Biology of the Bluntnose Sixgill

Adapted from a PowerPoint presentation given by R. Aidan Martin at a Sixgill Shark conference held at the Vancouver Aquarium, Vancouver, Canada, in November 2000

Identification

heavy body
single dorsal fin, located far posterior on body
anal fin present
caudal fin only slightly elevated above axis of body and with a weakly developed lower lobe
six pairs of gill slits
short, blunt snout
broadly arcuate mouth
six series of broad, comb-shaped teeth on each side of lower jaw
large, teardrop-shaped eyes
lateral line pale and conspicuous, with an odd ‘bump’ at base of the caudal fin
caudal peduncle short and stout, distance from rear of dorsal fin to upper caudal fin origin about equal to length of dorsal fin base
large size, averaging 8-12 feet (2.4 to 3.7 metres) long
one of only 11 sharks that regularly exceed 13 feet (4 metres) in length

Distribution

The Bluntnose Sixgill may be the most widely distributed of all sharks, rivalled only by the White Shark (Carcharodon carcharias)

Habitat

benthic on continental and insular shelves and upper slopes, from the surface and the intertidal down to a depth of at least 8,200 feet (2 500 metres)
young often close inshore, adults typically in deeper water below 300 feet (90 metres)
strongly bottom-associated, mostly over bathic soft bottoms and rocky reefs
a vertical migrator (moving surfaceward at night, returning to the depths before dawn)
seasonally (May-November) enters uncharacteristically shallow waters off British Columbia and Washington State, where it is typically encountered by divers at depths of 75 to 130 feet (23 to 39 metres) by day and as shallow as 10 feet (3 metres) at night
one report (Clark, pers. comm.) off a deep coral reef face in Palau during daylight hours
several local areas well known for shallow-water Sixgills
wide range and diversity of habitats suggest they may be encountered in many locations along our coast

Diet

Molluscs

snail (Gastropoda) – South Africa
Chokka Squid (Loligo vulgaris reynaudii) – South Africa
Angola Flying Squid (Todarodes angolensis) – South Africa

Crustaceans

crabs (Brachyura) – Cuba
shrimps (Caridia) – Cuba

Agnathans

Pacific Hagfish (Eptatretus tridentata) – California and British Columbia
sea lampreys (Petromyzontidae)

Teleost

Cape Anchovy (Engraulis capensis) – South Africa
Pacific salmon (Oncorhynchus spp.) – British Columbia
Spotted Hake (Urophycis regia) – Florida
Luminous Hake (Steindachneria argentea) – Florida
Atlantic hakes (Merluccius spp.) – South Africa and Spain
Pacific Hake (Merluccius productus) – California
cods (Gadidae)
lings (Gadidae)
Lingcod (Ophiodon elongatus) – British Columbia
Cabezon (Scorpaenichthys marmoratus) – British Columbia
Jacopever (Helicolenus dactylopterus) – South Africa
rockfish (Sebastes spp.) – British Columbia
grouper (Serranidae) – Florida
grenadier (Coelorinchus sp.) – South Africa
gurnards (Triglidae)
Dolphinfish (Coryphaena hipparus) – Cuba
Chub Mackerel (Scomber japonicus) – South Africa
Snoek (Thyrsites atun) – South Africa
small marlins (Makaira spp.) – Cuba
small Swordfish (Xiphias gladius) – Azores and Cuba
flounders (Pleuronectiformes)
anglerfishes (Lophiformes)

Chondrichthyans

Ratfish (Hydrolagus colliei) – California and British Columbia
Saint Joseph (Callorhinchus capensis) – South Africa
Bluntnose Sixgill Shark (Hexanchus griseus) – Cuba [scavenged]
Spiny Dogfish (Squalus acanthias) – South Africa, California and British Columbia
Longnose Dogfish (Squalus blainvillei) – Hawaii
Shortnose Dogfish (Squalus megalops) – South Africa
Prickly Shark (Echinorhinus cookei) – California
catsharks (Scyliorhinidae) – South Africa
Smoothhound Shark (Mustelus mutelus) – South Africa
other sharks – South Africa and Cuba
Atlantic Torpedo (Torpedo nobiliana) – Spain
Biscuit Skate (Raja clavata) – South Africa

