The cookiecutter shark, Isistius brasiliensis, is a small, cigar-shaped species found in deep, warm oceanic waters worldwide. It is named for the distinctive, circular wounds it leaves on its prey, which resemble cuts made by a cookie cutter. This shark typically grows to a maximum length of about 56 centimeters (22 inches). Its appearance is characterized by a dark brown body, large green eyes, and a prominent dark band or “collar” near its gill region. Despite its diminutive size, the shark possesses a specialized feeding mechanism that has led to its notoriety and the question of whether it poses a threat to humans.
Documented Interactions with Humans
Interactions between the cookiecutter shark and live humans are exceedingly rare, with only a handful of confirmed instances globally. The majority of these bites have occurred in Hawaiian waters. The victims have almost exclusively been long-distance channel swimmers who were in deep, open water during nighttime hours.
The wounds inflicted are non-fatal and are consistently described as small, circular, and concave, essentially a “scooped-out” plug of flesh. These clean, crater-like injuries typically measure between 8 and 13 centimeters in diameter. The bites are considered opportunistic or a case of mistaken identity, likely occurring when the shark encountered the swimmer while foraging near the surface at night.
In one of the first documented cases involving a live person, a swimmer was bitten twice on his chest and calf while attempting a channel crossing after sunset. The wounds required medical treatment, including skin grafts, due to the nature of the tissue removal. The rarity of these encounters means the cookiecutter shark is not considered a general threat, but the incidents highlight a specific risk profile for individuals entering its deep-water domain at night.
The Unique Feeding Strategy
The cookiecutter shark’s specialized feeding method is known as excision feeding. This approach allows the small shark to parasitize animals vastly larger than itself, including whales, dolphins, seals, and large pelagic fish like tuna and marlin. The shark’s mouth is highly adapted for this technique, featuring two distinct sets of teeth and fleshy, suctorial lips.
When feeding, the shark first attaches itself to the prey by closing its spiracles and retracting its tongue to create a vacuum seal with its lips. It then uses the small, sharp teeth in its upper jaw as an anchor, holding itself firmly in place against the prey’s surface. The lower jaw contains larger, triangular, and serrated teeth fused together at the base, forming a continuous, razor-sharp cutting blade.
The shark then rotates its body in a circular motion while using the lower blade-like teeth to excise a symmetrical, cone-shaped plug of flesh. This mechanism allows it to quickly remove a chunk of tissue and retreat without being caught by the larger animal. The cookiecutter shark’s indiscriminate nature extends beyond living prey; it has been documented leaving crater marks on inanimate objects. These targets include the rubber sonar domes of U.S. Navy submarines, deep-sea electrical cables, and various oceanographic instruments.
Deep-Sea Habitat and Encounter Rarity
The rarity of human interactions is directly related to the cookiecutter shark’s preferred habitat and daily behavior. It is primarily a creature of the mesopelagic zone, or “twilight zone,” of the open ocean. During the day, it retreats to depths typically residing between 1,000 and 3,700 meters (3,300 to 12,140 feet) below the surface.
The shark undertakes a diel vertical migration, one of the most extensive daily movements in the animal kingdom. It rises toward the surface at night to feed and then descends again before dawn. This nocturnal movement brings it into the upper water column, which is when the chance of encountering an open-ocean swimmer briefly exists.
The cookiecutter shark possesses a sophisticated camouflage strategy involving bioluminescence. Its entire underside, except for the dark collar near the gills, is covered in light-producing organs called photophores. These photophores produce a faint green glow, which helps the shark practice counterillumination, matching the downwelling light from the surface to conceal its silhouette from predators below. The dark, non-glowing collar is hypothesized to act as a lure, mimicking the silhouette of a smaller fish to attract larger prey.