Bigeye thresher

The **Bigeye thresher** (*Alopias superciliosus*) is a species of thresher shark that lives in temperate and tropical waters in most parts of the world. The bigeye thresher sharks eyes are well adapted to hunting prey in low light conditions. It also conducts diel vertical migrations between deeper and shallower water. Like most thresher sharks, almost half of their body length consists of the upper lobe on their tail fin.

## Description

The eyes of the bigeye thresher can measure up to 10 cm (3.9 in) across in adults. Each eye is taller than wide, with a bulbous upper portion. The orbits extend onto the dorsal surface of the head, allowing the eyes to orient upwards. There are also a pair of distinctive lateral grooves that extend from above the eyes to over the gill slits, giving it a "helmet"-like appearance. The snout is moderately long and bulbous, and there are no labial furrows at the corners of the mouth. The teeth are moderately large with a single, narrow cusp. There are 19–24 teeth in the upper jaw and 20–24 teeth in the lower jaw; their shapes are similar in both jaws. There are large and small dermal denticles, with the smaller ones more numerous and interspersed amongst the larger ones. The smaller denticles taper to a point.

Up to half the body length is taken up by the long upper lobe of the caudal fin, which is broader than in other threshers. The large pectoral fins have a curved anterior margin and broad tips. The first dorsal fin is placed further back than in the other thresher sharks, with the free rear tip located above or just before the pelvic fins. Its coloration is a deep, metallic violet to purplish brown above and creamy white below. This color rapidly fades to a dull gray after death. Most bigeye threshers are 3.3–4.0 m (10.8–13.1 ft) long and weigh 160 kg (350 lb). The largest known bigeye thresher measured 4.9 m (16 ft) long and weighed 364 kg (802 lb), and was caught near Tutukaka, New Zealand, in February 1981.

## Phylogeny and Taxonomy

British biologist Richard Thomas Lowe was the first to scientifically describe the bigeye thresher, in papers published in *Proceedings of the Royal Society of London* (1840) and *Transactions of the Zoological Society of London* (1849, sometimes given as 1839). He based his description on a specimen caught off Madeira in the eastern Atlantic Ocean. However, Lowe's description was subsequently overlooked by researchers and this species was known by different names until the 1940s, when new specimens from Cuba and Florida prompted its original scientific name to be resurrected. The specific epithet *superciliosus* is from the Latin *super* meaning "above", and *ciliosus* meaning "eyebrow", referring to the distinct lateral grooves above the eyes.

An allozyme analysis conducted by Blaise Eitner in 1995 showed that the closest relative of the bigeye thresher is the pelagic thresher (*A. pelagicus*), with which it forms a clade. Fossil remains of the bigeye thresher dating to the Middle Miocene (16.0–11.6 Ma) have been found in the Hokuriku region of Japan.

## Biology

### Diet

The bigeye thresher's teeth are larger than that of most thresher sharks, allowing them to hunt a wide variety of prey. These items include schooling fishsuch as mackerel and herring, benthicfishes such as hake and whiting, larger pelagic fishes such as lancetfish and small billfish, as well as other animals such as squid such as lycoteuthids and ommastrephids and possibly crab megalopae. The bigeye thresher hunts by stunning prey with its tail, and many have been found hooked to longlines by their tails with the bait fish in their stomachs. The shape of their eye sockets give them binocular vision in an upward direction to better aim their strikes. In the Mediterranean, they are strongly associated with schools of frigate mackerel (*Auxis rochei*), suggesting that these sharks follow concentrations of their prey from place to place. It is said that the bigeye thresher uses its long tail to smack down birds. The bigeye thresher hunts by stunning prey with its tail, and many have been found hooked to longlines by their tails.

## Distribution and habitat

The bigeye thresher has a very wide distribution, appearing in oceans across the world. It typically lives in temperate or tropical waters, however, some have been tracked to a depth of over 700 meters, at which point the water temperature would be around 5 degrees Celcius. It has been reported from New York to Florida, the Bahamas, Cuba, Venezuela, Tobago and southern Brazil. In the eastern Atlantic, it is known from Portugal, Madeira, Senegal, Guinea to Sierra Leone, Angola, and the Mediterranean Sea. In the western Indian Ocean, it occurs off South Africa, Madagascar, and the Arabian Sea. In the Pacific Ocean, it is known from southern Japan, Taiwan, New Caledonia, northwestern Australia, and New Zealand, and eastward to Hawaii, southern California, the Gulf of California, and west of the Galapagos Islands. Analysis of mitochondrial DNA has shown that Atlantic and Indo-Pacific populations are somewhat genetically divergent from each other.

Bigeye threshers are usually found over the continental shelf and in the open sea, though they are occasionally encountered in shallow coastal waters. They occur in surface temperatures of 16–25 °C (61–77 °F), but have been tracked as far down as 723 m (2,372 ft), where the temperature is only 5 °C (41 °F). Little is known of their geographical movements; one individual has been documented moving from New York to the Gulf of Mexico, a straight-line distance of 2,767 km (1,719 mi).

## Interactions with Humans

The bigeye thresher shark is rarely encountered by divers underwater and poses no danger to humans. This species is or was taken by longline fisheries operated by many countries, including the United States, Japan, Spain, Brazil, Uruguay, and Mexico, and constitutes about 10% of the pelagic shark catch. The bigeye thresher comprises 20% of the longline catch off Cuba, where it is attracted at night using cyalume sticks (chemical lights). It is also significant to Taiwanese fisheries, which land about 220 metric tons annually. The meat is marketed fresh, smoked, or dried and salted, though it is not highly regarded due to its mushy texture. The skin is used to make leather products, the liver oil for vitamins, and the fins for shark fin soup.

In the waters of the United States, this species is considered a nuisance bycatch of longlines, gillnets, and trawls. It is also occasionally caught in shark nets around beaches in South Africa. Along with the other thresher species, the bigeye thresher is listed as a game fish by the International Game Fish Association (IGFA), and is pursued by recreational anglers off the United States, South Africa, and New Zealand. The bigeye thresher is highly susceptible to over-exploitation due to its low lifetime fecundity. All three thesher shark species were assessed as Vulnerable by the International Union for Conservation of Nature (IUCN) in 2007.