Blacktip Reef Shark

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).

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.

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 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. 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 from brackish estuaries and lakes in Madagascar, and freshwater environments in Malaysia, though it is not able to tolerate low salinity to the same degree as the bull shark (C. leucas). At Aldabra in the Indian Ocean, blacktip reef sharks congregate in the channels between reef flats during low tide and travel to the mangroves when the water rises. There is equivocal evidence that sharks from the northern and southern extremes of its distribution are migratory. Blacktip reef shark (Carcharhinus melanopterus) Moorea.jpg Blacktip reef sharks (Carcharhinus melanopterus) Moorea.jpg Blacktip reef shark (Carcharhinus melanopterus) Moorea 4.jpg ese aquarium, 2010]]

Along with the grey reef shark (C. amblyrhinchos) and the whitetip 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. 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.

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, 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. Feeding activity may be greater at night than during the day.

Like the other members of its family, the blacktip reef shark is viviparous, though the details of its life history vary across its range. Its reproductive cycle is annual off northern Australia, with mating taking place from January to February, 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. Males and females mature sexually at lengths of and respectively off northern Australia, and and , respectively, off Aldabra. Males mature at long off Palmyra Atoll.

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. 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. 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 of spear fishers. The blacktip reef shark is a normal catch of coastal fisheries, such as those operating off Thailand and India, but is not targeted or considered commercially important. The meat (sold fresh, frozen, dried and salted, or smoked for human consumption), liver oil, and fins are used. The International 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 to overfishing have now been documented in many areas. This species has a low reproductive rate, limiting its capacity for recovering from depletion. Blacktip reef sharks are popular subjects of public aquarium exhibits, because of their stereotypically "shark-like" appearance, ability to breed in captivity and modest size, and are also attractions for ecotourism divers.