Bigeye Thresher Adult
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 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.
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).
Bigeye Thresher Structure
The bigeye
thresher has larger teeth than other threshers and feeds on a wider variety of prey. Known food items include schooling forage fish such as mackerel and herring, benthic fishes such as hake and whiting, larger pelagic fishes such as lancetfish and small billfish, squid such as lycoteuthids and ommastrephids, and possibly crab megalopae. They likely use their long tails to stun their prey prior to capture, as they are often found hooked by their tails on longlines and 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.