Marine Mammals

Harbour Seal (Phoca vitulina richardsi) – British Columbia
Cape Fur Seal (Arctocephalus pusullus pusillus) – South Africa
pinniped remains – California
Bottlenose Dolphin (Tursiops truncatus) – South Africa [scavenged]
whale blubber – California

Feeding Behavior

multicusped lower dentition suggests gouging or sawing flesh from food items too large to be swallowed whole
given speedy prey, probably relies on stealth to approach prey, launching a sudden ambush attack from close range (< 3 body lengths)
may lunge with jaws gaping, catching smaller prey at a jaw corner with a lateral swipe of the head
use of bottom topography to stalk and capture prey (i.e. Lingcod, pers obs.)

Predators

Steller’s Sea Lion (Eumetopias jubatus)?
Killer Whale (Orcinus orca)?
White Shark (Carcharodon carcharias)?

Ecological Competitors

Pacific Sleeper Shark (Somniosus pacificus)?
White Shark (Carcharodon carcharias) – possible competitive exclusion in South Africa

Sensory Biology

brain is proportionately small, with a smooth-surfaced, symmetrical cerebrum (Bauchot et al. 1995 and pers. obs.)
olfactory bulbs very large, comprising about 8% of total brain weight (Martin, unpubl. data)
facial and trigeminal nerves proportionately small in diameter, suggesting electroreception relatively unimportant
nuclei of lateral-line lobes proportionately small, suggesting acoustico-lateralis system of limited importance
epiphyseal ‘window’ over pineal organ may help regulate seasonal changes in reproductive hormones
attempts at maintenance in aquaria indicate that Bluntnose Sixgills become highly stressed even at moderate light levels; eyes well adapted to deep-sea light conditions
retinas completely populated by rods (no cones), with long rod outer segments (averaging 26.4 mm) and peak spectral sensitivity at about 460 nm (Cohen 1985)
iris non-contractile; tapetum lucidum present and migratory pigment cells absent (Martin, unpubl. data)

These data suggest that:

social organization is minimal and schooling behavior is unlikely
olfaction and vision are probably most important in prey detection and social interactions (courtship and agonistic encounters, respectively)

Reproduction

Length at Maturity: males 11 feet (3.4 metres), females 14 feet (4.3 metres)
Age at Maturity: unknown for both sexes (age determination hampered by poorly calcified vertebrae), but probably at least 11-14 years for males and 18 to 35 years for females
Mode: ovoviviparity
Gestation: unknown, but probably more than 2 years
Number of Pups: 22-108; large litter size suggests high juvenile mortality
Length at Birth: 28 inches (70 centimetres)
Maximum Length: 16 feet (4.8 metres)
Longevity: unknown, but probably more than 80 years

Social Behavior

As in other hexanchoids (but not chlamydoselachoids!) strong sexual dimorphism in teeth – male has more erect primary cusps than female, especially noticeable on anteriormost cusp of enlarged lower anterior teeth of male, which is about 50% longer than the secondary cusp that follows it

courtship and mating never observed
as a releaser, courting male probably gently ‘nips’ female’s gill region, pectoral fins, and flanks
seasonal appearance of scars on females in shallow-water
agonistic display features strong pectoral fin depression and exaggerated jaw gapes
has been known to snap aggressively at divers when touched
many of the same individuals appear to return to favored shallow-water sites, year-after-year – providing long-term opportunities to interact socially with one another

So What Are They Doing in Shallow Water?

My hypothesis:

light-sensitive sharks seasonally enter uncharacteristically shallow coastal waters of Pacific Northwest under cover of plankton bloom
arrival in highly productive shallow waters coincides with local salmon and herring runs pups are dropped in shallow coastal ‘nursery’ areas offering plenty of food but relative safety from predators
adults and subadults take advantage of rich feeding and opportunities for social interaction just seaward of nursery areas
when water clears in winter, the light-sensitive sharks head for deeper water (>300 feet or 100 metres deep)

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