Facts & Profile

Blacktip shark Carcharhinus limbatus

The blacktip shark (Carcharhinus limbatus) is a species of requiem shark, and part of the family Carcharhinidae. It is common to coastal tropical and subtropical waters around the world, including brackish habitats. Genetic analyses have revealed substantial variation within this species, with populations from the western Atlantic Ocean isolated and distinct from those in the rest of its range. The blacktip shark has a stout, fusiform body with a pointed snout, long gill slits, and no ridge between the dorsal fins. Most individuals have black tips or edges on the pectoral, dorsal, pelvic, and caudal fins. It usually attains a length of 1.5 m (4.9 ft).

Swift, energetic piscivores, blacktip sharks are known to make spinning leaps out of the water while attacking schools of small fish. Their demeanor has been described as "timid" compared to other large requiem sharks. Both juveniles and adults form groups of varying size. Like other members of its family, the blacktip shark is viviparous; females bear one to 10 pups every other year. Young blacktip sharks spend the first months of their lives in shallow nurseries, and grown females return to the nurseries where they were born to give birth themselves. In the absence of males, females are also capable of asexual reproduction.

Normally wary of humans, blacktip sharks can become aggressive in the presence of food and have been responsible for a number of attacks on people. This species is of importance to both commercial and recreational fisheries across many parts of its range, with its meat, skin, fins, and liver oil used. It has been assessed as Vulnerable by the IUCN, on the basis of its low reproductive rate and high value to fishers.

Description & appearance

The blacktip shark has a robust, streamlined body with a long, pointed snout and relatively small eyes. The five pairs of gill slits are longer than those of similar requiem shark species. The jaws contain 15 tooth rows on either side, with two symphysial teeth (at the jaw midline) in the upper jaw and one symphysial tooth in the lower jaw. The teeth are broad-based with a high, narrow cusp and serrated edges. The first dorsal fin is tall and falcate (sickle-shaped) with a short free rear tip; no ridge runs between the first and second dorsal fins. The large pectoral fins are falcate and pointed.

The coloration is gray to brown above and white below, with a conspicuous white stripe running along the sides. The pectoral fins, second dorsal fin, and the lower lobe of the caudal fin usually have black tips. The pelvic fins and rarely the anal fin may also be black-tipped. The first dorsal fin and the upper lobe of the caudal fin typically have black edges.Some larger individuals have unmarked or nearly unmarked fins. Blacktip sharks can temporarily lose almost all their colors during blooms, or "whitings", of coccolithophores.This species attains a maximum known length of 2.8 m (9.2 ft), though 1.5 m (4.9 ft) is more typical, and a maximum known weight of 123 kg (271 lb).

Distribution & habitat

The blacktip shark has a worldwide distribution in tropical and subtropical waters. In the Atlantic, it is found from Massachusetts to Brazil, including the Gulf of Mexico and the Caribbean Sea, and from the Mediterranean Sea, Madeira, and the Canary Islands to the Democratic Republic of the Congo. It occurs all around the periphery of the Indian Ocean, from South Africa and Madagascar to the Arabian Peninsula and the Indian subcontinent, to Southeast Asia. In the western Pacific, it is found from the Ryukyu Islands of Japan to northern Australia, including southern China, the Philippines and Indonesia. In the eastern Pacific, it occurs from Southern California to Peru. It has also been reported at a number of Pacific islands, including New Caledonia, Tahiti, the Marquesas, Hawaii, Revillagigedo, and the Galápagos.

Most blacktip sharks are found in water less than 30 m (98 ft) deep over continental and insular shelves, though they may dive to 64 m (210 ft). Favored habitats are muddy bays, island lagoons, and the drop-offs near coral reefs; they are also tolerant of low salinity and enter estuaries and mangrove swamps. Although an individual may be found some distance offshore, blacktip sharks do not inhabit oceanic waters. Seasonal migration has been documented for the population off the east coast of the United States, moving north to North Carolina in the summer and south to Florida in the winter.

Hunting & feeding

Fish make up some 90% of the blacktip shark's diet. A wide variety of fish have been recorded as prey for this species: sardines, menhaden, herring, anchovies, ladyfish, sea catfish, cornetfish, flatfish, threadfins, mullet, mackerel, jacks, groupers, snook, porgies, mojarras, emperors, grunts, butterfish, croakers, soles, tilapia, triggerfish, boxfish, and porcupinefish. They also feed on rays and skates, as well as smaller sharks such as smoothhounds and sharpnose sharks.

Rays, crustaceans, and cephalopods are occasionally taken. In the Gulf of Mexico, the most important prey of the blacktip shark is the Gulf menhaden (Brevoortia patronus), followed by the Atlantic croaker (Micropogonias undulatus). Off South Africa, jacks and herring are the most important prey. Hunting peaks at dawn and dusk. The excitability and sociability of blacktip sharks makes them prone to feeding frenzies when large quantities of food are suddenly available, such as when fishing vessels dump their refuse overboard.

Reproduction

As with other requiem sharks, the blacktip shark is viviparous. Females typically give birth to four to seven (range one to 10) pups every other year, making use of shallow coastal nurseries that offer plentiful food and fewer predators. Known nurseries include Pine Island Sound, Terra Ceia Bay, and Yankeetown along the Gulf Coast of Florida, Bulls Bay on the coast of South Carolina, and Pontal do Paraná on the coast of Brazil. Although adult blacktip sharks are highly mobile and disperse over long distances, they are philopatric and return to their original nursery areas to give birth. This results in a series of genetically distinct breeding stocks that overlap in geographic range.

Mating occurs from spring to early summer, and the young are born around the same time the following year after a gestation period of 10–12 months. Females have one functional ovary and two functional uteri; each uterus is separated into compartments with a single embryo inside each.The embryos are initially sustained by a yolk sac; in the 10th or 11th week of gestation, when the embryo measures 18–19 cm long (7.1–7.5 in), the supply of yolk is exhausted and the yolk sac develops into a placental connection that sustains the embryo until birth.The length at birth is 55–60 cm (22–24 in) off the eastern United States and 61–65 cm (24–26 in) off North Africa.The mortality rate in the first 15 months of life is 61–91%, with major threats being predation and starvation.The young remain in the nurseries until their first fall, when they migrate to their wintering grounds.

The growth rate of this species slows with age: 25–30 cm (9.8–11.8 in) in the first six months, then 20 cm (7.9 in) a year until the second year, then 10 cm (3.9 in) a year until maturation, then 5 cm (2.0 in) a year for adults. The size at maturity varies geographically: males and females mature at 1.4–1.5 m (4.6–4.9 ft) and 1.6 m (5.2 ft), respectively, in the northeastern Atlantic,1.3–1.4 m (4.3–4.6 ft) and 1.5–1.6 m (4.9–5.2 ft), respectively, in the Gulf of Mexico,1.5 and 1.6 m (4.9 and 5.2 ft) respectively off South Africa, and 1.7 and 1.8 m (5.6 and 5.9 ft), respectively, off North Africa.The age at maturation is 4–5 years for males and 7–8 years for females. The lifespan is at least 12 years.

In 2007, a 9-year-old female blacktip shark at the Virginia Aquarium and Marine Science Center was found to be pregnant with a single near-term female pup, despite having never mated with a male. Genetic analysis confirmed that her offspring was the product of automictic parthenogenesis, a form of asexual reproduction in which an ovum merges with a polar body to form a zygote without fertilization. Along with an earlier case of parthenogenesis in the bonnethead (Sphyrna tiburo), this event suggests that asexual reproduction may be more widespread in sharks than previously thought.