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Whitetip reef shark
The whitetip reef shark (Triaenodon obesus) is a species of requiem shark, in the family Carcharhinidae, and the only member of its genus. A small shark does not usually exceed 1.6 m (5.2 ft) in length, so this species is easily recognizable by its slender body and short but broad head, as well as tubular skin flaps beside the nostrils, oval eyes with vertical pupils, and white-tipped dorsal and caudal fins. One of the most common sharks found on Indo-Pacific coral reefs, the whitetip reef shark occurs as far west as South Africa and as far east as Central America. It is typically found on or near the bottom in clear water, at a depth of 8–40 m (26–131 ft).
During the day, whitetip reef sharks spend much of their time resting inside caves. Unlike other requiem sharks, which rely on ram ventilation and must constantly swim to breathe, this shark can pump water over its gills and lie still on the bottom. At night, whitetip reef sharks emerge to hunt bony fishes, crustaceans, and octopus in groups, their elongate bodies allowing them to force their way into crevices and holes to extract hidden prey. Individuals may stay within a particular area of the reef for months or years, frequently returning to the same shelter. This species is viviparous, in which the developing embryos are sustained by a placental connection to their mother.
Whitetip reef sharks are rarely aggressive towards humans, though they may investigate swimmers closely. However, spear fishers are at risk of being bitten by one attempting to steal their catch. This species is caught for food, though ciguatera poisoning resulting from its consumption has been reported. The IUCN has assessed the whitetip reef shark as Near Threatened, noting its numbers are dwindling due to increasing levels of unregulated fishing activity across its range. The slow reproductive rate and limited habitat preferences of this species renders its populations vulnerable to overfishing.
Description & appearance
A relatively small species, few whitetip reef sharks are longer than 1.6 m (5.2 ft). The maximum length this species attains is often given as 2.1 m (6.9 ft), though this was originally based on visual observations and may be dubious. The maximum reported weight is 18.3 kg (40 lb). The whitetip reef shark has a slim body and a short, broad head. The snout is flattened and blunt, with large flaps of skin in front of the nares that are furled into tubes. The eyes are small and oval with vertical pupils and prominent ridges above, and are often followed by a small notch. The mouth has a distinct downward slant (imparting a disgruntled expression to the shark), with short furrows at the corners. There are 42–50 tooth rows in the upper jaw and 42–48 tooth rows in the lower jaw. Each tooth has a single narrow, smooth-edged cusp at the center, flanked by a pair of much smaller cusplets.
The first dorsal fin is positioned well back on the body, closer to the pelvic than the pectoral fins. The second dorsal and anal fins are large, about half to three-quarters as high as the first dorsal fin. The broad, triangular pectoral fins originate at or slightly before the level of the fifth gill slit. There is no ridge between the first and second dorsal fins. The lower lobe of the caudal fin is half the length of the upper, which has a strong notch near the tip. The dermal denticles are small and overlapping, usually with 7 horizontal ridges, giving the skin a smooth feel. The coloration is grayish to brownish above and white below, with a pattern of scattered small, dark spots unique to each individual. The tips of the first dorsal fin and upper caudal fin lobe, and sometimes also the second dorsal fin and lower caudal fin lobe, are bright white.
Distribution & habitat
The whitetip reef shark is distributed widely across the entire Indo-Pacific region. It was once thought to have formerly existed in the Atlantic Ocean, based on fossil teeth found in North Carolina dating to the Miocene epoch. However, more recent research indicates that the teeth belonged to a mackerel shark, and that this species has never colonized the Atlantic. In the Indian Ocean, it occurs from northern KwaZulu-Natal, South Africa to the Red Sea and the Indian subcontinent, including Madagascar, Mauritius, the Comoros, the Aldabra Group, the Seychelles, Sri Lanka, and the Chagos Archipelago. In the western and central Pacific, it occurs from off southern China, Taiwan, and the Ryukyu Islands, to the Philippines, Southeast Asia, and Indonesia, to northern Australia, and is also found around numerous islands in Melanesia, Micronesia, and Polynesia, as far as Hawaii to the north and the Pitcairn Islands to the southeast. In the eastern Pacific, it occurs from Costa Rica to Panama, and off the Galápagos Islands.
