The blacktip reef shark (Carcharhinus melanopterus) is a species of requiem shark, in the family Carcharhinidae, which can be easily identified by the prominent black tips on its fins (especially on the first dorsal fin and its caudal fin). Among the most abundant sharks inhabiting the tropical coral reefs of the Indian and Pacific 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 enter brackish and freshwater environments. It typically attains a length of . 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 active predator of small bony fish, cephalopods, and crustaceans, and has also been known to feed on sea snakes and seabirds. 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 is viviparous, with females giving birth to two to five young on a biennial, annual, or possibly biannual cycle. Reports of the gestation 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 by high 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, and liver oil, but is not considered to be a commercially significant species. The International Union for Conservation of Nature has assessed the blacktip reef shark as Vulnerable. 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.
Taxonomy
French naturalists Jean René Constant Quoy and Joseph Paul Gaimard originally described the blacktip reef shark during the 1817–1820 exploratory voyage of the corvette Uranie. In 1824, their account was published as part of Voyage autour du monde...sur les corvettes de S.M. l'Uranie et la Physicienne, Louis de Freycinet's 13-volume report on the voyage. The type specimen was a -long juvenile male caught off the island of Waigeo, west of New Guinea.
Subsequent authors moved the blacktip reef shark to the genus Carcharhinus; in 1965 the International Commission on Zoological Nomenclature designated it as the type species for the genus. In some earlier literature, the scientific name of this shark was mistakenly given as C. spallanzani, now recognized as a synonym of the spottail shark (C. sorrah).
Phylogeny
Like most other members of its genus, the phylogenetic position of the blacktip reef shark remains indeterminate. Based on morphology, Jack Garrick proposed in 1982 that the closest relative of the blacktip reef shark was the nervous shark (C. cautus). 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 1998 allozyme analysis by Gavin Naylor again yielded ambiguous results, finding that the blacktip reef shark forms a polytomy (irresolvable group) with 10 other Carcharhinus species.
Distribution and habitat
The blacktip reef shark is found throughout nearshore waters of the tropical and subtropical Indo-Pacific. however a number of sightings and captures of this species have been reported from the inshore waters of Ishigaki Island of Okinawa Prefecture in southern Japan. A Lessepsian migrant, this shark has colonized the eastern Mediterranean Sea by way of the Suez Canal.
Although it has been reported from a depth of , The teeth of adult males are more abruptly curved than those of females.
The pectoral fins are large and narrowly falcate (sickle-shaped), tapering to points. The sizable first dorsal 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 the anal 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 long, though rarely individuals may reach or possibly . 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. Nybelinia queenslandensis, Otobothrium alexanderi, and Platybothrium jondoeorum, a myxosporidian in the genus Unicapsula, and the monogenean Dermophthirius melanopteri. One of the few documented examples of infectious disease in a shark was a fatal case of hemorrhagic septicemia in a blacktip reef shark, caused by the bacterium Aeromonas salmonicida subsp. salmonicida.
Feeding
thumb|upright=1.2|left|alt=Many black-tipped dorsal fins visible above churning water, and a small fish mid-jump at the upper center|The primary food of the blacktip reef shark is small fish, such as mullet.
As often the most abundant apex predator within its ecosystem, the blacktip reef shark plays a major role in structuring inshore ecological communities. Squid, octopus, cuttlefish, shrimp, crabs, and mantis shrimp are also taken, as well as carrion and smaller sharks and rays, though this is rare. Sharks off Palmyra Atoll have been documented preying on seabird chicks that have fallen out of their nests into the water. Electroreception is another means by which this shark can locate prey; its ampullae of Lorenzini have a sensitivity of approximately 4 nV/cm and an effective range of . 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 a feeding frenzy. as well as off Moorea in French Polynesia, where mating occurs from November to March. 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. 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. If accurate, the shorter reproductive cycles of these subpopulations may be a consequence of warmer water. Off Moorea, 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. The litter size is 2–5 (typically 4), and is not correlated with female size. Growth is initially rapid; one documented captive shark grew an average of per year in its first two years of life. The growth rate slows to around per year in juveniles and adults. 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.