Sixgill sharks are a group of sharks that have six gills as opposed to the typical five seen in sharks.
There are three species of sixgill sharks, which belong to the genus Hexanchus and are characterised by saw-like lower teeth.
It was originally thought that the genus comprised just two species: the bluntnose sixgill shark (Hexanchus griseus) and the bigeye sixgill (Hexanchus nakamurai).
They are distributed worldwide in tropical and temperate waters.
The bluntnose sixgill shark is one of three shark species with six gills. Its eyes are bright green and it feeds nocturnally, ambushing its prey. Because it lives in deep waters, not much is known about it
According to a study led by Toby Daly-Engel, a researcher at the Florida Institute of Technology, many verified records for these species are lacking, and misidentification is common.
A newly identified species of sixgill shark, the Atlantic sixgill shark (Hexanchus vitulus), settled the debate as to whether bigeye sixgill sharks from the Atlantic constitute a separate species in February 2018.
The three species can be described as follows:
Bluntnose sixgill shark (Hexanchus griseus): This large, deepwater shark has comb-like, yellow lower teeth, a long tail, and is the largest of the sixgill species, with an average length of 15 to 16 feet (4.6 to 4.9 metres).
It has a blunt, rounded snout, and a single dorsal fin. Its skin colour is dark, ranging from greyish brown to black on the back, and fading lighter towards the belly.
Its eyes are bright green and it feeds nocturnally, ambushing its prey. Because it lives in deep waters, not much is known about it.
Bigeye sixgill shark (Hexanchus nakamurai): A little-known, moderately large shark up to 5.9 feet (1.8 metres) long. It is slender and has a small dorsal fin far along its back, and large reflective green eyes. It is usually dark to light grey on top, fading to white underneath.
Atlantic sixgill shark (Hexanchus vitulus): Measures up to six feet (1.8 metres) long and lives at extreme ocean depths.
It is currently thought to be restricted to the Atlantic and looks very similar to the bigeye sixgill shark.