean reef sharks are capable of everting their stomachs, which likely serves to cleanse indigestible particles, parasites, and mucus from the stomach lining.
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.
The Black-tip Shark (Carcharhinus melanopterus) is a species of shark of the family Carcharhinidae, easily identified by the black tips of its fins, especially on the first dorsal fin and the caudal fin. It is one of the most abundant sharks in the tropical coral reefs of the Indian Ocean and Pacific Ocean. This species prefers shallow coastal waters and frequently exposes its first dorsal fin in these areas. Most Black-tipped Sharks live on reef margins and sandy bottoms, but they are also known to support brackish or freshwater environments. This species generally reaches a length of 1.6 m. Black-tip Sharks are sedentary and live in very small areas and may remain in the same area for several years. They are active predators of small bone fish, cephalopods and crustaceans, and are also known to feed on marine snakes and seabirds. The data collected concerning the life cycle of the Black-tip Shark are sometimes contradictory and there appear to be significant differences depending on the geographical location within the range of the species. Like other members of its family, this shark is viviparous and females give birth to between two and five young babies every two years, every year or sometimes twice a year. Indeed, according to its habitat the gestation period of this shark can be 7-9 months, 10-11 months or 16 months. Newborns live in coastal waters and in shallower waters than adults, often forming large groups in areas flooded by high tides. Shy and capricious, the Black-tip Shark is difficult to approach and rarely represents a danger to humans, unless it is excited by food. However, bathers in shallow waters can sometimes have their legs bitten by mistake. This shark is fished for its meat, fins and liver oil, but is not considered to be a commercially important species. The International Union for Conservation of Nature assessed the near threatened species. Although the species as a whole remains widespread and relatively common, overfishing of this shark and its slow rate of reproduction has led to its decline in a number of localities.
The Caribbean reef shark is the most common shark on or near coral reefs in the Caribbean. It is a tropical inshore, bottom-dwelling species of the continental and insular shelves. Although C. perezi mainly inhabits shallow waters, it has been recorded to reach depths to at least 98 feet (30 m). Caribbean reef sharks are commonly found close to drop-offs on the outer edges of coral reefs and also may lie motionless on the bottom of the ocean floor. This phenomenon has also been observed in caves off the coast of Mexico and off the Brazilian archipelago of Fernando de Noronha.
The Caribbean Reef Shark is known to become aggressive in the presence of food, but they are mostly only considered dangerous to humans because of its size. This shark was fished in Belize for almost the entire 20th century. They were used to make local delicacies in addition to liver oil (mostly used in cosmetics). Their low reproduction rate combined with a high level of hunting and fishing have caused the numbers to dwindle. The shark is now considered to be near threatened. Many countries and organizations have banned the commercial fishing of this species.
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.
Blacktip reef sharks are fast, pursuit predators that prefer reef fishes, but also feeds on stingrays, crabs, mantis shrimps and other crustaceans, cephalopods, and other mollusks. In the Maldives, this species has been documented feeding cooperatively on small schooling fishes, herding them against the shore and feeding en masse. Feeds heavily on sea snakes in northern Australia. A large individual (1.6 m) was observed attacking a green sea turtle, Chelonia mydas, in North Male’ Atoll, Maldives.
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Although still abundant at Cocos Island and other relatively pristine sites, grey reef sharks are susceptible to localized depletion due to their slow reproductive rate, specific habitat requirements, and tendency to stay within a certain area. The IUCN has assessed the grey reef shark as Near Threatened; this shark is taken by multispecies fisheries in many parts of its range and used for various products such as shark fin soup and fishmeal. Another threat is the continuing degradation of coral reefs from human development. There is evidence of substantial declines in some populations. Anderson et al. (1998) reported, in the Chagos Archipelago, grey reef shark numbers in 1996 had fallen to 14% of 1970s levels. Robbins et al. (2006) found grey reef shark populations in Great Barrier Reef fishing zones had declined by 97% compared to no-entry zones (boats are not allowed). In addition, no-take zones (boats are allowed but fishing is prohibited) had the same levels of depletion as fishing zones, illustrating the severe effect of poaching. Projections suggested the shark population would fall to 0.1% of pre-exploitation levels within 20 years without additional conservation measures. One possible avenue for conservation is ecotourism, as grey reef sharks are suitable for shark-watching ventures, and profitable diving sites now enjoy protection in many countries, such as the Maldives.
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