How the Deep-Sea Cookie Shark Hunts, Glows, and Bites

The cookiecutter shark is a small, cigar-shaped deep-sea shark that feeds by gouging nearly perfect oval plugs of flesh from animals far larger than itself, including whales, tuna, dolphins, and even the occasional human swimmer. Belonging to the genus Isistius, these sharks rarely exceed half a meter in length, yet they leave distinctive crater-like wounds on some of the ocean’s biggest creatures. Despite being widely distributed across tropical and subtropical waters worldwide, cookiecutter sharks remain poorly understood because they spend most of their lives in the mesopelagic zone, hundreds of meters below the surface.

How the Bite Works

The cookiecutter shark’s most remarkable trait is the way it feeds. Its lower jaw is lined with a band of large, razor-sharp, triangular teeth that interlock like a saw blade, while the upper jaw carries smaller, narrower teeth that act more like hooks. When the shark latches onto a larger animal, it uses powerful suction from its fleshy lips to anchor itself, then sinks in its lower teeth and rotates its entire body. This twisting motion carves out a clean, roughly symmetrical oval plug of flesh and blubber. Researchers who simulated the bite using a 3D-printed jaw-and-dental model confirmed that the body rotation is necessary to produce the nearly symmetrical oval wounds found on prey species including marine mammals, fishes, and squids.¹ The resulting wound looks almost as if someone pressed a melon baller into the animal’s body.

The lower teeth are shed as an entire row at once rather than individually, which is unusual among sharks. This means the cookiecutter shark replaces its cutting edge all at once and swallows the old set, presumably to recycle the calcium. The upper teeth, by contrast, are replaced one at a time, similar to most other sharks. This arrangement keeps the lower saw blade functional as a unit, which is essential for a feeding style that depends on a continuous cutting arc.

Three Species, One Strange Strategy

The genus Isistius currently contains three recognized species: Isistius brasiliensis (the common cookiecutter shark), I. plutodus (the largetooth cookiecutter shark), and I. labialis.² Of these, I. brasiliensis is by far the most commonly encountered and best studied, while I. labialis is known from very few specimens.

The differences between the two better-known species are more than cosmetic. I. brasiliensis has a more rounded, proportionally longer snout compared to I. plutodus, and its second dorsal fin is roughly the same height as the first. In I. plutodus, the second dorsal fin is noticeably taller than the first. The teeth tell the bigger story: I. plutodus has far fewer tooth rows but individually much larger lower teeth, roughly six times the size of its upper teeth, compared to a three-to-one ratio in I. brasiliensis.³ That difference in tooth proportions suggests I. plutodus may take larger, deeper bites relative to its body size, though direct feeding observations in the wild are essentially nonexistent for either species.

What Gets Bitten

The list of animals bearing cookiecutter shark scars is startlingly long. Large whales, including sei, fin, Bryde’s, and sperm whales, have been documented with the characteristic crater wounds.⁴ Short-finned pilot whales in Hawaiian waters show high rates of scarring consistent with cookiecutter shark bites, and research there has provided evidence that the sharks may perform seasonal migrations or shift their diets over the course of a year.⁵ Tuna species are also frequent targets; studies have documented predation on yellowfin tuna, skipjack tuna, and little tunny, with bite marks appearing even on relatively young fish.⁶

Beyond living marine animals, cookiecutter sharks have been known to bite rubber sonar domes on submarines and the insulation on undersea cables. The U.S. Navy reportedly had to reinforce sonar dome coverings after repeated bite damage in the 1970s. This indiscriminate biting of rubbery surfaces suggests the sharks may initially target objects based partly on texture or electrical signature rather than visual identification alone.

Predator or Parasite?

