Bluntnose sixgill shark (Hexanchus griseus) COSEWIC assessment and status report: chapter 3

Species Information

Name and classification

The bluntnose sixgill shark (Hexanchus griseus) is one of four species belonging to the family Hexanchidae sometimes referred to as cow sharks. There are two other sixgill shark species in the genus Hexanchus; H. nakamurai and H. vitulus, neither of which is found in Canada’s Pacific waters. The bluntnose sixgill shark is also commonly referred to as sixgill shark, six-gill shark, mud shark, cow shark, shovelnosed shark, grey shark, and gray shark (Froese and Pauly 2005). Early historical documents in Canada refer often refer to this species as mud shark. In French they are called requin griset.

Morphological description

The bluntnose sixgill shark is an easily recognizable shark with several characteristics not often found in other shark species (Mecklenburg et al. 2002). The name, sixgill, refers to the presence of six gill slits whereas most other shark species have only five (Figure 1). A second obvious characteristic is the presence of only a single dorsal fin compared to two in all other shark species normally found on Canada’s Pacific coast. Their colour is a dark brown or grey to black dorsally with the colour becoming lighter towards their underside. Their head is broad and depressed with a blunt snout. Their eyes are conspicuously bright green and moderately large (Ebert 2003). The upper and lower teeth of the bluntnose sixgill shark are strikingly different, with the lower teeth being quite large, low and wide with several cusplets (8-12) and the upper teeth being smaller and singularly pointed (Mecklenburg et al. 2002). The single dorsal fin is located far back on the body and positioned above and in between the pelvic and anal fins on the ventral side. Like many benthic sharks, the caudal fin of the bluntnose sixgill shark has a small lower lobe.

Figure 1. Diagram of the bluntnose sixgill shark. Source: Compagno 1984.

Figure 1. Diagram of the bluntnose sixgill shark. Source: Compagno 1984. (See long description below)
Long description of Figure 1

Figure 1 is an illustration of the Bluntnose Sixgill Shark, showing its distinct presence of six gill slits, a single dorsal fin located far back on the body, and the colouring being darker dorsally becoming lighter towards its underside. The head is broad and depressed with a blunt snout. The image shows the caudal fin having a small lower lobe. A line representing the maximum recorded length of 350 centimetres (cm) (male) and 480 cm (female) is provided at the bottom of the illustration.

Genetic description

The population structure of bluntnose sixgill sharks has not been adequately studied anywhere in its global range. The small amount of research that does exist is primarily from the northeast Pacific. In Canada, a single tagging study (n=214) was undertaken along the west coast of Vancouver Island in 1994 but no information on population structure was ascertained from this small study (McFarlane, personal communication (pers. comm.) 2006). A combined tagging and genetic study has been ongoing in Puget Sound since 2001 (Larson et al. 2005). As of January 2006, a total of 45 sharks have been tagged with a visible stainless-steel dart tag (Larson and Christiansen 2003; Christiansen, pers. comm. 2006). Sixteen of the tagged sharks have been resighted at the observation area with one shark having been resighted four times over a period at large of nearly 700 days. Preliminary genetic results from over 200 samples suggest that the long term effective population size is at least 7900 individuals (Larson et al. 2005), but it is not possible to infer current levels of population abundance from the genetic estimate. Overall, the population structure is not known.

Designatable units

The population structure of bluntnose sixgill sharks in Canada’s Pacific waters is unknown. There are no known obstacles to migration or dispersal, and therefore for the purposes of this report, bluntnose sixgill sharks are considered to be one designatable unit throughout Canadian waters.

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