| Whitetip reef shark | |
|---|---|
 | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Division: | Selachii |
| Order: | Carcharhiniformes |
| Family: | Carcharhinidae |
| Genus: | Triaenodon J. P. Müller &Henle, 1837 |
| Species: | T. obesus |
| Binomial name | |
| Triaenodon obesus (Rüppell, 1837) | |
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| Range of the whitetip reef shark | |
| Synonyms | |
Carcharias obesusRüppell, 1837 | |
Thewhitetip reef shark (Triaenodon obesus) is aspecies ofrequiem shark, in thefamily Carcharhinidae, and the only member of itsgenus. A small shark that does not usually exceed 1.6 m (5.2 ft) in length, this species is easily recognizable by its slender body and short but broad head, as well as tubular skin flaps beside thenostrils, oval eyes with vertical pupils, and white-tippeddorsal andcaudal fins. One of the most common sharks found onIndo-Pacificcoral reefs, the whitetip reef shark occurs as far west asSouth Africa and as far east asCentral 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 itsgills and lie still on the bottom. At night, whitetip reef sharks emerge to huntbony fishes,crustaceans, andoctopus 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 isviviparous, in which the developingembryos are sustained by aplacental 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, thoughciguatera poisoning resulting from its consumption has been reported. TheIUCN 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 tooverfishing.
The whitetip reef shark was first described by the German naturalistEduard Rüppell asCarcharias obesus, in the 1837Fische des Rothen Meere (Fishes of the Red Sea).[2] His choice of thespecific epithetobesus was curious, given that this shark is actually quite slender.[3] Later in 1837,Johannes Müller andFriedrich Henle moved this species into its own genusTriaenodon, from theGreektriaena meaning "trident", andodon meaning "tooth". As Rüppell did not originally designate aholotype, in 1960 a 31-cm-long specimen caught offJeddah,Saudi Arabia, was made the specieslectotype.[2] Othercommon names for this shark include blunthead shark, light-tip shark, reef whitetip shark, and whitetip shark.[4]
Once placed in the familyTriakidae, the whitetip reef shark is now recognized by most authors as belonging to the familyCarcharhinidae on the basis ofmorphological characters, such as a fullnictitating membrane, well-developed precaudal pit, strong lowercaudal fin lobe, and scroll-likeintestinal valves.[5] Morphological andmolecular phylogenetic analyses suggest the whitetip reef shark is grouped with thelemon sharks (Negaprion) and thesliteye shark (Loxodon) in occupying an intermediate position on the carcharhinidevolutionary tree, between mostbasal genera (Galeocerdo,Rhizoprionodon, andScoliodon) and the mostderived (Carcharhinus andSphyrna).[6]
The whitetip reef shark is distributed widely across the entireIndo-Pacific region. It was once thought to have existed in theAtlantic Ocean, based onfossil teeth found inNorth Carolina dated to theMiocene epoch, although subsequent research has suggested that the teeth are of amackerel shark, and that the whitetip has never colonized the Atlantic.[7] In theIndian Ocean, whitetip reef sharks can be found off of northernKwaZulu-Natal,South Africa and all along theEast African coast, from theHorn of Africa to theRed Sea and along theIndian subcontinent (includingSri Lanka). They are found near many major island chains as well, includingMadagascar,Mauritius,Mayotte, theComoros, theAldabra Group, theSeychelles and theChagos Archipelago.
