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Characteristics, distribution and ecology
Taxonomische classification
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Author: (Bleeker, 1856)

Field Marks:
A medium-sized to large grey shark with a moderately long, broadly rounded snout, usually round eyes, no interdorsal ridge, narrow-cusped, serrated upper anteroposterior teeth, usually 14/13 on each side, large second dorsal fin with a short rear tip, and a broad black band on the posterior margin of the caudal fin.

Diagnostic Features:
A moderate-sized fairly stocky species (to 2.4 m). Snout fairly long and broadly rounded; internarial width 1 to 1.4 times in preoral length; eyes usually round and fairly large, their length 2 to 2.7% of total length; anterior nasal flaps hardly expanded as very low triangular lobes; upper labial furrows short and inconspicuous; hyomandibular line of pores just behind mouth corners not conspicuously enlarged; gill slits moderate sized, the third 2.8 to 4.2% of total length and less than 2/5 of first dorsal base; usually 14/13 rows of anteroposterior teeth in each jaw half but varying from 13 to 14/13 to 14; upper teeth with narrow, strongly serrated, semierect to oblique, high cusps, and crown feet with coarser serrations and often distal cusplets; lower teeth with erect or semioblique, narrow serrated cusps and transverse roots. Usually no interdorsal ridge. First dorsal fin moderate-sized and semifalcate, with a narrowly rounded or pointed apex and posterior margin curving ventrally from fin apex; origin of first dorsal fin usually over or just in front of pectoral free rear tips; inner margin of first dorsal moderately long, but less than a half dorsal base; second dorsal fin moderately large and high, its height 2.7 to 3.4% of total length, its inner margin fairly long and 1.2 to 1.6 times its height; origin of second dorsal about over anal origin; pectoral fins moderately large, narrow andfalcate, with narrowly rounded or pointed apices, length of anterior margins about 18 to 21% of total length; 211 to 221 total vertebral centra, 110 to 119 precaudal centra. Colour grey above, white below; first dorsal plain or irregularly white-edged, entire posterior margin of caudal (terminal, pre- and postventral margins) with a conspicuous broad black margin, pectorals, second dorsal, anal, and pelvic fins with blackish of dusky tips.

Geographical Distribution:
Indian Ocean: Madagascar, Mauritius, Scychelles, ? India. Western central Pacific: Singapore, Thailand, Viet Nam, possibly China, The Philippines, Indonesia, New Guinea, Australia (Queensland and Western Australia) east to the Hawaiian Islands and the Tuamotu Archipelago; including New Caledonia and Lord Howe Island, Palau, Caroline Islands, Marshall Islands, Line Islands, Guam, Solomon Islands, Phoenix Islands, Gilbert Islands, Pitcairn Island, Johnston Island, Wake Island and Tahiti.

Habitat and Biology:
A coastal-pelagic and inshore species frequenting continental and insular shelves of the Indo-West Pacific and oceanic waters adjacent to them; common on coral reefs, often in deeper areas near drop-offs to the open sea, in atoll passes, and in shallow lagoons adjacent to areas of strong currents. This shark is often found cruising near the bottom but will visit the surface, especially to investigate food sources; occurs at a depth from the surface and intertidal down to at least 100 m. Sonic-tagged individuals have been shown to venture several kilometres offshore at depths less than 100 m. This is one of the three commonest reef sharks in the Indo-Pacific (the others being the blacktip and whitetip reef sharks); it prefers low, small coral islands and has a preference for their leeward sides. It shows microhabitat separation from the blacktip reef shark; around islands where both species occur, the blacktip occupies shallow flats while the grey reef shark is usually found in deeper areas, but where the blacktip is absent the grey reef shark is commonly found on the flats.

This is an active, strong-swimming social species, that forms daytime schools or aggregations in favoured areas such as reef passes, lagoons, or places near passes. Especially prominent are groups of juveniles on probable pupping and nursery grounds. At night these groups disperse, with individuals moving to different areas. Although this shark is active during the day, it is more active nocturnally.

Viviparous, with a yolk-sac placenta; number of young 1 to 6 per litter. Gestation period about 12months. Individuals mature at about 7 to 7.5 years, with a maximum age of at least 25 years.

Feeds on reef bony fishes, particular small fishes less than 30 cm long, but also squid, octopi, crabs, lobsters and shrimp. It feeds mostly off but near the bottom, but can capture bottom prey. It complements the whitetip reef shark, as it is far more adept at catching off-bottom fish than the whitetip, but the latter is far more competant in extracting prey from crevices and holes in reefs.

This shark is prone to investigate novel events in circumstances where food stimuli are not present. In seldom-frequented areas divers may be approached very closely by several of these sharks when they first enter the water, butthe sharks soon disperse and seldom reappear except at a distance. Repeated dives at the same locality will seldom bring forth the local sharks, which apparently have sated their curiosity. When feeding stimuli are present or when these sharks are accosted they can be aggressive and dangerous despite their usually modest size. Spearfishing will bring these sharks in to boldly contest the catch and, although they generally can discriminate between speared fish and spearfisher, several attacks on people have occurred, including at least one fatal attack. These may be mistaken-identity attacks, especially when a speared fish moves very close to a diver, or when divers stupidly attach fish to their belts, as this shark does not normally take mammalian prey. Observations and subsequent experiments by divers (in some cases using small submarines) revealed that this shark performs what is apparently a threat-display when approached too closely, or when startled by unusual sounds or quick movements, under conditions when no feeding stimuli are present; presence of food stimuli apparently depresses this display. This consists of an exaggerated swimming pattern in which the shark wags its head and tail in broad sweeps, arches its back, lifts its head, depresses its pectoral fins and sometimes swims in a horizontal spiral. The display varies in intensity from merely a component of flight from the accosting diver to a series of figure-8 loops in front of the aggressor. Using a small shark-shaped 'Shark Observation Submersible' to approach grey reef sharks, Dr Donald R. Nelson was able to elicit threat display from the sharks while other divers filmed the behaviour from a safe distance. When persistantly approached by the sub, some of the displaying sharks fled, but a few terminated the display and attacked the sub at high speed, biting one or more times and then fleeing. The speed of the attacks and the damage to the sub was impressive, and is a mute warning that these sharks should be treated with respect and not cornered or harassed by divers. The threat-display behaviour of this shark is thought by some researchers to possibly intimidate potential predators on it.

Size:
Maximum possibly 233 to 255 cm, adult males maturing at 130 to 145 cm, adult females maturing at 122 to 137 cm; most adults of either sex below 190 cm, but one male reportedly 255 cm;size at birth between 45 and 60 cm.

Interest to Fisheries:
Fished in Thailand, but details of fisheries there and elsewhere not recorded.

Type material:
Holotype: Rijksmuseum van Natuurlijke Historie, Leiden, RNH 7377, 1540 mm female. Type Locality: Java Sea near Salambo Islands.

Grey reef shark (Carcharhinus amblyrhynchos)