Latin name

Alopias superciliosus

Other name

Alopias superciliosus

Identification

A characteristic feature of the Bigeye Thresher is the long upper caudal fin blade, which can be as long as the body. They have a wider blade than other threshers. This species has a stocky, torpedo-shaped body and a narrow head with a conical, convex 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. The furrows at the corners of the mouth are absent. The mouth has 19-24 upper and 20-24 lower rows of teeth. The teeth are fairly large, pointed, without serrations. The diameter of the eye is up to 10 cm in adult sharks. The eyes are oval and vertically elongated. The orbits extend over the dorsal surface of the head, allowing the shark to look upwards. The skin is covered with interspersed large and small placoid scales. The small scales taper towards the tip.

Bigeye threshers have aerobic red muscle responsible for heat production, distributed along the flanks and located just under the skin rather than deep within the body. The sides lack a system of counter-current blood vessels (rete mirabile) to reduce metabolic energy loss, so these sharks are unable to maintain an elevated body temperature. They do, however, have an ocular rete mirabile that protects the eyes and brain from temperature fluctuations. During their daily vertical migrations, water temperature fluctuations can reach 15-16°C.

Features of fish fins

The bigeye thresher's long, broad pectoral fins taper to rounded tips with a slightly concave caudal margin. The first dorsal fin is displaced posteriorly compared to other threshers and is closer to the base of the pelvic fins. The pelvic fins are about the same size as the first dorsal fin and 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 colouration of these sharks is an intense purple or brownish purple with a metallic tinge. After death the colour fades quickly to a dull grey. The belly is creamy white. The white colouration does not extend to the base of the pectoral and ventral fins - this distinguishes the Pelagic Thresher from the similar Thresher shark, which has a patch at the base of the pectoral fins.

Distribution

Bigeye threshers are found in the tropical waters of the Indo-Pacific and Atlantic Oceans. In the western Atlantic, they are found from New York to Florida, the Bahamas, Cuba, Venezuela and southern Brazil. In the eastern Atlantic, they are found off the coasts of Portugal, Madeira, Senegal, Guinea, Sierra Leone, Angola and the Mediterranean. In the western Indian Ocean, these sharks are found off the coast of South Africa, Madagascar and the Arabian Sea. In the Pacific Ocean, they inhabit the coastal waters of southern Japan, Taiwan, New Caledonia, northwestern Australia, New Zealand, east of Hawaii and southern California. They are also found in the Gulf of California and off the Galapagos Islands.

Habitat

This species is found both on the continental shelf and offshore. They sometimes come close to the coast. Although they prefer temperatures between 16°C and 25°C, they are found at depths of up to 723 m where water temperatures do not exceed 5°C.

Size

The bigeye thresher averages 3.3-4 m in length and 160 kg in weight. The maximum recorded length and mass (4.9 m and 364 kg) were recorded for an individual caught near Tutukaka, New Zealand, in February 1981.

Behavior

The size and position of the eyes in this species of shark are adapted to search for the silhouette of prey in low light conditions. They are one of a small group of sharks that make diurnal vertical migrations. They spend the day at depths of 300-500m below the thermocline, where temperatures range from 6°C to 12°C, and at night they ascend to depths of 100m or less. These migrations are due to the fact that sharks hunt at night and hide at depth from predators during the day. During the day, sharks swim at a measured pace, while at night they make rapid ascents and dives.

Food and feeding habits

They are active predators, using their tail to stun their prey. Bigeye threshers have larger teeth than other members of the genus. They prey on small gregarious fish such as mackerel and herring, demersal fish such as hake, pelagic fish such as lancetfish and small billfish, squid Lycoteuthidae and Ommastrephidae and possibly crabs. They circle the school and seal it off with tail strikes. Because of this hunting tactic, they are sometimes caught with their tails on longline hooks or entangled in nets. The shape of their eye sockets gives Bigeye threshers binocular vision in an upward direction, allowing them to see their target better. In the Mediterranean they follow schools of frigate mackerel (Auxis rochei), probably following the largest aggregation of prey.

Reproduction

Reproduction in this species is not seasonal. They reproduce by oviparity. There are 2, very rarely 3 or 4 newborns in a litter, 1.35-1.4 m long. The exact gestation period is unknown. Fertilisation and embryonic development occur intrauterine. The embryo is initially nourished by the egg yolk. After the yolk sac is emptied, it begins to eat egg capsules produced by the mother (intrauterine oophagy). Cannibalism, which is characteristic of common sand sharks, is not observed in pelagic fox sharks. Externally, juveniles resemble adult sharks, but their head and eyes are proportionally larger. The inner walls of the oviduct are covered with a thin layer of epithelium due to damage from the sharp placoid scales of the embryo. This feature is not observed in other members of the fox shark genus.

Males mature at a length of 2.7-2.9 metres, corresponding to an age of 9-10 years, and females mature at a length of 3.3-3.6 metres, corresponding to an age of 12-14 years. The maximum recorded lifespan for males and females is 19 and 20 years respectively. Females are thought to produce only 20 offspring during their lifetime.

Fishing

Bigeye threshers are of interest to sport fishermen in the USA, South Africa and New Zealand. They are fished commercially in the USA, Japan, Spain, Brazil, Uruguay and Mexico, accounting for up to 10% of the total pelagic shark catch. Off the coast of Cuba, where they are caught by night baiting with chemical lights, sharks account for up to 20% of the longline catch. They are also important to the commercial fishery in Taiwan, with an annual catch of 220 tonnes.

Relationship with a person

Despite its large size, the species is considered safe for humans. Divers rarely encounter Bigeye threshers. There are no recorded attacks on humans by these sharks.

The meat of this fish is marketed in fresh, smoked and salt-dried forms, although it is not highly prized due to its soft, mushy texture. The skin is used for leather, the liver fat for vitamins and the fins for soup.