Latin name

Alopias pelagicus

Other name

Alopias pelagicus

Identification

The Pelagic Thrasher is characterized by a long upper caudal fin blade that can equal the length of the body. They have a stocky, torpedo-shaped body and a short, narrow head with a conical, pointed snout. There are 5 pairs of short gill slits. The last 2 slits are above the long and narrow pectoral fins. The mouth is small and arched. There are no furrows at the corners of the mouth. The mouth has 21-22 rows of teeth on each side of the symphysis on the upper and lower jaws. The teeth are small with serrated edges. The eyes are very large in young sharks, but decrease proportionally with age. The third eyelid is absent. The skin of fox sharks is covered with small, soft, placoid scales, the surface of which is covered with 3 ridges. 

Features of fish fins

Dorsal spines (total): 0; Anal spines: 0. 

The pectoral fins are long, broad and straight, tapering to rounded tips. The first dorsal fin is small and located between the bases of the pectoral and pelvic fins. The pelvic fins are about the same size as the first dorsal fin. Males have thin, long pterygopodia. The second dorsal and anal fins are tiny. There is a crescent-shaped dorsal and ventral notch in front of the caudal fin. There is a small ventral notch near the edge of the upper lobe. The lower lobe is short but well developed.

Fish colouring

The coloration is an intense dark blue. The belly is white. The white coloration does not extend to the base of the pectoral and pelvic fins - this distinguishes these sharks from the similar Thresher shark, which has a spot at the base of the pectoral fins.

Distribution

Pelagic threshers are often confused with threshers, so their range may be wider than currently known. They are widespread in the Indo-Pacific, with reports of their presence off South Africa, the Red Sea, the Arabian Sea (off Somalia, Pakistan, and between Oman and India), China, southeastern Japan, northeastern Australia, New Caledonia, Tahiti, Hawaii, the Gulf of California, and the Galapagos Islands.

Habitat

Tropical marine pelagic-oceanic oceanodromous fishes. Their depth range is from 0 to 300 meters. They inhabit the open sea, but sometimes come close to the shore in areas with narrow continental shelves. They can be seen on underwater cliffs covered with coral reefs or near the tops of seamounts in the Red Sea, Sea of Cortez, off Indonesia and Micronesia. They also enter large lagoons in the Tuamotu Islands.

Size

The pelagic thresher is the smallest member of the family, reaching an average length of 3 m and weight of 88.4 kg. The maximum recorded length for males is 3.5 m and for females 3.8 m. 

The growth rate of pelagic fox sharks slows with age. Up to one year of age, young sharks grow 9 cm per year, in the second and third year of life this figure is 8 cm, 5-6 year old sharks grow 6 cm per year, 7-10 year old sharks grow 4 cm per year, annual growth at the age of 10-12 years is 3 cm, and sharks older than 13 years do not grow more than 2 cm per year. Males mature at 2.7-2.8 m in length, corresponding to 7-8 years of age, and females mature at 2.8-2.9 m in length, corresponding to 8-9 years of age. The maximum recorded life span for females and males is 16 and 14 years, respectively. 

Behavior

These sharks are excellent swimmers. It is not uncommon for them to jump completely out of the water and make up to 5 turns on their axis. They have a band of aerobic red muscle on the sides of their body just under the skin that can contract powerfully for long periods of time. However, unlike threshers, they do not have the muscle structure to store metabolic body heat energy (rete mirabile). They do have an ocular rete mirabile that protects the eyes and brain from temperature changes. Juvenile foxes can be preyed upon by large sharks and killer whales. 

Food and feeding habits

They feed mainly on small schooling fish. Analysis of the stomach contents of captured sharks has shown that they are barracudas, lightfishes and snake mackerels, which live in the mesopelagic zone. There is some competition between pelagic threshers and other large pelagic fish such as marlins, jacks and tunas, which also hunt near the surface. Before attacking, they circle the school and seal it with tail strikes. This hunting tactic sometimes results in their tail getting caught on a hook or tangled in a net.

Reproduction

Reproduction in pelagic fox sharks is not seasonal. They reproduce by oviparity. A litter contains 2, rarely one, newborn up to 1.6 m long. The size of the pups can be up to 43% of the length of the mother. The large size reduces the risk of the pups becoming prey for predators. The exact gestation period is unknown, probably less than a year. Fertilization and embryonic development occur in the uterus. The embryo is nourished by the yolk sac until it reaches 12 centimeters in length. After the yolk sac is emptied, the embryo begins to feed on egg capsules produced by the mother (intrauterine oophagy). Each capsule is 5.5 cm long and 1.2 cm wide and contains 20-30 eggs. The teeth of the embryos are peg-shaped and adapted to bite through the capsule, whereas in developed embryos they are covered with soft tissue and the capsules are swallowed whole. The teeth become functional soon after birth. Cannibalism, common in sandbar sharks, is not observed in pelagic fox sharks.

Fishing

Pelagic threshers are of interest to sport fishermen. This species is fished commercially. They are caught with longlines and less commonly with gillnets. They are also caught as bycatch in tuna fisheries. The meat is highly prized. The skin is dressed and the liver fat is used for vitamin production.

Relationship with a person

Despite its large size, the species is considered safe for humans. Pelagic fox sharks have small teeth and are also shy and will immediately swim away if humans appear.