The smalltooth cookiecutter shark (Isistius brasiliensis) is a deep-sea species known for its unique, parasitic feeding style. They are named for the perfectly circular plugs of flesh they excise from larger marine animals, leaving a wound that looks as though it was cut with a cookie cutter. Despite their specialized teeth and formidable feeding method, the actual danger they pose to humans is extremely low. Their small size and preference for the ocean’s dark, deep zones mean human encounters are rare and typically accidental.
Anatomy and Deep-Sea Life
The cookiecutter shark is a small, cigar-shaped dogfish, typically growing to a maximum length of about 20 inches (50 cm). A dark collar marking around its gill slits earned it the earlier common name of cigar shark. Adapted to the deep sea, the species possesses a large, oil-filled liver that can account for up to 35% of its total body weight, helping maintain neutral buoyancy.
A remarkable feature is the sharkâs bioluminescence, produced by tiny organs called photophores that cover its entire underside. This greenish glow is part of a camouflage technique known as counter-illumination, which helps the shark blend with the faint downwelling sunlight when viewed from below. A small patch near the throat lacks these light organs, creating a dark silhouette. This dark patch is thought to mimic a smaller fish, luring larger prey close enough for the shark to strike.
The shark’s habitat is primarily in the mesopelagic and bathypelagic zones, ranging from roughly 280 feet to over 11,500 feet deep in tropical and subtropical waters worldwide. They follow a diel vertical migration pattern, staying deep during the day and moving closer to the surface at night to feed. This preference for deep water inherently limits interaction with humans, who typically remain in shallow, sunlit surface waters.
The Unique Suction-Bite Feeding Method
The cookiecutter shark’s mouth is highly specialized for its ectoparasitic feeding strategy. It possesses suctorial lips and a modified pharynx, allowing it to attach firmly to the side of its prey. Once attached, the shark creates a vacuum seal by retracting its tongue and closing its spiracles.
Its teeth are sharply differentiated. Small, pointed upper teeth act as anchors to hold the shark in place. The lower jaw holds a single row of 25 to 31 large, triangular, blade-like teeth. Using these lower teeth like a bandsaw, the shark rotates its body to carve out a perfect, circular plug of flesh. The resultant wound is typically concave and symmetrical, giving the shark its common name.
This specialized bite allows the shark to feed on animals significantly larger than itself without killing them, operating as a facultative ectoparasite. The shark swallows the plug of tissue whole, leaving the host to heal from the distinctive, deep, circular wound. An unusual behavior is that the shark sheds its entire set of lower teeth at once and often swallows them, potentially to recycle the calcium.
Documented Targets and Human Risk Assessment
The primary targets of the cookiecutter shark are large marine animals, including whales, dolphins, seals, and large pelagic fish like tuna and swordfish. Characteristic bite marks have been observed on victims as large as great white sharks and various cetaceans. These bites are generally non-lethal to the massive hosts but represent a loss of tissue and energy.
The shark is also known for attacking inanimate objects in the deep ocean. They have left signature circular marks on the rubber sonar domes of nuclear submarines and on deep-sea cables. It is hypothesized that the thick rubber material of the sonar domes may confuse the shark into viewing the object as potential prey.
For humans, the risk is extremely low, primarily because of the shark’s oceanic, deep-water habitat. Historically, most reports of bites involved post-mortem scavenging on deceased individuals. However, a handful of documented cases exist of unprovoked bites on live humans, most notably on long-distance swimmers in deep, open water at night.
These rare incidents occur during vertical migration periods when the shark is closer to the surface, and a human swimmer may be perceived as a novel, large prey item. The recorded bites have been described as small, open, round, and concave wounds, consistent with their feeding method, but they are not life-threatening. While the shark possesses a unique bite, the danger to the average person is negligible.