| Blacktip reef shark | |
|---|---|
_Moorea_5.jpg) | |
| Moorea,French Polynesia | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Chondrichthyes |
| Subclass: | Elasmobranchii |
| Division: | Selachii |
| Order: | Carcharhiniformes |
| Family: | Carcharhinidae |
| Genus: | Carcharhinus |
| Species: | C. melanopterus |
| Binomial name | |
| Carcharhinus melanopterus | |
 | |
| Range of the blacktip reef shark | |
| Synonyms | |
Carcharias elegansEhrenberg, 1871 * ambiguous synonym | |
Theblacktip reef shark (Carcharhinus melanopterus) is aspecies ofrequiem shark, in thefamily Carcharhinidae, which can be easily identified by the prominent black tips on its fins (especially on the firstdorsal fin and itscaudal fin). Among the most abundant sharks inhabiting thetropicalcoral reefs of theIndian andPacific Oceans, this species prefers shallow, inshore waters. Its exposed first dorsal fin is a common sight in the region. The blacktip reef shark is usually found over reef ledges and sandy flats, though it has also been known to enterbrackish andfreshwater environments. It typically attains a length of 1.6 m (5.2 ft). Like other sharks, the females are larger than the males.
The blacktip reef shark has extremely small home ranges and exhibits strong site fidelity, remaining within the same local area for up to several years at a time. It is an activepredator of smallbony fish,cephalopods, andcrustaceans, and has also been known to feed onsea snakes andseabirds. Accounts of the blacktip reef shark's life history have been variable and sometimes contradictory, in part reflecting geographical differences within the species. Like other members of its family, this shark isviviparous, with females giving birth to two to five young on a biennial, annual, or possibly biannual cycle. Reports of thegestation period range from 7–9, through 10–11, to possibly 16 months.Mating is preceded by the male following closely behind the female, likely attracted by her chemical signals. Newborn sharks are found further inshore and in shallower water than adults, frequently roaming in large groups over areas flooded byhigh tide.
Timid and skittish, the blacktip reef shark is difficult to approach and seldom poses a danger to humans unless roused by food. However, people wading through shallow water are at risk of having their legs mistakenly bitten. This shark is used for its meat, fins, andliver oil, but is not considered to be a commercially significant species. TheInternational Union for Conservation of Nature has assessed the blacktip reef shark asVulnerable. Although the species as a whole remains widespread and relatively common,overfishing of this slow-reproducing shark has led to its decline at a number of locales.
FrenchnaturalistsJean René Constant Quoy andJoseph Paul Gaimard originally described the blacktip reef shark during the 1817–1820 exploratory voyage of thecorvetteUranie. In 1824, their account was published as part ofVoyage autour du monde...sur les corvettes de S.M. l'Uranie et la Physicienne,Louis de Freycinet's 13-volume report on the voyage. Thetype specimen was a 59 cm (23 in)-long juvenile male caught off the island ofWaigeo, west ofNew Guinea.[3] Quoy and Gaimard chose the nameCarcharias melanopterus, from the Greekmelas meaning "black" andpteron meaning "fin" or "wing", in reference to this shark's prominent fin markings.[4]
Subsequent authors moved the blacktip reef shark to thegenusCarcharhinus; in 1965 theInternational Commission on Zoological Nomenclature designated it as thetype species for the genus.[3] In some earlier literature, the scientific name of this shark was mistakenly given asC. spallanzani, now recognized as asynonym of thespottail shark (C. sorrah).[5] Othercommon names for this species include blackfin reef shark, black-finned shark, blacktip shark, reef blacktip shark, and guliman.[6]
Like most other members of its genus, thephylogenetic position of the blacktip reef shark remains indeterminate. Based onmorphology,Jack Garrick proposed in 1982 that the closest relative of the blacktip reef shark was thenervous shark (C. cautus).[7]Leonard Compagno's 1988 morphological analysis suggested affinity not only between this species and the nervous shark, but also four other species, and could not resolve their relationships further. A 1998allozyme analysis by Gavin Naylor again yielded ambiguous results, finding that the blacktip reef shark forms apolytomy (irresolvable group) with 10 otherCarcharhinus species.[8]