In the western and central Pacific Ocean, whitetips occur offSouth China,Taiwan, and theRyukyu Islands, south to thePhilippines and throughout the entireCoral Triangle, including the coastlines ofIndonesia,East Timor andPapua New Guinea. Their range continues south to the northernAustralia and theGreat Barrier Reef. Whitetips are also found around numerous Pacificatolls and islands, includingMelanesia,Micronesia, andPolynesia, north toHawaii and south to thePitcairn Islands. They have also been sighted nearMidway andJohnston Atolls andLaysan. In the Eastern Pacific, they may be found off the west coasts ofCosta Rica andPanama south to theGalápagos Islands, and as far north asIsla Isabel,Isla Socorro, and the southern tip ofBaja California (Cabo San Lucas andCabo Pulmo National Marine Park).[2]
Associated almost exclusively withcoral reefhabitats, whitetip reef sharks are most often encountered around coral heads and ledges with high vertical relief, and additionally over sandy flats, inlagoons, and near drop-offs to deeper water.[8] They prefer very clear water, and rarely stray far from the bottom during the day.[5] The species is most common at a depth of 8–40 m (26–131 ft).[2] On occasion, they may enter water less than 1 m deep when foraging; there is an exceptional record of a whitetip reef shark being captured from a depth of 330 m (1,080 ft) off the Ryukyu Islands.[5]
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.[5] The maximum reported weight is 18.3 kg (40 lb).[4] 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 thenares that are furled into tubes. The eyes are small and oval with verticalpupils 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.[2]
The firstdorsal fin is positioned well back on the body, closer to thepelvic than thepectoral fins. The second dorsal andanal 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 fifthgill 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.[2] Thedermal 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.[5]
The whitetip reef shark is one of the three most common sharks inhabiting the reefs of the Indo-Pacific, the other two being theblacktip reef shark (Carcharhinus melanopterus) and thegrey reef shark (Carcharhinus amblyrhynchos). The habitat preferences of this species overlap those of the other two, though it does not tend to frequent very shallow water like the blacktip reef shark, nor the outer reef like the grey reef shark.[2] The whitetip reef shark swims with strong undulations of its body, and unlike other requiem sharks can lie motionless on the bottom and actively pump water over its gills forrespiration.[2] This species is most active at night or duringslack tide, and spends much of the day resting inside caves singly or in small groups, arranged in parallel or stacked atop one another. Off Hawaii, these sharks may be found sheltering inside underwaterlava tubes, while off Costa Rica they are often seen lying in the open on sandy flats.[9]
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 atJohnston Atoll found that none of the sharks examined had moved more than 3 km (1.9 mi) away from their original capture location over periods of up to a year.[5] Another study atRangiroa Atoll inFrench Polynesia found that, after more than three years, around 40% of the originally tagged sharks were still present on the same reef where they were first captured. An individual shark may rest inside the same cave for months to years. The daytimehome range of a whitetip reef shark is limited to approximately 0.05 km2 (0.019 sq mi); at night this range increases to 1 km2 (0.39 sq mi).[9] These sharks are notterritorial and share their home ranges with others of their species; they do not performthreat displays.[2][8]
Important predators of the whitetip reef shark includetiger sharks (Galeocerdo cuvier),Galapagos sharks (Carcharhinus galapagensis), and possibly alsosilvertip sharks (Carcharhinus albimarginatus), though they usually occur at depths greater than those favored by whitetip reef sharks. An 80 cm (31 in) long whitetip reef shark has also been found in the stomach of agiant grouper (Epinephelus lanceolatus), though these groupers are unlikely to be significant predators of this species due to their rarity.[5] Knownparasites of the whitetip reef shark include thecopepodParalebion elongatus and the praniza (parasitic)larvae of theisopodGnathia grandilaris.[10][11] While resting during the day, these sharks have been observed being cleaned by thewrasseBodianus diplotaenia and thegobyElacatinus 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-cleaninghyperiidamphipods; the mechanical stimulation from the moving amphipods are thought to have evoked this behavior through their similarity to actual cleaner organisms.[12]
