Attacked by a Dogfish


Cookiecutter Shark (Isistius brasiliensis)

The diminutive Cookiecutter Shark (Isistius brasiliensis) grows to a length of about 1.5 feet (50 centimetres) and is brilliantly bioluminescent. It has powerful jaws armed with tiny, hook-like upper teeth and very sharp, bandsaw-like lower teeth. These lower teeth interlock, allowing them to be shed and replaced as a unit, ensuring that the shark has a continuous cutting edge at all times.

The Cookiecutter Shark preys primarily on pelagic squids almost as large as itself, but also takes other free-swimming mesopelagic creatures, including bristlemouths and copepod-like crustaceans. It is most notorious, however, for cheekily scooping golfball-sized plugs of flesh from a wide variety of large open ocean fishes and cetaceans as well as for damaging fishing, research, and naval equipment up to and including the sonar domes of nuclear submarines, effectively rendering these vessels blind. There is even a report of Cookiecutters scavenging the body of a drowned fisherman.

When attacking large prey, the Cookiecutter is believed to act like across between a mousetrap and a pint-sized vampire. First seducing unsuspecting prey with its curious greenish glow, the little shark latches on by a co-ordinated action of its specialized suctorial lips and large, mobile basihyal ('tongue'). Then with its teeth firmly attached the Cookiecutter is rotated around by laminar flow of water created by its prey's momentum, allowing its bandsaw-like lower teeth to scoop out a conical plug of flesh. This procedure creates a characteristic 'crater wound', about 2 inches (5 centimetres) across and 2.5 inches (7 centimetres) deep; these wounds usually heal in about 2 to 3 months, eventually leaving a whitish, semi-circular scar.

A 1992 Paper by ichthyologists Shigeru Shirai and Kazuhiro Nakaya described the functional morphology of the feeding apparatus of the Cookiecutter Shark. They found that, like other squaloids, the jaws of the Cookiecutter are short and set transversely into the head but also showed many unusual specializations. Shirai and Nakaya concluded that these specializations were well suited to making a strong oral vacuum, scooping out the flesh of prey, and rotating the body itself. Despite the appealing drama of the standard story of how the little Cookiecutter inflicts its damage on large prey, the fact that most any edged object rotated against a soft one would produce a circular concavity, and Shirai and Nakaya's exquisite anatomical work, I do not find either scenario completely convincing. It seems to me that, once attached, the suctorial lips and hook-like upper teeth of the Cookiecutter Shark would not readily be rotated by the shark or by water movement any more than a fork stuck in a roast could be spun by a blast of air from a blowdryer.

Due to the relatively firm manner in which they are suspended from the skull, the jaws of squaloid sharks are only modestly protrusible. Perhaps most significant, however, is the unusual arrangement of the hyoid arches in squaloids. In the Great White and most other sharks that regularly gouge flesh from prey too large to be swallowed whole, the hyoid arches are folded back behind the jaws, the left and right one forming an angle of about 45 degrees with the long axis of the shark. But in many squaloids including the Cookiecutter the hyoid arches are set perpendicular to the body's longitudinal axis, forming a relatively stiff right-angle with the long axis of the skull. I did not know what to make of this until I was bitten by a squaloid with a jaw and hyoid arch arrangement similar to that of the Cookiecutter: the Spiny Dogfish (Squalus acanthias).


Spiny Dogfish (Squalus acanthias)

The Spiny Dogfish is perhaps the best understood shark, at least from an anatomical, physiological, and fisheries biology perspective. But no one knows much about its behavioral ecology. To study the body language and social hierarchy of the Spiny Dogfish under a competitive feeding context, I set up a concentrated bait station in shallow water off the coast of British Columbia, Canada. During the night dive that followed, the water was swarming with the little beasties, whose cat-like eyes shone fiery green in the beam of my underwater light. All was going according to plan as the Dogfish competed vigorously at the bait. Suddenly, I felt something bump into my right upper arm. At the same instant, I felt a very curious sensation: a kind of rapid forward-and-back vibration that so reminded me of my electric razor that despite the fact that I was underwater I fully expected to hear a mechanical BUZZZ. I looked down to see a 2-foot (60-centimetre) Dogfish clamped onto my wetsuit. When the vibration stopped, I saw a nearly circular patch of neoprene was hanging on by a mere thread or two. The significance of this didn't dawn on me until after I had surfaced.

The lower teeth of the Spiny Dogfish are noticeably larger than the upper, with broad overlapping bases and strongly oblique cusps. This forms a nearly continuous cutting edge much like that formed by the lower tooth band of a Cookiecutter. While it seems likely that the Cookiecutter would twist and writhe in order to sever the last recalcitrant strands of skin, I don't think it relies on rotation to actually cut through the flesh itself. Instead, I propose that the Cookiecutter vibrates its hyoid arches forward-and-back, causing the jaws to slice through tuna flesh or whale blubber as efficiently as an electric carving knife.

 

ReefQuest Centre for Shark Research
Text and illustrations R. Aidan Martin
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