The blacktip reef shark is found throughout nearshore waters of the tropical and subtropicalIndo-Pacific.[5] In theIndian Ocean, it occurs fromSouth Africa to theRed Sea, includingMadagascar,Mauritius, and theSeychelles, and from there eastward along the coast of theIndian subcontinent toSoutheast Asia, includingSri Lanka, theAndaman Islands, and theMaldives. In thePacific Ocean, it is found from southernChina and thePhilippines toIndonesia, northernAustralia andNew Caledonia, and also inhabits numerous oceanic islands, including theMarshall,Gilbert,Society, andHawaiian Islands andTuamotu.[9] Contrary to what most sources state, there is a report suggesting that the specimens of this species fromJapanese waters might be fromTaiwan,[10] however a number of sightings and captures of this species have been reported from the inshore waters ofIshigaki Island ofOkinawa Prefecture in southernJapan.[11][12] ALessepsian migrant, this shark has colonized the easternMediterranean Sea by way of theSuez Canal.[9]
Although it has been reported from a depth of 75 m (246 ft),[6] the blacktip reef shark is usually found in water only a few meters deep, and can often be seen swimming close to shore with its dorsal fin exposed.[3] Younger sharks prefer shallow, sandy flats, while older sharks are most common around reef ledges and can also be found near reef drop-offs. This species has also been reported frombrackishestuaries andlakes in Madagascar, andfreshwater environments inMalaysia, though it is not able to tolerate lowsalinity to the same degree as thebull shark (C. leucas).[3] AtAldabra in the Indian Ocean, blacktip reef sharks congregate in the channels between reef flats duringlow tide and travel to themangroves when the water rises.[13] There is equivocal evidence that sharks from the northern and southern extremes of its distribution aremigratory.[3]
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A robustly built species with a streamlined "typical shark" form, the blacktip reef shark has a short, wide, rounded snout and moderately large, oval eyes. Eachnostril has a flap of skin in front that is expanded into a nipple-shaped lobe. Not counting small symphysial (central) teeth, the tooth rows number 11–13 (usually 12) on either side of the upper jaw and 10–12 (usually 11) on either side of the lower jaw. The upper teeth are upright to angled and narrowly triangular in shape, bearing serrations that are more coarse on the bases; the lower teeth are similar, but more finely serrated.[3][5] The teeth of adult males are more abruptly curved than those of females.[14]
Thepectoral fins are large and narrowly falcate (sickle-shaped), tapering to points. The sizable firstdorsal fin is high with a curving S-shaped rear margin, and originates over the free rear tips of the pectoral fins. The second dorsal fin is relatively large with a short rear margin, and is placed opposite theanal fin. There is no ridge between the dorsal fins. This shark is a pale grayish-brown above and white below, with an obvious white band on the sides extending forward from above the anal fin. All the fins have black tips highlighted by lighter-colored borders, which are especially striking on the first dorsal fin and lower caudal fin lobe. Most blacktip reef sharks are no more than 1.6 m (5.2 ft) long, though rarely individuals may reach 1.8 m (5.9 ft) or possibly 2.0 m (6.6 ft).[3] The maximum weight on record with theInternational Game Fish Association is 13.6 kg (30 lb).[6]
Along with thegrey reef shark (C. amblyrhinchos) and thewhitetip reef shark (Triaenodon obesus), the blacktip reef shark is one of the three most common sharks inhabiting coral reefs in the Indo-Pacific. This species predominates in shallow habitats, while the other two are mostly found deeper. Fast-swimming and active, the blacktip reef shark may be encountered alone or in small groups; large "social" aggregations have also been observed.[3][15] For the most part, juvenile and adult sharks are not segregated by sex, save for the movements of pregnant females to give birth. Individuals exhibit strong fidelity to particular areas, where they may remain for several years.[16]
A tracking study offPalmyra Atoll in the central Pacific has found the blacktip reef shark has a home range of around 0.55 km2 (0.21 sq mi), among the smallest of any shark species. The size and location of the range does not change with time of day. Within this range, 3–17% of the area constitute favored hunting patches that are disproportionately occupied by the resident shark. The sharks spend most of their time swimming back and forth along reef ledges, making occasional short forays onto sandy flats. Their average swimming speed decreases when the tide rises at night, possibly because the influx of cooler water reduces theirmetabolism, or the accompanying movement ofprey fishes makes foraging easier.[17] Blacktip reef sharks at Aldabra tend to be more mobile than those at Palmyra, with recorded individual movements of up to 2.5 km (1.6 mi) over 7 hours.[13]
