The whitetip reef shark (Triaenodon obesus) is a species of requiem shark, in the family Carcharhinidae. It is the only extant species of genus Triaenodon. A small shark that does not usually exceed in length, 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 .
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 Vulnerable, 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.
Taxonomy and phylogeny
thumb|left|Early illustration of a whitetip reef shark from Systematische Beschreibung der Plagiostomen (1841).
The whitetip reef shark was first described by the German naturalist Eduard Rüppell as Carcharias obesus, in the 1837 Fische des Rothen Meere (Fishes of the Red Sea). His choice of the specific epithet obesus was curious, given that this shark is actually quite slender. Later in 1837, Johannes Müller and Friedrich Henle moved this species into its own genus Triaenodon, from the Greek triaena meaning "trident", and odon meaning "tooth". As Rüppell did not originally designate a holotype, in 1960 a 31-cm-long specimen caught off Jeddah, Saudi Arabia, was made the species lectotype.
Once placed in the family Triakidae, the whitetip reef shark is now recognized by most authors as belonging to the family Carcharhinidae on the basis of morphological characters, such as a full nictitating membrane, well-developed precaudal pit, strong lower caudal fin lobe, and scroll-like intestinal valves.
Distribution and habitat
thumb|left|alt=Photo of a whitetip reef shark resting amongst many brightly colored corals, its head concealed in a cave|The whitetip reef shark almost exclusively inhabits coral reefs
The whitetip reef shark is distributed widely across the entire Indo-Pacific region. It was once thought to have existed in the Atlantic Ocean, based on fossil teeth found in North Carolina dated to the Miocene epoch, although subsequent research has suggested that the teeth are of a mackerel shark, and that the whitetip has never colonized the Atlantic. In the Indian Ocean, whitetip reef sharks can be found off of northern KwaZulu-Natal, South Africa and all along the East African coast, from the Horn of Africa to the Red Sea and along the Indian subcontinent (including Sri Lanka). They are found near many major island chains as well, including Madagascar, Mauritius, Mayotte, the Comoros, the Aldabra Group, the Seychelles and the Chagos Archipelago.
In the western and central Pacific Ocean, whitetips occur off South China, Taiwan, and the Ryukyu Islands, south to the Philippines and throughout the entire Coral Triangle, including the coastlines of Indonesia, East Timor and Papua New Guinea. Their range continues south to the northern Australia and the Great Barrier Reef. Whitetips are also found around numerous Pacific atolls and islands, including Melanesia, Micronesia, and Polynesia, north to Hawaii and south to the Pitcairn Islands. They have also been sighted near Midway and Johnston Atolls and Laysan. In the Eastern Pacific, they may be found off the west coasts of Costa Rica and Panama south to the Galápagos Islands, and as far north as Isla Isabel, Isla Socorro, and the southern tip of Baja California (Cabo San Lucas and Cabo Pulmo National Marine Park).
Description
thumb|right|upright=1.2|alt=Frontal view of a whitetip reef shark, which has a wedge-shaped snout, oval eyes, and tubular flaps of skin next to the nostrils|The "face" of a whitetip reef shark is distinctive, with a broad snout, tubular nasal flaps, and oval eyes with vertical pupils.
A relatively small species, few whitetip reef sharks are longer than . The maximum length this species attains is often given as , though this was originally based on visual observations and may be dubious.
Whitetip reef sharks generally remain within a highly localized area; only rarely do they undertake long movements, wandering for a while before settling down somewhere new. One study at Johnston Atoll found that none of the sharks examined had moved more than away from their original capture location over periods of up to a year. While resting during the day, these sharks have been observed being cleaned by the wrasse Bodianus diplotaenia and the goby Elacatinus puncticulatus. Unusually, there is also a report of seven whitetip reef sharks adopting a cleaning posture (mouth agape and gills flared) in the midst of a swarm of non-cleaning hyperiid amphipods; the mechanical stimulation from the moving amphipods are thought to have evoked this behavior through their similarity to actual cleaner organisms.
Feeding
thumb|The lower jaw and teeth of whitetip reef shark
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. It is especially sensitive to natural and artificial low-frequency sounds in the 25–100 Hz range, which evoke struggling fish. 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. Despite their nocturnal habits, whitetip reef sharks will hunt opportunistically in daytime. Off Hawaii, they follow Hawaiian monk seals (Monachus schauinslandi) and attempt to steal their catches. 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. The newborns measure long and have relatively longer caudal fins than adults. This shark develops slowly compared to other requiem sharks; newborns grow at a rate of per year while adults grow at a rate of per year.
Human interactions
The white tip reef shark is more harmless and is seldom aggressive unless provoked. They are also fearless and curious, as the whitetip reef sharks may approach swimmers closely to investigate. However, these sharks readily attempt, and quite boldly, to steal catches from spear fishers, which has resulted in several people being bitten in the process. Whitetip reef sharks are well-suited to ecotourism diving, and with conditioning they can be hand-fed by divers.
The whitetip reef shark is taken by fisheries operating off Pakistan, India, Sri Lanka, Madagascar, and likely elsewhere, using longlines, gillnets, and trawls. The meat and liver are eaten, though sharks from certain areas present a substantial risk of ciguatera poisoning (especially the liver, which contains a much higher concentration of the toxin than the meat). Its restricted habitat, low dispersal, and slow reproduction are factors that limit this shark's capacity for recovering from overfishing. In June 2018 the New Zealand Department of Conservation classified the whitetip reef shark as "Vagrant" under the New Zealand Threat Classification System.
Other Triaenodon species
The T. willei is the only described extinct species of this genus, found in Europe, in Belgium, between the Eocene period (in Lutetian to Ypresian stages).
See also
- List of sharks
References
External links
- Whitetip reef shark (Triaenodon obesus) at FishBase
- Whitetip reef shark (Triaenodon obesus) at Florida Museum of Natural History Ichthyology Department