Blacktip reef shark

Appearance

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. Each nostril has a flap of skin in front that is expanded into a nipple-shaped lobe.

Not counting small symphysial teeth, the tooth rows number 11–13 on either side of the upper jaw and 10–12 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. The teeth of adult males are more abruptly curved than those of females.

The pectoral fins are large and narrowly falcate, 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 1.6 m long, though rarely individuals may reach 1.8 m or possibly 2.0 m. The maximum weight on record is 13.6 kg.

Distribution

The blacktip reef shark is found throughout nearshore waters of the tropical and subtropical Indo-Pacific. In the Indian Ocean, it occurs from South Africa to the Red Sea, including Madagascar, Mauritius, and the Seychelles, and from there eastward along the coast of the Indian Subcontinent to Southeast Asia, including Sri Lanka, the Andaman Islands, and the Maldives.

In the Pacific Ocean, it is found from southern China and the Philippines to Indonesia, northern Australia and New Caledonia, and also inhabits numerous oceanic islands, including the Marshall, Gilbert, Society, and Hawaiian Islands and Tuamotu. Contrary to what most sources state, there are no confirmed records of this species from Japanese waters, and purported Japanese specimens likely came from Taiwan. A Lessepsian migrant, this shark has colonized the eastern Mediterranean Sea by way of the Suez Canal.

Behavior

Along with the grey reef shark and the whitetip reef shark, 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.

Habitat

Although it has been reported from a depth of 75 m, 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.

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.

Food

As often the most abundant apex predator within its ecosystem, the blacktip reef shark plays a major role in structuring inshore ecological communities. Its diet is composed primarily of small teleost fishes, including mullet, groupers, grunters, jacks, mojarras, wrasses, surgeonfish, and smelt-whitings.

Groups of blacktip reef sharks in the Indian Ocean have been observed herding schools of mullet against the shore for easier feeding. Squid, octopus, cuttlefish, shrimp, and mantis shrimp are also taken, as well as carrion and smaller sharks and rays, though this is rare.

Off northern Australia, this species is known to consume sea snakes, including ''Acrochordus granulatus'', ''Hydrelaps darwiniensis'', ''Hydrophis'' spp. and ''Lapemis hardwickii''. Sharks off Palmyra Atoll have been documented preying on seabird chicks that have fallen out of their nests into the water. Miscellaneous items that have been found inside the stomachs of this species include algae, turtle grass, coral, hydrozoa, bryozoa, rats, and stones.

Evolution

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 , Madagascar by Fourmanoir , and the Red Sea by Gohar and Mazhar , 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.

When receptive to mating, a female blacktip reef shark swims slowly in a sinusoidal 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 and follows her with his snout oriented towards her vent. 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. 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 his claspers into her cloaca. Copulation lasts for several minutes, after which the sharks separate and resume their regular behavior. 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 female has a single functional ovary and two functional uteruses, divided into separate compartments for each embryo. Newly ovulated egg cases measure 3.9 cm by 2.6 cm; after hatching the embryos are sustained by a yolk sac during the first stage of development.

After two months, the embryo measures 4 cm long and has well-developed external gills. After four months, the yolk sac has begun to be converted into a placental 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 and has resorbed its external gills; the placenta is fully formed, though some yolk remains until seven months into gestation.

Parturition occurs from September to November, with females making use of shallow nursery areas interior of the reef. Newborn pups measure 40–50 cm long in the Indian Ocean and off northern Australia, while free-swimming pups as small as 33 cm long have been observed in the Pacific islands.

The litter size is 2–5, and is not correlated with female size. 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 or seaweed beds. Growth is initially rapid; one documented captive shark grew an average of 23 cm per year in its first two years of life. The growth rate slows to around 5 cm per year in juveniles and adults. Males and females mature sexually at lengths of 95 cm and 97 cm respectively off northern Australia, and 105 cm and 110 cm, respectively, off Aldabra. Males mature at 97 cm long off Palmyra Atoll.