In the deep ocean, far from sunlight, a small shark has been quietly carving perfect circles of flesh out of animals many times its size. For decades, those neat, crater-like wounds on whales, tuna, and even other sharks were a forensic puzzle that hinted at a predator we rarely see but often notice after the fact. Scientists have now pulled together behavioral data, traditional knowledge, and new ecological modeling to explain how this animal pulls off its surgical attacks and why those circles matter far beyond the shock value of the scars.
The culprit is the cookiecutter shark, a cigar-shaped hunter that behaves less like a classic apex predator and more like a parasite with teeth. By decoding its nightly movements, its glow-in-the-dark camouflage, and its unusual jaw mechanics, researchers have finally clarified how this shark bites such precise plugs of flesh and how those bites ripple through marine food webs and human fisheries.
The shark behind the circles
The animal responsible for those almost cartoonishly round wounds is the cookiecutter shark, known to science as Isistius brasiliensis. Despite its modest size, this shark leaves a calling card that is impossible to miss: a sharply defined, circular crater where a plug of tissue has been removed from a much larger animal. Descriptions of Cookiecutter Sharks, Isistius, Description, Behavior, Cookiecutter, Quoy and Gaimard emphasize how this small, brownish shark appears to blend into the dark water column, with a stout body, large eyes adapted to low light, and a mouth packed with triangular lower teeth that are shed as a single unit.
Unlike the streamlined silhouettes of great whites or makos, Isistius brasiliensis looks almost unassuming, which makes the violence of its bite even more surprising. The shark’s lower jaw forms a rigid, saw-like arc, while the upper teeth are smaller and more grasping, a configuration that is highly specialized for gouging rather than slicing. As accounts of the cookiecutter shark note, this species targets everything from large fish and marine mammals to smaller prey such as squid, functioning as a kind of mobile hole punch in the open ocean.
How a perfect circle bite actually works
The mystery of the perfect circle starts with how the cookiecutter shark approaches its victim. Rather than charging in at high speed, it uses suction, lips, and a twisting motion to remove a plug of flesh in a single, brutal maneuver. Educational explainers on how the animal feeds describe how it first latches on with its lips, then uses its powerful jaws and body rotation to carve out a disc of tissue, a process vividly illustrated in a short video that warns viewers, Don’t be fooled by its adorable name—the cookiecutter shark attacks by suctioning its lips to its victim’s flesh and spinning away with a chunk.
That rotational gouge explains why the wounds are so clean and circular, more like a biopsy than a ragged bite. When scientists examined injuries on large sharks and marine mammals, they noticed that the edges were smooth and uniform, with a depth and diameter that matched the arc of the cookiecutter’s lower jaw. The precision of the bite, combined with the shark’s habit of targeting soft, muscular areas like flanks or bellies, turns each attack into a near-perfect circle, a biological signature that has now been matched to the mechanics of its specialized teeth and suction-based strike.
From strange scars to a named culprit
Long before the behavior of the cookiecutter shark was documented in detail, its work was visible on the bodies of other animals. Marine biologists studying great white sharks began to notice circular scars on the heads and flanks of otherwise healthy individuals, marks that did not match hooks, boat strikes, or other common injuries. One widely shared explanation of these wounds walks viewers through a close-up of a white shark’s head and points out a round depression, explaining that That is a scar left by another shark species- the cookie cutter shark, not a fishing injury or parasite.
Earlier forensic work on shark carcasses and live animals converged on the same conclusion. When a researcher examined a great white with several circular wounds, he noted that a wound from a hook should leave more of a hole and would not be as smooth, and that only a small shark with a specialized jaw leaves a bite like that. Detailed coverage of that case identified the attacker as a cookie-cutter, cementing the link between the mysterious circles and Isistius brasiliensis in the scientific record.
A parasite that feeds on giants
Once the culprit was identified, the scale of its ambition became clear. Cookiecutter sharks routinely feed on animals far larger than themselves, including great whites, swordfish, dolphins, and whales. A detailed account of interactions with large predators notes that Small cookiecutter sharks feed on flesh of great whites and that jaws collected from these sharks are from Isistius brasiliensis, underscoring how a relatively tiny fish can repeatedly wound one of the ocean’s most formidable hunters.
Biologists now describe the cookiecutter as a facultative parasite, an animal that does not kill its host outright but instead removes small pieces of tissue over time. Analyses of its ecology point out that the impact of this parasitism on prey species, in terms of resources diverted from growth or reproduction, is uncertain, but that the shark’s diet includes a wide range of large vertebrates and smaller prey such as squid. That dual role, as both predator and parasite, is highlighted in overviews of the cookiecutter shark, which stress that its circular bites may weaken animals without necessarily killing them, subtly reshaping energy flows in the deep sea.
Glow in the dark and built for ambush
To understand how a small shark can get close enough to bite a whale or a great white, researchers have focused on its bioluminescence and vertical movements. Cookiecutter sharks are counter-illuminators, using light-producing organs on their bellies to match the faint glow of the water above them, which makes them nearly invisible from below. A detailed profile titled Cookiecutter Sharks Use Glow In The Dark Ability To Attract Prey, Its Own Size, Background, The Cookiecutter, Isistius explains that this glow-in-the-dark ability helps the shark attract prey up to 10 times its own size, turning its body into a deceptive lure in the midwater gloom.
