Egofelix Magazine Presents: Natural History of the Sixgill Shark (Hexanchus griseus)

Natural History of the Sixgill Shark (Hexanchus griseus)

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Where to find them

Hornby Island is a rare and ideal spot for scuba divers to observe and study the placid sixgill shark up close. Sixgills are found in all of the world’s oceans, but in most locations, they live at depths reaching 2500 m (8000′), and hence are inaccessible to humans under most circumstances.

However, during the months of May through September, sixgills are commonly spotted at the wall off Flora Islet between 25 and 30 m (80 – 100′), and have even been reported as shallow as 10 m (30′).

With the onset of autumn the sightings decrease, but research suggests that the sharks stay relatively close by and simply move into deeper water. Sixgills apparently also migrate inshore seasonally at Monterey Canyon, San Francisco Bay and Tyler Rock on the west coast of BC.

Why here?

No reason for these shark’s ascent to the shallows is readily apparent. At the depths where recreational divers encounter them, sixgills display complete disinterest in preferred prey items, and many of the sharks are not of a size that would indicate sexual maturity (greater than 4 m/400 kg (13′ /880 lb.) in females or 3 m/200 kg (10′ /440 lb.) in males).

This seems to rule out migration for breeding or eating, but other factors must be taken into consideration, for example, sixgills may feed nocturnally and thus are less likely to be observed by divers while eating. The possibility of nocturnal feeding patterns is supported by the fact that sixgills are apparently more active at dawn and dusk, and divers have encountered them in very shallow water (10 m/30′) while night diving at various locations.

Behavior

Sixgills are quite mysterious in their activities; much information presented here about their habits (dietary, reproductive and otherwise) is based on speculation, inference and unconfirmed reports by divers. In fact, in over two decades of encounters with sixgills at Flora Islet, divers have never observed the sharks doing anything other than just cruising along.

At first observation they may appear sluggish, but any diver who has tried keeping up with one knows that they swim at quite a good pace. They generally stay close to the bottom or hug the shelves of the wall, and they become agitated if separated from this point of reference.

Sixgills are usually solitary, but have been seen on more than one occasion in groups of up to five. Up to a dozen sharks are sometimes observed in the same area. Individual sharks in the same vicinity do not appear to be engaged in any common activity such as breeding or hunting.

These sharks seem to have little interest in (or appetite for) humans. Their only acknowledgment of the presence of divers is their casual and careful avoidance of physical contact, which is maintained by the shark swiveling its head to keep the interlopers in view and steering its body to preserve its “bubble” of personal space. There are no known reports of sixgills attacking humans, but common sense should be exercised. They are large, powerful animals, but they need not be feared, only respected.

Anatomy

The sixgill commonly attains a length of 4.8 m (16′), and there have been reports of it reaching 7.6m (25′). The sixgill gets its name from a physical feature which sets it apart from other sharks: instead of the more advanced five-gill arrangement which has evolved in most sharks, the sixgill has six gill slits. This unusual evolution has left the sixgill shark found in today’s oceans virtually unchanged from fossil forms dating back 150 million years. The closely related sevengill shark Notorynchus cepedianus has a similarly prehistoric body plan.

Other physical features that identify the sixgill include its very wide body (it is also called “cow shark” and “bulldog shark”) and the fact that it has only one dorsal fin (most sharks have two) which is placed very far back towards its long, raked tail. Sixgills range from dull grey to brown in colour dorsally, with lighter white to yellow ventrally.

Another identifier of the sixgill is the fact that its mouth contains two different types of teeth. Its upper teeth are triangular in shape, and the lower ones are edged with saw-like serrations.

Diet

Stomach content analysis suggests that sixgills favour both bony and cartilaginous fishes. Preferred items in the latter group include rays, ratfish (Hydrolagus colliei) and dogfish (Squalus acanthias). The remains of crustaceans and sea mammals such as seals, sea lions and whales have also been reported in the sixgill’s gut. Sixgills apparently scavenge and they are known to take bait from dogfish longlines.

Although they are generally slow moving, sixgills can move with incredible speed for short periods, and may use this ability coupled with their drab coloration to capture their prey using an element of surprise.

Sharks and Submersibles

At Flora, SCUBA divers are allowed the rare chance to swim beside a sixgill shark. But this encounter is always all too brief, it is limited by the diver’s air supply, exposure to chill, and depth and decompression limits. Submersibles extend or eliminate these limits, and in doing so are invaluable in helping us learn more about the sixgill’s “other life”.

In September 1996 a camera crew came to Hornby to film a PBS “New Explorers” broadcast called “Walking Among the Sharks”. Deep Rover, the Aquarius, and the Newtsuit carried scientists and film crew into two shark encounters during the four-day shoot. Deep Rover was also used in 1988 to film a French National Television adventure special narrated by Krov Menuin.

Points of Interest

Although no reproductive activities between sixgills have been reported at Flora, other research indicates that these sharks give birth to pups 65 – 70cm in length, in litters which can number from 20 to over 100 (Ebert, D.A., South African Journal of Marine Sciences, 1986, 14:213-218).

Sixgills are extremely primitive in design; they are virtually unchanged from fossil forms 150 million years old (Harvey-Clark, Chris, “Shark Watching: Swan Song or the Sixgills”, Marine Forum, Fall 1993, 12). Fossils remains of the sixgill and other sharks are found on Hornby’s beaches.

Flora Islet was recently purchased by the Provincial Government from a private owner and is slated to become part of Helliwell Provincial Park. Community members on Hornby are currently working to upgrade the protection provided to marine life in this unique area.

References

  1. Harvey-Clark, Chris “Sixgill Shark Information Sheet” 1994.
  2. Weber, Edward “Primeval Predator” Diving & Snorkeling Spring 1990: 72.
  3. Robbie and Bob Zielinski, long-time shark divers “extraordinaire”.


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