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.[5] This species feeds mainly on bony fishes, includingeels,squirrelfishes,snappers,damselfishes,parrotfishes,surgeonfishes,triggerfishes andgoatfishes, as well asoctopuses,spiny lobsters, andcrabs.[2] The whitetip reef shark is highly responsive to theolfactory,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.[8][13][14] It is especially sensitive to natural and artificial low-frequency sounds in the 25–100 Hz range, which evoke struggling fish.[9]
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.[15] 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.[8] 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 afeeding frenzy.[8] Despite their nocturnal habits, whitetip reef sharks will hunt opportunistically in daytime.[5] OffBorneo, this species gathers around reef drop-offs to feed on food brought up by the rising current.[16] OffHawaii, they followHawaiian monk seals (Monachus schauinslandi) and attempt to steal their catches.[6] A whitetip reef shark can survive for six weeks without food.[5]
Like other members of its family, the whitetip reef shark isviviparous; once the developingembryos exhaust their supply ofyolk, theyolk sac is converted into aplacental connection through which the mother delivers nourishment for the remainder ofgestation. Mature females have a single functionalovary, on the left side, and two functionaluteruses. The reproductive cycle is biennial.[17]
Mating is initiated when up to five males follow closely behind a female and bite at her fins and body, possibly cued bypheromones indicating the female's readiness.[18] 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 hisclaspers forward, inflates the associated siphon sac (a subcutaneous abdominal organ that takes in seawater that is used to flushsperm 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 reflectmate choice on her part. The male has a limited time in which to achievecopulation, as while he is holding the female's pectoral fin in his mouth he is being deprived ofoxygen. 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.[19][20]
After agestation 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.[17] Parturition occurs from May to August (autumn and winter) in French Polynesia, in July (summer) offEnewetak Atoll, and in October (summer) off Australia.[2][17] Females give birth while swimming, making violent twists and turns of their bodies; each pup takes under an hour to fully emerge.[21] 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 at a rate of 2–4 cm (0.79–1.57 in) per year.[5]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 theMaldives, suggesting regional variation in maturation size.[22] On theGreat Barrier Reef, males live to 14 years and females to 19 years; the maximum lifespan of this shark may be upwards of 25 years.[5][17] In 2008, a whitetip reef shark produced a single pup through possiblyasexual means at the Nyiregyhaza Centre inHungary; previous instances of asexual reproduction in sharks have been reported in thebonnethead (Sphyrna tiburo) and theblacktip shark (Carcharhinus limbatus).[23]
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 fromspear fishers, which has resulted in several people being bitten in the process.[5] In some places, local whitetip reef sharks have learned to associate the sound of aspeargun discharge or a boat dropping anchor with food and respond within seconds.[9] As of 2008, theInternational Shark Attack File lists two provoked and three unprovoked attacks to this species.[24] Whitetip reef sharks are well-suited toecotourism diving, and with conditioning they can be hand-fed by divers.[2] InHawaiian mythology, the fidelity (i.e. "loyalty") of whitetip reef sharks to certain areas of the reef for years at a time may have inspired belief inʻaumākua, the spirits of family ancestors that take animal form and protect their descendants.[25]
The whitetip reef shark is taken byfisheries operating offPakistan,India,Sri Lanka,Madagascar, and likely elsewhere, usinglonglines,gillnets, andtrawls. The meat andliver are eaten, though sharks from certain areas present a substantial risk ofciguatera poisoning (especially the liver, which contains a much higher concentration of the toxin than the meat).[2][5] TheInternational Union for Conservation of Nature (IUCN) has assessed this species as Vulnerable, as its numbers have dropped in recent decades due to increasing, and thus far unregulated, fishing pressure in the tropics.[22] Its restricted habitat, low dispersal, and slow reproduction are factors that limit this shark's capacity for recovering fromoverfishing.[1] On the Great Barrier Reef, populations of whitetip reef sharks in fishing zones have been reduced by 80% relative to no-entry zones. Furthermore, populations in no-take zones, where boats are allowed but fishing prohibited, exhibit levels of depletion comparable to fishing zones due topoaching. Demographic models indicate that these depleted populations will continue to decline by 6.6–8.3% per year without additional conservation measures.[17] In June 2018 the New ZealandDepartment of Conservation classified the whitetip reef shark as "Vagrant" under theNew Zealand Threat Classification System.[26]
