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Species of shark

The grey reef shark or gray reef shark (Carcharhinus amblyrhynchos, sometimes misspelled amblyrhynchus or amblyrhinchos) is a species of requiem shark, in the family Carcharhinidae. One of the most common reef sharks in the Indo Pacific, it is found as far east as Easter Island and as far west as South Africa. This species is most often seen in shallow water near the drop offs of coral reefs. It has the typical "reef shark" shape, with a broad, round snout and large eyes. It can be distinguished from similar species by the plain or white tipped first dorsal fin, the dark tips on the other fins, the broad, black rear margin on the tail fin, and the lack of a ridge between the dorsal fins. Most individuals are less than long. The grey reef shark is a fast swimming, agile predator that feeds primarily on free swimming bony fishes and cephalopods. Its aggressive demeanor enables it to dominate many other shark species on the reef, despite its moderate size. Many grey reef sharks have a home range on a specific area of the reef, to which they continually return. However, they are social rather than territorial. During the day, these sharks often form groups of five to 20 individuals near coral reef drop offs, splitting up in the evening as the sharks begin to hunt. Adult females also form groups in very shallow water, where the higher water temperature may accelerate their growth or that of their unborn young. Like other members of its family, the grey reef shark is viviparous, meaning the mother nourishes her embryos through a placental connection. Litters of one to six pups are born every other year. The grey reef shark was the first shark species known to perform a threat display, a stereotypical behavior warning that it is prepared to attack. Other common names used for this shark around the world include black vee whaler, bronze whaler, Fowler's whaler shark, graceful shark, graceful whaler shark, grey shark, grey whaler shark, longnose blacktail shark, school shark, and shortnose blacktail shark. Some of these names are also applied to other species. Based on morphological characters, vertebral counts, and tooth shapes, Garrick (1982) concluded the grey reef shark is most closely related to the silvertip shark (C. albimarginatus). This interpretation was supported by a 1992 allozyme phylogenetic analysis by Lavery.

The Grey Reef Shark has a streamlined, moderately stout body with a long, blunt snout and large, round eyes. The upper and lower jaws each have 13 or 14 teeth (usually 14 in the upper and 13 in the lower). The upper teeth are triangular with slanted cusps, while the bottom teeth have narrower, erect cusps. The tooth serrations are larger in the upper jaw than in the lower. The first dorsal fin is medium sized, and no ridge runs between the second dorsal fin and it. The pectoral fins are narrow and falcate (sickle shaped). Grey Reef Sharks that spend time in shallow water eventually darken in color, due to tanning. 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. and less commonly within lagoons. On occasion, this shark may venture several kilometers out into the open ocean. The consumptive influence of grey reef sharks on reef fish communities is likely to vary depending on whether sharks forage within the reef environment, or on pelagic resources (like they have been observed to do at Palmyra Atoll). On the infrequent occasions when they swim in oceanic waters, grey reef sharks often associate with marine mammals or large pelagic fishes, such as sailfish (Istiophorus platypterus). One account has around 25 grey reef sharks following a large pod of bottlenose dolphins (Tursiops sp. ), along with 25 silky sharks (C. falciformis) and a single silvertip shark. Grey reef sharks are themselves prey for larger sharks, such as the silvertip shark. Known parasites of this species include the nematode Huffmanela lata and several copepod species that attach to the sharks' skin, and juvenile stages of the isopods Gnathia trimaculata and G. grandilaris that attach to the gill filaments and septa (the dividers between each gill).

Grey reef sharks feed mainly on bony fishes, with cephalopods such as squid and octopus being the second most important food group, and crustaceans such as crabs and lobsters making up the remainder. The larger sharks take a greater proportion of cephalopods. These sharks hunt individually or in groups, and have been known to pin schools of fish against the outer walls of coral reefs for feeding. They excel at capturing fish swimming in the open, and they complement hunting whitetip reef sharks, which are more adept at capturing fish inside caves and crevices.

During mating, the male grey reef shark bites at the female's body or fins to hold onto her for copulation.

Grey reef sharks are active at all times of the day, with activity levels peaking at night. Little evidence of territoriality is seen in the grey reef shark; individuals tolerate others of their species entering and feeding within their home ranges. Off Hawaii, individuals may stay around the same part of the reef up to three years,

Social aggregation is well documented in grey reef sharks. In the northwestern Hawaiian Islands, large numbers of pregnant females have been observed slowly swimming in circles in shallow water, occasionally exposing their dorsal fins or backs. These groups last from 11:00 to 15:00, corresponding to peak daylight hours. Similarly, at Sand Island off Johnston Atoll, females form aggregations in shallow water from March to June. The number of sharks per group differs from year to year. Each day, the sharks begin arriving at the aggregation area at 09:00, reaching a peak in numbers during the hottest part of the day in the afternoon, and dispersing by 19:00. Individual sharks return to the aggregation site every one to six days. These female sharks are speculated to be taking advantage of the warmer water to speed their growth or that of their embryos. The shallow waters may also enable them to avoid unwanted attention by males. 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.

The "hunch" threat display of the grey reef shark is the most pronounced and well known agonistic display (a display directed toward competitors or threats) of any shark. Investigations of this behavior have been focused on the reaction of sharks to approaching divers, some of which have culminated in attacks. The display consists of the shark raising its snout, dropping its pectoral fins, arching its back, and curving its body laterally. While holding this posture, the shark swims with a stiff, exaggerated side to side motion, sometimes combined with rolls or figure 8 loops. The intensity of the display increases if the shark is more closely approached or if obstacles are blocking its escape routes, such as landmarks or other sharks. If the diver persists, the shark may either retreat or launch a rapid, open mouthed attack, slashing with its upper teeth. There have been several known attacks on spearfishers, possibly by mistake, when the shark struck at the speared fish close to the diver. This species will also attack if pursued or cornered, and divers should immediately retreat (slowly and always facing the shark) if it begins to perform a threat display. Although still abundant in 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 endangered; 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. 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.