Bluntnose Sixgill Shark

Most modern sharks have five pairs of gill slits, and the presence of six, or in a few species seven, is considered a primitive characteristic, meaning it more closely resembles the condition in the ancient sharks from which all modern sharks descended. The cow sharks, the family that includes the bluntnose sixgill, retain this and several other features, such as a single dorsal fin and a particular jaw and tooth structure, that place them among the most anatomically conservative of living sharks. The extra gill slit itself is not known to give any dramatic advantage in the modern ocean, but it has simply been retained in this lineage while most other shark groups reduced to five.

It is one of the largest predatory sharks, with typical adults ranging from around 3 to nearly 5 metres and the very largest confirmed individuals exceeding 5.5 metres in length and weighing on the order of 600 kilograms, placing it among the biggest sharks after species like the whale shark, basking shark, and great white. Its heavy, thick body adds to the impression of bulk, and because it usually lives in deep water and is rarely seen alive by people, the size of a large sixgill often comes as a surprise on the rare occasions when one is filmed by a submersible or encountered by deep divers.

They occur in deep water in temperate and tropical seas around the world, generally spending the day at depths of a few hundred to over a thousand metres along continental shelves and slopes. However, they undertake a daily vertical migration, moving up through the water column at night to feed, and in a small number of locations, such as parts of the Pacific Northwest of North America, this brings them into water shallow enough that scuba divers can occasionally encounter them after dark, which has made those sites well known among divers hoping to see a large, ancient, deep-sea shark up close.

It is currently assessed as Near Threatened, reflecting a species that is still widely distributed but under gradual pressure. It is caught deliberately in some longline and other fisheries for its meat, liver oil, and fins, taken by recreational anglers, and caught unintentionally as bycatch in fisheries targeting other deep-water species. Because the sixgill grows slowly, takes many years to reach breeding age, and has a long reproductive cycle, its populations have very little capacity to rebound, so even a modest, steady level of fishing mortality is enough to cause slow long-term declines, which is the concern underlying its listing.