Associated almost exclusively with coral reef habitats, whitetip reef sharks are most often encountered around coral heads and ledges with high vertical relief, and additionally over sandy flats, in lagoons, and near drop-offs to deeper water. They prefer very clear water and rarely swim far from the bottom. This species is most common at a depth of 8–40 m (26–131 ft). On occasion, they may enter water less than 1 m deep, and there is an exceptional record of a whitetip reef shark being captured from a depth of 330 m (1,080 ft) in the Ryukyu Islands.
Hunting & food
With its slender, lithe body, the whitetip reef shark specializes in wriggling into narrow crevices and holes in the reef and extracting prey inaccessible to other reef sharks. Alternatively, it is rather clumsy when attempting to take food suspended in open water.This species feeds mainly on bony fishes, including eels, squirrelfishes, snappers, damselfishes, parrotfishes, surgeonfishes, triggerfishes and goatfishes, as well as octopuses, spiny lobsters, and crabs. The whitetip reef shark is highly responsive to the olfactory, acoustic, and electrical cues given off by potential prey, while its visual system is attuned more to movement and/or contrast than to object details. It is especially sensitive to natural and artificial low-frequency sounds in the 25–100 Hz range, which evoke struggling fish.
Whitetip reef sharks hunt primarily at night, when many fishes are asleep and easily taken. After dusk, groups of sharks methodically scour the reef, often breaking off pieces of coral in their vigorous pursuit of prey. Multiple sharks may target the same prey item, covering every exit route from a particular coral head. Each shark hunts for itself and in competition with the others in its group. Unlike blacktip reef sharks and grey reef sharks, whitetip reef sharks do not become more excited when feeding in groups and are unlikely to be stirred into a feeding frenzy. Despite their nocturnal habits, whitetip reef sharks will hunt opportunistically in daytime. Off Borneo, this species gathers around reef drop-offs to feed on food brought up by the rising current. Off Hawaii, they follow Hawaiian monk seals (Monachus schauinslandi) and attempt to steal their catches. A whitetip reef shark can survive for six weeks without food.
Like other members of its family, the whitetip reef shark is viviparous; once the developing embryos exhaust their supply of yolk, the yolk sac is converted into a placental connection through which the mother delivers nourishment for the remainder of gestation. Mature females have a single functional ovary, on the left side, and two functional uteruses. The reproductive cycle is biennial.
Mating is initiated when up to five males follow closely behind a female and bite at her fins and body, possibly cued by pheromones indicating the female's readiness. Each male attempts to seize the female by engulfing one of her pectoral fins; at times two males might grasp a female on both sides simultaneously. Once engaged, the sharks sink to the bottom, whereupon the male (or males) rotates one of his claspers forward, inflates the associated siphon sac (a subcutaneous abdominal organ that takes in seawater that is used to flush sperm into the female), and attempts to make contact with the female's vent. In many cases, the female resists by pressing her belly against the bottom and arching her tail; this may reflect mate choice on her part. The male has a limited time in which to achieve copulation, as while he is holding the female's pectoral fin in his mouth he is being deprived of oxygen. On the other hand, if the female is willing, the pair settles side-by-side with their heads pressed against the bottom and their bodies at an upward angle.
After a gestation period of 10–13 months, females give birth to litters of 1–6 (usually 2–3) pups. The number of offspring is not correlated with female size; each female produces an estimated average of 12 pups over her entire lifetime. Parturition occurs from May to August (autumn and winter) in French Polynesia, in July (summer) off Enewetak Atoll, and in October (summer) off Australia. Females give birth while swimming, making violent twists and turns of their bodies; each pup takes under an hour to fully emerge. The newborns measure 52–60 cm (20–24 in) long and have relatively longer caudal fins than adults. This shark develops slowly compared to other requiem sharks; newborns grow at a rate of 16 cm (6.3 in) per year while adults grow as a rate of 2–4 cm (0.79–1.57 in) per year. Sexual maturity is reached at a length of around 1.1 m (3.6 ft) and an age of 8–9 years, though mature males as small as 95 cm (37 in) long have been recorded from the Maldives, suggesting regional variation in maturation size. On the Great Barrier Reef, males live to 14 years and females to 19 years; the maximum lifespan of this shark may be upwards of 25 years. In 2008, a whitetip reef shark produced a single pup through possibly asexual means at the Nyiregyhaza Centre in Hungary; previous instances of asexual reproduction in sharks have been reported in the bonnethead (Sphyrna tiburo) and the blacktip shark (Carcharhinus limbatus).
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Whitetip reef shark
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