Spinner shark

Appearance

The average spinner shark is 2 m long and weighs 56 kg; this species attains a maximum known length and weight of 3 m and 90 kg.

Indo-Pacific sharks are generally larger than those from the northwest Atlantic. This species has a slim, streamlined body with a distinctive, long, pointed snout. The eyes are small and circular. Prominent, forward-pointing furrows occur at the corners of the mouth.

The tooth rows number 15–18 in each half of the upper jaw and 14–17 in each half of the lower jaw, with two and one tiny symphysial teeth, respectively. The teeth have long, narrow central cusps and are finely serrated in the upper jaw and smooth in the lower jaw. The five pairs of gill slits are long.

The first dorsal fin is relatively small and usually originates behind the free rear tip of the pectoral fins. No ridge exists between the first and second dorsal fins. The pectoral fins are moderately short, narrow, and falcate. The body is densely covered with diamond-shaped dermal denticles with seven shallow horizontal ridges.

The coloration is gray above, sometimes with a bronze sheen, and white below, with a faint white band on the sides. Young individuals have unmarked fins; the tips of the second dorsal fin, pectoral fins, anal fin, and lower caudal fin lobe are black in larger individuals. The spinner shark differs from the blacktip shark in that its first dorsal fin is slightly more triangular in shape and is placed further back on the body. Adults can also be distinguished by the black tip on the anal fin.

Distribution

Some uncertainty exists in the distribution data for the spinner shark due to confusion with the blacktip shark. In the Western Atlantic Ocean, it occurs from North Carolina to the northern Gulf of Mexico, including the Bahamas and Cuba, and from southern Brazil to Argentina. In the Eastern Atlantic, it occurs from off North Africa to Namibia. In the Indian Ocean, it is found from South Africa and Madagascar, to the Red Sea and the Gulf of Aden, to India and nearby islands, to Java and Sumatra.

In the Pacific Ocean, it occurs off Japan, Vietnam, Australia, and possibly the Philippines. Parasitological evidence suggests that Indian Ocean spinner sharks have passed through the Suez Canal into the Mediterranean Sea, becoming Lessepsian migrants.

The spinner shark has been reported from the ocean surface to a depth of 100 m, though it prefers water less than 30 m deep, and occupies all levels of the water column. This species may be found from coastal waters to well offshore, over continental and insular shelves. Juveniles have been known to enter bays, but avoid brackish conditions. The northwest Atlantic subpopulation is known to be migratory; in spring and summer, they are found in warm inshore waters, and in winter, they move south into deeper water.

Behavior

The spinner shark is a fast, active swimmer that sometimes forms large schools, segregated by age and sex. Young individuals prefer cooler water temperatures than adults. Off South Africa, females are found close to shore year-round, while males only appear during the summer.

Smaller spinner sharks may be preyed upon by larger sharks. Known parasites of the spinner shark include the copepods ''Kroyeria deetsi'', ''Nemesis pilosus'', and ''N. atlantica'', which infest the shark's gills, ''Alebion carchariae'', which infests the skin, ''Nesippus orientalis'', which infests the mouth and gill arches, and ''Perissopus dentatus'', which infests the nares and the rear margins of the fins.

Reproduction

Like other requiem sharks, the spinner shark is viviparous. Adult females have a single functional ovary and two functional uteri; each uterus is divided into compartments, one for each embryo. The embryos are initially sustained by a yolk sac. When the embryo grows to around 19 cm long, the supply of yolk has been exhausted and the empty yolk sac develops into a placental connection through which the mother provides nutrients for the remainder of gestation. This species has the smallest ova relative to the fully developed embryo of any viviparous shark known. Females give birth to three to 20 pups every other year, after a gestation period of 11–15 months.

Mating occurs from early spring to summer, and parturition in August off North Africa, from April to May off South Africa, and from March to April in the northwestern Atlantic. Young are birthed in coastal nursery areas such as bays, beaches, and high-salinity estuaries in water deeper than 5 m.

The length at birth is 66–77 cm in the northwestern Atlantic, 61–69 cm off Tunisia, and 60 cm off South Africa. Spinner sharks are relatively fast-growing sharks: 30 cm per year for newborns, 25 cm per year for one-year-olds, 10 cm per year for adolescents, and 5 cm per year for adults. In the northwestern Atlantic, males mature at 1.3 m long and females at 1.5–1.6 m long, corresponding to ages of 4–5 years and 7–8 years, respectively. Off South Africa, males mature at 1.8 m and females at 2.1 m . Spinner sharks generally do not reproduce until they are 12–14 years old. The maximum lifespan has been estimated at 15–20 years or more.

Food

Spinner sharks feed primarily on small bony fish, including tenpounders, sardines, herring, anchovies, sea catfish, lizardfish, mullets, bluefish, tunas, bonito, croakers, jacks, mojarras, and tongue-soles.

They have also been known to eat stingrays, cuttlefish, squid, and octopus. Groups of spinner sharks are often found pursuing schools of prey at high speed. Individual prey are seized and swallowed whole, as this shark lacks cutting dentition.

This species employs an unusual tactic when feeding on schools of small fish; the shark charges vertically through the school, spinning on its axis with its mouth open and snapping all around it. The shark's momentum at the end of these spiraling runs often carries it into the air, giving it its common name. The blacktip shark also performs this behavior, though not as often.

Off Madagascar, spinner sharks follow migrating schools of mackerel, tunas, and jacks. Like blacktip sharks, they congregate around shrimp trawlers to feed on the discarded bycatch, and may be incited into feeding frenzies.