A sixgill shark has six gill slits.
The name refers to six pairs of gill openings on each side of the head, an unusual feature among living sharks. Most modern sharks have five gill slits, so the extra opening is an immediately useful clue when identifying members of the sixgill group, especially the bluntnose, bigeyed and Atlantic sixgill sharks.
Sixgill sharks belong to the family Hexanchidae and are often associated with deep water. The bluntnose sixgill shark, Hexanchus griseus, is the largest living species in the group and can reach roughly 5.5 m. It has a single dorsal fin positioned far back near the tail, another feature that distinguishes it from many familiar coastal sharks.
The term “sixgill shark” can refer broadly to sharks in the genus Hexanchus, rather than one single species. Taxonomy has also changed: genetic work helped restore the Atlantic sixgill shark, Hexanchus vitulus, as a species after it had previously been grouped with the bigeyed sixgill shark. Regardless of those species distinctions, six gill slits remain the defining anatomical trait.