Bluntnose Sixgill Shark
HEXANCHUS GRISEUS (BONNATERRE 1788)
FAMILY HEXANCHIDAE (COW SHARKS)
Identification: A huge shark with a broad head and blunt snout with a body that is soft and very stout, a broad mouth, small eyes that appear emerald green in life. This shark is characterized by 6 wide gill slits, a single dorsal fin that is situated posteriorly on the body, above the anal fin. The anal fin is smaller than the dorsal fin and the caudal fin is long and unforked, with a distinct sub-terminal notch. The distinctive lower teeth are very wide, comb-like, multi-cusped, and arranged in six rows on each side of the jaw. This is a uniformly dark shark, sometimes with a slightly lighter underside, particularly in juveniles.
Size: The maximum total length reported for males is approximately 430 cm, and for females is 482 cm. A large female sighted from a submersible was estimated to be about 550 cm. Anecdotal reports suggest this species can obtain much larger sizes.
Distribution: This species has a circumglobal distribution, inhabiting continental and insular shelves and slopes, as well as seamounts. Juveniles are known to inhabit inshore bays, notably Monterrey Bay and the Puget Sound/Georgia Basin system.
Habitat: This deepwater species is found from surface waters to a maximum depth of 2500 m, although most of their time is spent between 100 and 1000 m. This species is normally found associated with continental and insular slopes, and seamounts. Juveniles are commonly encountered inshore in shallow, cold water of west coast bays, including San Francisco Bay, with a deep connection to offshore waters. Recent tagging data collected by scientists from the Virginia Institue of Marine Science Shark Ecology Program indicate that sixgill sharks undergo daily vertical migrations, diving deeper during the day and moving into shallower water at night, presumably to feed. Their tagging data also suggests that certain individuals are resident to small areas of insular slopes off Hawaii.
Biology: An aplacental viviparous species, the sixgill shark is one of the more fecund shark species, with reported litter sizes ranging from 47 to 108 pups. The gestation period of this species is unknown, but may be up to 24 months. Very little is known about the behavior of this species, although they have been observed to react negatively to high light levels. Bluntnose sixgill sharks feed on a variety of prey items, including carrion, but the diet primarily consists of other chondrichthyans, teleosts, and secondarily on invertebrates, including cephalopods. The size and type of prey items increases ontogenetically with growth. Large sixgills (> 3 m total length) are known to consume billfishes and small cetaceans such dolphins. They are a major predator of spiny dogfish (Squalus acanthias) in the Puget Sound/Georgia Basin system.
General interest: Only four other species of sharks in the world have six gills: the bigeye sixgill shark (Hexanchus nakamurai), the sixgill sawshark (Pliotrema warreni), and two species of frill sharks (Chlamydoselachus spp.). Of these, only the bigeye sixgill may be confused with the bluntnose sixgill shark. However, H. nakamurai is easily distinguished from H. griseus by the longer snout, larger eyes, slimmer body, distinct lower caudal lobe (i.e. caudal is forked) and more anteriorly placed dorsal fin.
Sixgill sharks are commonly encountered by submarines and are therefore a “regular” star in many documentaries which showcase submarine photographs and video footage. Additionally, juvenile sixgills sharks are abundant in the shallow waters of several west coast bays and recreational SCUBA divers often encounter them in these areas. In fact, many dive shops offer specialty dives with sixgill sharks in cities near the Puget Sound/Georgia Basin system.
By Chip Cotton
Virginia Institute of Marine Science
College of William and Mary
Gloucester Point, VA 23062