One of the persistent debates about the cookiecutter shark is whether to call it a predator or a parasite. Most parasites maintain an ongoing relationship with a host, feeding over time without killing it. The cookiecutter shark takes a single bite and leaves. Yet the encounter is almost never fatal to the larger animal, which is more consistent with a parasitic interaction than a predatory one. Researchers who studied the crater wounds on large whales addressed this directly, choosing to use the terms “predation” and “prey” because there is no evidence the relationship is anything other than transitory, even though the bite is highly unlikely to kill the victim.⁷

Some biologists prefer the term “ectoparasitic predator” as a compromise, acknowledging that the feeding strategy borrows from both categories. In practice, the label matters less than the ecological reality: cookiecutter sharks extract resources from organisms many times their size and almost never kill them. For the prey species, the bite is a nuisance wound, a small cost of living in tropical oceans. For the shark, it is a remarkably efficient way to access high-calorie tissue without the risks of attacking prey its own size or larger in a conventional predatory encounter.

Life in the Deep and the Nightly Rise

Cookiecutter sharks are mesopelagic, spending daylight hours at depths that can exceed 1,000 meters. At night, they migrate vertically toward the surface, following the massive daily movement of deep-sea organisms known as the diel vertical migration. This migration brings them into contact with surface-dwelling and near-surface animals that would otherwise be beyond their reach. Although the species is widely distributed throughout the world’s tropical and subtropical oceanic waters, it remains very poorly understood due to this deep-water lifestyle.⁸

The shark’s underside has a band of bioluminescent photophores that glow green. One hypothesis holds that this glowing belly mimics the silhouette of a small fish when viewed from below, potentially luring curious predators into striking range. When a larger fish moves in to investigate what it thinks is easy prey, the cookiecutter shark turns the tables and takes its plug. This lure-based strategy has not been conclusively proven in the wild, but it fits with what is known about the shark’s anatomy and the light conditions at the depths where encounters happen.

Cookiecutter sharks also have unusually large eyes relative to their body size, consistent with a life spent hunting in low-light conditions. Their green bioluminescence, combined with the light-absorbing dark collar around their gill region, may create a visual illusion for prey species looking up from below: the collar breaks the glow pattern in a way that resembles the outline of a much smaller fish.

Encounters with Humans

Cookiecutter shark bites on living humans were essentially unheard of until recently. In a five-month period in 2019, three long-distance swimmers were bitten while attempting to cross the Ka’iwi Channel between the Hawaiian Islands of O’ahu and Moloka’i. All three sustained nonfatal circular wounds measuring between 8 and 13 centimeters in diameter. The bites occurred while swimming over waters deeper than about 600 meters (roughly 2,000 feet) at night. Transport times to the emergency department averaged 73 minutes because the injuries happened on the open ocean, but all three swimmers were hemodynamically stable on arrival and did not require blood transfusions.⁹

These cases were the first documented series of cookiecutter shark bites on live humans. Prior to 2019, the total number of confirmed bites on living people worldwide was vanishingly small. As of 2023, seven cases had been reported globally, six of them in Hawai’i and five among channel swimmers specifically.¹⁰ The bites are painful and produce deep, circular wounds that heal slowly because of the volume of tissue removed, but none of the recorded cases have been life-threatening.

Why Darkness Matters

An analysis of 129 successful solo channel crossings in Hawai’i found a striking pattern: swimmers bitten by cookiecutter sharks were overwhelmingly in the water during periods when both the moon and the sun were below the horizon. About 12 percent of swimmers in the fully dark group were bitten, compared to roughly 1 percent in the group that had at least some moonlight or sunlight in the sky. The relative risk was about 12.6, meaning swimmers in pitch-dark conditions were roughly thirteen times more likely to be bitten.¹¹ All five documented bites on channel swimmers occurred after sundown.

Interestingly, the use of electronic shark deterrent devices and artificial illumination (lights used by support boats during nighttime crossings) did not show a statistically significant difference in bite rates. The shark deterrent devices commonly used by open-water swimmers are designed to repel larger coastal species and may not produce the right frequency or field strength to deter a deep-sea species like the cookiecutter shark. The practical takeaway for channel swimmers is that moon phase and swim timing appear to be the most meaningful controllable risk factors, though the absolute risk remains very low given how few bites have occurred in over six decades of recorded channel crossings.