Blacktip reef sharks, particularly small individuals, fall prey to larger fishes, includinggroupers,grey reef sharks,tiger sharks (Galeocerdo cuvier), and members of their own species. At Palmyra Atoll, adults avoid patrolling tiger sharks by staying out of the central, deeper lagoon.[17] Their knownparasites include thetapewormsAnthobothrium lesteri,[18]Nybelinia queenslandensis,[19]Otobothrium alexanderi,[20] andPlatybothrium jondoeorum,[21] amyxosporidian in the genusUnicapsula,[22] and themonogeneanDermophthirius melanopteri.[23] One of the few documented examples ofinfectious disease in a shark was a fatal case ofhemorrhagicsepticemia in a blacktip reef shark, caused by thebacteriumAeromonas salmonicida subsp.salmonicida.[24]
As often the most abundantapex predator within itsecosystem, the blacktip reef shark plays a major role in structuring inshoreecological communities.[16] Its diet is composed primarily of smallteleost fishes, includingmullet, groupers,grunters,jacks,mojarras,wrasses,surgeonfish, andsmelt-whitings. Groups of blacktip reef sharks in the Indian Ocean have been observed herdingschools of mullet against the shore for easier feeding.[25]Squid,octopus,cuttlefish,shrimp, andmantis shrimp are also taken, as well ascarrion and smallersharks and rays, though this is rare.[3][9] Off northern Australia, this species is known to consumesea snakes, includingAcrochordus granulatus,Hydrelaps darwiniensis,Hydrophis spp. andLapemis hardwickii.[26] Sharks off Palmyra Atoll have been documented preying onseabird chicks that have fallen out of theirnests into the water.[16] Miscellaneous items that have been found inside thestomachs of this species includealgae,turtle grass,coral,hydrozoa,bryozoa,rats, and stones.[13][16]
Researchers working atEnewetak Atoll in the Marshall Islands have found the blacktip reef shark can be readily attracted by splashing or striking metal tools against hard objects underwater, as well as by the scent of both healthy and injured fish.[27] As with most sharks, the blacktip reef shark does not have anycone cells in itsretina, limiting its ability to discriminate colors and fine details. Instead, its vision is adapted for sensitivity to movement or contrast under low light conditions, which is further enhanced by the presence of a reflectivetapetum lucidum. Experiments have shown that this shark is capable of detecting small objects up to 1.5–3 m (5–10 ft) away, but is unable to clearly discern the shape of the object.[13][28]Electroreception is another means by which this shark can locate prey; itsampullae of Lorenzini have a sensitivity of approximately 4 nV/cm and an effective range of 25 cm (10 in).[29] Similar to the grey reef shark, this species becomes more excited and "confident" in the presence of other individuals of its species, and in extreme situations can be roused into afeeding frenzy.[27] Feeding activity may be greater at night than during the day.[13]
Like the other members of its family, the blacktip reef shark isviviparous, though the details of its life history vary across its range. Its reproductive cycle is annual off northernAustralia, with mating taking place from January to February,[30] as well as offMoorea inFrench Polynesia, where mating occurs from November to March.[31] The cycle is biennial off Aldabra, where intense competition within and between species for food may constrain females to only bearing young every other year.[13] Earlier accounts from the Indian Ocean by Johnson (1978), Madagascar by Fourmanoir (1961), and the Red Sea by Gohar and Mazhar (1964), indicated a biannual cycle in these regions with two breeding seasons per year from June to July and December to January.[31][32][33] If accurate, the shorter reproductive cycles of these subpopulations may be a consequence of warmer water.[31]
