Alopias superciliosus

The bigeye thresher (Alopias superciliosus) is a species of thresher shark, family Alopiidae, found in temperate and tropical oceans worldwide. Like other thresher sharks, nearly half its total length consists of the elongated upper lobe of the tail fin. Its common name comes from its enormous eyes, which are placed in keyhole-shaped sockets that allow them to be rotated upward. This species can also be distinguished by a pair of deep grooves on the top of its head, from which its scientific name is derived. The large eyes of the bigeye thresher are adapted for hunting in low light conditions. It is one of the few sharks that conduct a diel vertical migration, staying in deep water during the day and moving into surface waters at night to feed. To protect its sensitive brain and eyes from the temperature changes accompanying these movements, the bigeye thresher has a vascular exchange system called the rete mirabile around those organs. This species feeds mainly on fish and squid, which are stunned via whip-like strikes of the long tail. Bigeye threshers are ovoviviparous, usually bearing litters of two pups. The embryos are oophagous and feed on ova produced by the mother while inside the uterus. This shark is caught by commercial fisheries across its range; the meat is not highly regarded but the skin, fins, and liver oil are valued. It has been assessed as Vulnerable by the International Union for Conservation of Nature (IUCN). 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. The bigeye thresher has a virtually circumtropical distribution. In the western Atlantic Ocean, 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). 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.