Grey reef shark

Appearance

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 . 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 there is no ridge running between it and the second dorsal fin. The pectoral fins are narrow and falcate .

The coloration is grey above, sometimes with a bronze sheen, and white below. The entire rear margin of the caudal fin has a distinctive, broad, black band. There are dusky to black tips on the pectoral, pelvic, second dorsal, and anal fins. Individuals from the western Indian Ocean have a narrow, white margin at the tip of the first dorsal fin; this trait is usually absent from Pacific populations. Grey reef sharks that spend time in shallow water eventually darken in color, due to tanning. Most grey reef sharks are less than 1.9 m long. The maximum reported length is 2.6 m and the maximum reported weight is 33.7 kg .

Distribution

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. 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.

Generally a coastal, shallow-water species, grey reef sharks are mostly found in depths of less than 60 m . However, they have been known to dive to 1,000 m . They are found over continental and insular shelves, preferring the leeward sides of coral reefs with clear water and rugged topography. They are frequently found near the drop-offs at the outer edges of the reef, particularly near reef channels with strong currents, and less commonly within lagoons. On occasion, this shark may venture several kilometers out into the open ocean.

usually near dropoffs, outer slopes and reef channels with strong currents near the outer edges of reefs.

Behavior

Along with the blacktip reef shark and the whitetip reef shark , the grey reef shark is one of the three most common sharks inhabiting Indo-Pacific reefs. They actively expel most other shark species from favored habitats, even species larger in size. In areas where this species co-exists with the blacktip reef shark, the latter species occupies the shallow flats, while the former stays in deeper water. Areas with a high abundance of grey reef sharks tend to contain few sandbar sharks , and ''vice versa''; this may be due to their similar diets causing competitive exclusion.

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 . There is an account of around 25 grey reef sharks following a large pod of bottlenose dolphins , along with 25 silky sharks and a single silvertip shark. Rainbow runners have been observed rubbing against grey reef sharks, using the sharks' rough skin to scrape off parasites.

Grey reef sharks are prey for larger sharks, such as the silvertip shark. At Rangiroa Atoll in French Polynesia, great hammerheads feed opportunistically on grey reef sharks that are exhausted from pursuing mates. 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 .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 . 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.

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 . 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.Social aggregation is well documented in grey reef sharks. In the northwestern Hawaiian Islands, large numbers of pregnant adult 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 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.

Habitat

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. 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.

Generally a coastal, shallow-water species, grey reef sharks are mostly found in depths of less than 60 m . However, they have been known to dive to 1,000 m . They are found over continental and insular shelves, preferring the leeward sides of coral reefs with clear water and rugged topography. They are frequently found near the drop-offs at the outer edges of the reef, particularly near reef channels with strong currents, and less commonly within lagoons. On occasion, this shark may venture several kilometers out into the open ocean.

usually near dropoffs, outer slopes and reef channels with strong currents near the outer edges of reefs.Along with the blacktip reef shark and the whitetip reef shark , the grey reef shark is one of the three most common sharks inhabiting Indo-Pacific reefs. They actively expel most other shark species from favored habitats, even species larger in size. In areas where this species co-exists with the blacktip reef shark, the latter species occupies the shallow flats, while the former stays in deeper water. Areas with a high abundance of grey reef sharks tend to contain few sandbar sharks , and ''vice versa''; this may be due to their similar diets causing competitive exclusion.

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 . There is an account of around 25 grey reef sharks following a large pod of bottlenose dolphins , along with 25 silky sharks and a single silvertip shark. Rainbow runners have been observed rubbing against grey reef sharks, using the sharks' rough skin to scrape off parasites.

Grey reef sharks are prey for larger sharks, such as the silvertip shark. At Rangiroa Atoll in French Polynesia, great hammerheads feed opportunistically on grey reef sharks that are exhausted from pursuing mates. 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 .

Food

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. Their sense of smell is extremely acute, being capable of detecting one part tuna extract in 10 billion parts of sea water. In the presence of a large quantity of food, grey reef sharks may be roused into a feeding frenzy; in one documented frenzy caused by an underwater explosion that killed several snappers, one of the sharks involved was attacked and consumed by the others.

Evolution

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 and two functional uteruses. One to four pups 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 long. Sexual maturation occurs at around seven years of age, when the males are 1.3–1.5 m long and females are 1.2–1.4 m 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.