Did you know that there are sharks with six (some even seven) instead of the usual five gill slits on each side? The order is named Hexanchiformes and contains only six living species. They appear like ancient extinct sharks with few modern adaptations. One of them is the bluntnose sixgill shark (closely related to the bigeyed sixgill shark).
They are widely distributed in the Pacific, Atlantic and Indian Oceans and the Mediterranean Sea and usually live at freezing and dark depths of 300 to 6,150 ft -90 to 1,875 m- (as shown here). Known to migrate vertically at night, sometimes bluntnose sixgills seem to stay in shallow waters (despite their usual sensitivity to light). For instance, between August and November each year, several specimen can be found at depths of 70 to 90 ft -21 to 27 m- during the day and as shallow as 10 ft -3 m- during the night, at least at certain locations off the coast of British Columbia, Canada. They are generally curious about divers and, at these depths, can be observed extensively. It is assumed that they followed their prey or sought out safe nursery grounds for their young, utilize plankton blooms to shade their eyes.
Bluntnose sixgills are ovoviviparous and, after probably more than two years gestation, giving birth to large litters of 22 to 108 pups. Newborn sharks are relatively big (28 in -70 cm- long) and developed, but slow and therefore vulnerable. The teeth show a strong sexual dimorphism (the male shark has more erect primary cusps than the female), which fits with the among sharks popular courting behavior of “love nips” (proven by special scars only females exhibit). They are slow-growing and therefore live long (like the Greenland shark).
Like all sharks, bluntnose sixgills have specialized senses. Since their portion of the brain are proportionately small, it seems that electroreception and hearing are relatively unimportant. But their olfactory bulbs are very large (about 8% of total brain weight), and their eyes are well adapted to deep-sea light conditions. Their iris is non-contractile and their retinas are completely populated by rods (and no cones, thus no color vision) “with very long rod outer segments to give it maximal sensitivity at those wavelengths of light that predominate in the deep-sea”. They are predators, but scavenge on whale carcasses, too.
Growing to 16 ft -4.9 m- in length, bluntnose sixgills are captured in commercial fisheries (often incidentally), but also targeted by sport fisheries. Young ones are known to snap and trash when touched or captured, but older ones are lethargic and don’t fight. Bluntnose sixgills pose no danger to humans, despite being one of only 11 shark species that regularly exceed 13 ft -4 m- in length. The catch is used for fresh, smoked, salted or dried food products, as well as liver oil, fish meal and pet food. Their population is near threatened worldwide and severely depleted regionally, e.g. in the Northeast Pacific. Since visiting divers inject between five and ten million tourist dollars into the local economy of British Columbia each year due to the shallow-water bluntnose sixgills there, in 1997 the British Columbia government made it illegal for recreational or commercial fishermen to possess even a single carcass of this species caught by any means. I hope that more governments decide to protect these sluggish “Jurassic monsters”.
Sources: here, here, here and here