Why the Wounds Puzzled Scientists for So Long

Before the cookiecutter shark was identified as the culprit, the circular crater wounds found on whales, dolphins, and large fish were a genuine mystery. Observers variously attributed them to the physical displacement of barnacles, infections from parasitic copepods, bites from lampreys or remoras, and even microbial infections.¹² The confusion was understandable. A small, deep-water shark that rises to the surface at night, takes a single symmetrical bite, and disappears back into the abyss is not the kind of predator people expect to find. The wounds looked too uniform, too circular, and too clean to come from a conventional bite. It was only when researchers matched the wound geometry to the jaw shape and tooth arrangement of Isistius species that the mystery was resolved.

Even today, scientists cannot always determine which Isistius species made a particular wound. Bite scars on a whale in the South Atlantic could be from I. brasiliensis or I. plutodus, and distinguishing them from scar morphology alone is unreliable. The identification of the biting agent to species level typically requires either catching the shark in the act (which almost never happens) or analyzing tooth marks under conditions that allow measurement of individual tooth dimensions.

A Fossil Record That Goes Back Millions of Years

The cookiecutter shark’s distinctive lower teeth, with their triangular shape and interlocking bases, are recognizable even as fossils. Fossil teeth of Isistius triangulus, an extinct relative, have been recovered from creek deposits in Florida. Researchers screenwashed nearly 95 kilograms of matrix material and produced 186 whole or partial lower teeth of the species, representing the first documented fossil records of the genus from that state.¹³ The preservation of so many teeth in one location suggests that the site, informally called Cookiecutter Creek, was once an environment where these sharks were relatively common.

The existence of fossil Isistius teeth tells us that the plug-feeding strategy is not a recent evolutionary experiment. This lineage has been gouging oval chunks from larger animals for millions of years, a remarkable degree of ecological consistency. The tooth design, with its saw-like lower row built for a rotational cutting motion, appears to be a deeply conserved adaptation rather than something that arose recently. Whatever selective pressures originally favored this feeding style, they have been stable enough to maintain it across geological time scales and multiple species within the genus.

Nonlethal Impacts on Commercially Important Fish

For fisheries, cookiecutter shark bites are more of an economic nuisance than an ecological threat. Tuna and swordfish brought to market with oval crater wounds are cosmetically damaged, and the wounds can reduce the usable flesh from a given fish. In regions where cookiecutter sharks are abundant, scarring rates on commercially caught tuna can be high enough to attract notice from fishers and processors. The bite does not kill the fish, but in species destined for high-value sashimi markets, any wound that affects flesh quality translates directly to lower prices.

The impact extends to aquaculture in some regions as well. Caged fish in offshore pens located over deep water have occasionally shown cookiecutter-style wounds, presumably from sharks rising during their nightly vertical migration. Because the bites are non-fatal, the main concern is infection at the wound site rather than direct mortality. For wild fish, the energy cost of healing a cookiecutter wound is real but appears to be manageable for most species. The frequency with which scarred animals are caught in apparently good health suggests that the wounds heal over the course of weeks without major lasting effects on fitness.

What the Shark’s Glow Might Really Be For

The bioluminescence of the cookiecutter shark has drawn more speculation than hard evidence. The counter-illumination hypothesis, where the glowing belly cancels out the shark’s silhouette against downwelling light, is well supported in other mesopelagic species. But the cookiecutter shark’s dark collar around its throat complicates the picture. If the sole purpose of the glow were camouflage, a dark patch that breaks the light pattern would be counterproductive. The alternative explanation is that the collar creates the illusion of a small fish silhouette, essentially functioning as a lure. The rest of the body glows to match the ambient light, while the dark collar stands out as an apparent small prey item. A curious predator approaches what looks like a bite-sized target and instead gets bitten.

Testing this in the wild is extraordinarily difficult. Cookiecutter sharks do not survive well in captivity, and observing natural feeding behavior at depth, at night, in open ocean conditions is beyond the reach of most research budgets. What researchers have are the anatomical facts (the photophores, the collar, the teeth, the suction lips) and the wound evidence on prey species. The lure hypothesis fits these facts elegantly, but elegant and proven are different things. Until someone captures feeding behavior on camera at depth, the bioluminescence story remains one of the more compelling but unconfirmed hypotheses in shark biology.