Bigeyed sixgill shark

A Bigeyed sixgill shark (Hexanchus nakamurai)

The **bigeyed sixgill shark** (*Hexanchus nakamurai*) is a member of the family Hexanchidae, often referred to as **cow sharks**. It is the smallest living member of the genus *Hexanchus* and one of only three living members of this genus alongside the much larger bluntnose sixgill shark and its much closer relative, the Atlantic sixgill shark.

## Description

This shark's dorsal surface has a brownish-gray color, and is sharply separated from the light coloring of its ventral surface. The eyes are a fluorescent green while the shark is alive. The body of this shark is small, slim, and fusiform in shape. As the name suggests, this shark has six gill slits, considered an unusual trait among most shark species. The head is narrow and somewhat flattened, and the mouth contains 5 rows of large, comb-shaped teeth. This shark's single dorsal fin is pushed back towards the caudal fin, and is behind the pelvic fins. The upper caudal fin is much longer than the lower, with a deep notch near the tip. All fins have thin white margins on the edge. In juveniles, the upper caudal fin has a black tip.

Length at birth for this species is 34–45 cm (13-17 inches); adults average 1.2 m (4 feet) for both genders, with a maximum of 1.8 m (6 feet). Mature adults weigh around 20 kg (40 lb.).

## Habitat and Distribution

Bigeyed sixgills are probably found in both Indian and Pacific seas, as recordings in the Atlantic are now determined to be the separate species *Hexanchus vitulus,* the smaller Atlantic sixgill shark, though this is yet unknown. So far, they have been found to reside in the Mediterranean Sea, Indian Ocean, and the western Pacific Ocean. These sharks live near the sea floor between 90 and 600 m deep, but may be vertical migrators that move closer to the surface at night to take advantage of prey. They make their home on continental shelves, insular shelves, and upper slopes. These sharks do not inhabit abyssal environments, as their cousin, the bluntnose sixgill does.

## Biology

Little is known about the feeding habits of the bigeyed sixgill. Based on the few stomach examinations of these sharks, they are thought to feed mainly on small to moderately sized bony fish and crustaceans. Like all sharks of the order Hexanchiformes found so far, this species is ovoviviparous, meaning the eggs are retained inside the female's body until they hatch. The bigeyed sixgill, after an unknown gestation period, gives birth to litters of up to 13 pups. Because of the size of this shark, their main predators are probably larger sharks.

This shark's mating habits are unknown, as courtship has never been seen.

### Parasites

As with other sharks, bigeyed sixgill sharks harbour a number of parasite species. Monogeneans of the genus *Protocotyle* (family Hexabothriidae) have been found only on the gills of species of *Hexanchus*. The species *Protocotyle euzetmaillardi* was recently described from the gills of *Hexanchus nakamurai* off New Caledonia, in the South Pacific Ocean.

## Conservation

Like many deepwater animals, the bigeyed sixgill is listed as Data Deficient by the IUCN Red List. Based on what little is known of the species, these sharks are not suffering major declines to their population, but pressure is growing from deepwater fisheries. According to the International Shark Attack File, the bigeyed sixgill is harmless, as its natural environment is far from people, and no attacks on humans by the species have ever been reported.