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.
My home in the coral reefs is being damaged by ocean acidification—which occurs when the ocean absorbs carbon and becomes acidified. I love living among thriving reefs, but increasing acidification degrades the physical structure of these reefs, putting my habitat and food supply at risk. This affects all the creatures living among the reef—not just my team of fellow blacktip reef sharks.
There is little evidence of territoriality in the grey reef shark; individuals will tolerate others of their species entering and feeding within their home ranges. Off Hawaii, individuals may stay around the same part of the reef for up to three years, while at Rangiroa, they regularly shift their locations by up to 15 km (9.3 mi). Individual grey reef sharks at Enewetak become highly aggressive at specific locations, suggesting they may exhibit dominant behavior over other sharks in their home areas.
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The Caribbean reef shark occurs throughout the tropical western Atlantic Ocean, from North Carolina in the north to Brazil in the south, including Bermuda, the northern Gulf of Mexico, and the Caribbean Sea. However, it is extremely rare north of the Florida Keys. It prefers shallow waters on or around coral reefs, and is commonly found near the drop-offs at the reefs' outer edges. This shark is most common in water shallower than 30 m (98 ft), but has been known to dive to 378 m (1,240 ft).
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.
Caribbean reef sharks are sometimes seen resting motionless on the sea floor or inside caves; it is the first active shark species in which such a behavior was reported. In 1975, Eugenie Clark investigated the famed "sleeping sharks" inside the caves at Isla Mujeres off the Yucatan Peninsula, and determined that the sharks were not actually asleep as their eyes would follow divers. Clark speculated that freshwater upwellings inside the caves might loosen parasites on the sharks and produce an enjoyable "narcotic" effect. If threatened, Caribbean reef sharks sometimes perform a threat display, in which they swim in a short, jerky fashion with frequent changes in direction and repeated, brief (1–1.2 second duration) drops of the pectoral fins. This display is less pronounced than the better-known display of the grey reef shark (C. amblyrhynchos).
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.
The Caribbean Reef Shark also finds its food in the reefs such as bony fishes, large crustaceans and cephalopods. This shark is also known to feed on yellow sting-rays and eagle rays quite frequently. A unique feature of these predators is that they are capable of reverting or purging their own stomachs. This helps purge the parasites, mucus or any other objects on the stomach lining.
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.
Corals, including some major extinct groups Rugosa and Tabulata, have been important reef builders through much of the Phanerozoic since the Ordovician Period. However, other organism groups, such as calcifying algae, especially members of the red algae Rhodophyta, and molluscs (especially the rudist bivalves during the Cretaceous Period) have created massive structures at various times. During the Cambrian Period, the conical or tubular skeletons of Archaeocyatha, an extinct group of uncertain affinities (possibly sponges), built reefs. Other groups, such as the Bryozoa have been important interstitial organisms, living between the framework builders. The corals which build reefs today, the Scleractinia, arose after the Permian–Triassic extinction event that wiped out the earlier rugose corals (as well as many other groups), and became increasingly important reef builders throughout the Mesozoic Era. They may have arisen from a rugose coral ancestor. Rugose corals built their skeletons of calcite and have a different symmetry from that of the scleractinian corals, whose skeletons are aragonite. However, there are some unusual examples of well-preserved aragonitic rugose corals in the late Permian. In addition, calcite has been reported in the initial post-larval calcification in a few scleractinian corals. Nevertheless, scleractinian corals (which arose in the middle Triassic) may have arisen from a non-calcifying ancestor independent of the rugosan corals (which disappeared in the late Permian).