That camouflage works hand in hand with a daily migration pattern. During the day, cookiecutters stay deep, out of reach of most surface predators, then rise toward the surface at night to feed. Their glowing underside can mimic the downwelling light, while a darker collar near the head may create the illusion of a smaller fish, drawing curious predators close enough for a sudden bite. This combination of vertical movement, light-based disguise, and specialized jaws is what allows the shark to approach giants without being seen until the moment its lips clamp down and the circle is cut.
Tracking the night patterns
For years, those behaviors were inferred from scattered observations and bite marks, but new research has started to map the cookiecutter’s nightly routine in much finer detail. A recent study from the University of Hawaiʻi analyzed a large dataset of interactions between these sharks and pelagic animals, using both scientific monitoring and traditional knowledge to understand when and where bites occur. The project, described in a university release that notes how University of Hawaiʻi researchers have bridged ʻike Hawaiʻi and modern science, was led by Suca, J. and focused on patterns that might otherwise remain unseen in the open ocean.
Coverage of the same work for a broader audience emphasizes that the study was Published in Marine Ecology Progress Series and that it examined Night patterns in cookiecutter activity, finding that bites on pelagic animals and fishing gear cluster in darker, low-illumination periods. By tying bite frequency to moon phase, time of night, and depth, the researchers showed that the shark’s perfect circles are not random at all, but part of a predictable schedule that can be modeled and, potentially, mitigated where they intersect with human activity.
When perfect circles hit human fisheries
The same traits that make cookiecutter sharks so effective against whales and great whites also bring them into conflict with commercial fishing. Longline fleets that target high-value species like swordfish have reported gear damage and lost catch when cookiecutters attack hooked fish before they can be hauled aboard. A technical analysis of these interactions notes that, Specifically, Xiphias gladius, the swordfish targeted by nighttime shallow-set longlines, were examined for patterns in cookie cutter shark bites, revealing operational and environmental factors that increase the risk of damage.
Those findings dovetail with the broader ecological work from Hawaiʻi, which shows that the sharks are most active near the surface during the same dark hours when many fleets set their lines. By aligning fishing schedules, gear depth, and illumination conditions with the known peaks in cookiecutter activity, managers may be able to reduce the number of fish that reach the boat bearing circular wounds instead of intact fillets. The perfect circles that once seemed like a bizarre curiosity have become a practical management concern, pushing scientists and fishers to treat the shark as a predictable, if unusual, player in pelagic ecosystems.
Science meets ʻike Hawaiʻi
One of the most striking aspects of the recent breakthroughs is how they blend Western marine biology with Indigenous knowledge. Researchers at the University of Hawaiʻi have worked with Hawaiian language experts and cultural practitioners to interpret traditional observations of strange marks on fish and marine mammals, and to name the shark in a way that reflects its behavior. A detailed university feature explains that ʻŌlelo in science helped the team develop a new Hawaiian name for the cookiecutter shark, nahunai, and that this collaboration revealed patterns that might otherwise remain unseen.
A companion video piece, produced by the University of Hawaii, Hawaiian research team, shows scientists and cultural experts discussing how ʻike Hawaiʻi, or Hawaiian knowledge, can guide questions about when and where the sharks appear. By treating oral histories and place-based observations as data alongside satellite tags and catch records, the project reframed the cookiecutter not just as a biological oddity but as a familiar presence in Hawaiian waters, one that local communities have noticed and named long before its Latin designation entered the literature.
Cracking the mystery in the lab and at sea
While field observations and Indigenous knowledge have mapped where and when cookiecutters strike, laboratory work and comparative anatomy have filled in the details of how those perfect circles are carved. Researchers examining preserved specimens have documented the arrangement of the shark’s teeth, the musculature of its jaws, and the structure of its lips, all of which contribute to the suction-and-twist feeding style. Popular science coverage of the latest synthesis of this work notes that Scientists Solved the Mystery of the Shark That Bites Perfect circles by combining behavioral observations, anatomical studies, and ecological data, and that the cookiecutter is, in their words, a truly weird shark.
At sea, tagging and remote cameras have begun to capture glimpses of the shark in action, although its deep, nocturnal habits still make direct observation rare. The convergence of physical evidence on carcasses, scars on live animals, and the shark’s own morphology has now convinced researchers that no other species is responsible for the distinctive circular wounds. What began as a scattered set of strange scars has become a coherent story of a small predator that uses light, timing, and a specialized bite to feed on giants, a story that is still being refined as new data from Hawaiʻi and other ocean basins come in.
Why the circles matter for ocean health
Understanding the cookiecutter’s perfect circles is not just a matter of satisfying curiosity. Each bite represents a transfer of energy, a small but repeated tax on the bodies of whales, sharks, and large fish that already face pressures from climate change, pollution, and industrial fishing. Scientific summaries of the species stress that the full impact of its parasitism on prey species, in terms of resources diverted from growth or reproduction, remains uncertain, but that its broad diet and global distribution make it an important, if underappreciated, component of pelagic ecosystems. By quantifying how often and where the bites occur, researchers can start to assess whether cookiecutters are simply background players or whether their cumulative effect shapes the health and behavior of their hosts.
For me, the most compelling part of this story is how a pattern that once looked like a curiosity on a whale’s flank has become a lens on the deep ocean itself. The circles point to a shark that glows to hide in plain sight, that migrates with the night, and that has been quietly interacting with some of the ocean’s most iconic animals for as long as they have shared the water. Thanks to the combined efforts of anatomists, fisheries scientists, and Hawaiian knowledge holders, the mystery of the shark that bites perfect circles is no longer about who is doing the biting, but about what those bites tell us about a changing sea and the intricate relationships that hold it together.
More from MorningOverview