When receptive to mating, a female blacktip reef shark swims slowly in asinusoidal pattern near the bottom with her head pointed down; observations in the wild suggest female sharks release chemical signals that allow males to track them. Once the male finds her, he closes to around 15 cm (5.9 in) and follows her with his snout oriented towards her vent.[34] A courting male may also bite the female behind her gills or on her pectoral fins; these mating wounds heal completely after 4–6 weeks.[31] After a period of synchronous swimming, the male pushes the female on her side and positions her so her head is against the bottom and her tail is raised. Once the female is in position, the male inserts one of hisclaspers into hercloaca.Copulation lasts for several minutes, after which the sharks separate and resume their regular behavior.[34] OffMoorea, individual older females mate and give birth at a consistent time every year, often to within a week's precision, whereas younger females exhibit more variability in their timing. Younger females are also more likely to fail to become pregnant after mating.[31]
Thegestation period has been reported as 10–11 months long in the Indian Ocean and Pacific islands,[13][31] and 7–9 months long off northern Australia.[30] Earlier authors, such as Melouk (1957), have estimated a gestation period as long as 16 months, though the validity of this figure has subsequently been challenged.[31] The female has a single functionalovary (on the right) and two functionaluteruses, divided into separate compartments for eachembryo. Newlyovulated egg cases measure 3.9 cm (1.5 in) by 2.6 cm (1.0 in); after hatching the embryos are sustained by ayolk sac during the first stage of development. After two months, the embryo measures 4 cm (1.6 in) long and has well-developed external gills. After four months, the yolk sac has begun to be converted into aplacental connection that attaches to the uterine wall; at this time, the embryo's dark fin markings develop. By five months, the embryo measures 24 cm (9.4 in) and has resorbed its external gills; the placenta is fully formed, though some yolk remains until seven months into gestation.[13]
Parturition occurs from September to November, with females making use of shallow nursery areas interior of the reef.[17][30][31] Newborn pups measure 40–50 cm (16–20 in) long in the Indian Ocean and off northern Australia, while free-swimming pups as small as 33 cm (13 in) long have been observed in the Pacific islands.[16][35] The litter size is 2–5 (typically 4), and is not correlated with female size.[9][13] Young blacktip reef sharks commonly form large groups in water barely deep enough to cover their bodies, over sand flats or in mangrove swamps close to shore. During high tide, they also move onto flooded coral platforms orseaweed beds.[17][27][36] Growth is initially rapid; one documented captive shark grew an average of 23 cm (9.1 in) per year in its first two years of life.[37] The growth rate slows to around 5 cm (2.0 in) per year in juveniles and adults.[17] Males and femalesmature sexually at lengths of 95 cm (37 in) and 97 cm (38 in) respectively off northern Australia,[30] and 105 cm (41 in) and 110 cm (43 in), respectively, off Aldabra.[13] Males mature at 97 cm (38 in) long off Palmyra Atoll.[17]
Under most circumstances, the blacktip reef shark has a timid demeanor and is easily frightened away by swimmers. However, its inshore habitat preferences bring it into frequent contact with humans, and thus it is regarded as potentially dangerous.[3] As of early 2009, 11 unprovoked attacks and 21 attacks total (none fatal) were listed on theInternational Shark Attack File that are attributable to the blacktip reef shark.[38] Most attacks involve sharks biting the legs or feet of waders, apparently mistaking them for their natural prey, and do not result in serious injury.[3] In the Marshall Islands, native islanders avoid blacktip reef shark attacks by swimming rather than wading through shallow water, as a way of discouraging these sharks is to submerge one's body. The blacktip reef shark has also been known to become aggressive in the presence of bait, and may pose a threat while attempting to steal the catches ofspear fishers.[3]
The blacktip reef shark is a normal catch of coastalfisheries, such as those operating offThailand andIndia, but is not targeted or considered commercially important.[9] The meat (sold fresh, frozen, dried and salted, or smoked for human consumption),liver oil, andfins are used.[6] TheInternational Union for Conservation of Nature has assessed the blacktip reef shark as Vulnerable and declining. Though it remains widespread and common overall, substantial local declines due tooverfishing have now been documented in many areas. This species has a low reproductive rate, limiting its capacity for recovering from depletion.[9][16] Blacktip reef sharks are popular subjects ofpublic aquarium exhibits, because of their stereotypically "shark-like" appearance, ability to breed in captivity and modest size, and are also attractions forecotourism divers.[10][14]
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