In late October this year, seven-year-old Jack Tolley became the second person in recorded medical history to be bitten by a cookiecutter shark in Australia.
The rarely seen shark left a 73-millimetre, disc-shaped wound on the boy's leg, which required surgery and a brief hospital stay. According to local reports, Tolley recovered well (despite narrowly avoiding a skin graft), and has been curiously researching the fish that bit him ever since.
"For a seven-year-old, he was very brave," advanced-care paramedic Wayne Harper, who treated the injury, told the Townsville Bulletin.
The incident, which occurred just before a family snorkelling excursion in North Queensland, was a surprising one – and that's because it took place in shallow water. Cookiecutter sharks are little-known animals, but past studies suggest they prefer incredibly deep habitats and ascend to feed only after dark. Nearly all past bites on humans have occurred during moonlit nights.
"It is certainly interesting," says shark scientist Dr David Ebert, who recently published a study on cookiecutter sharks off the coast of California. "Especially since these sharks are typically associated with being far offshore. Something must have been wrong with it to be so close."
Cookiecutter sharks (Isistius brasiliensis and Isistius plutodus) aren't new to science. We've known about their trademark bites – the inspiration behind their common name – since the 1970s. The conical craters left by their circular jaws have been observed on the bodies of all kinds of animals, from elephant seals to great whites. But only a small number of bites on humans have ever been documented.
Given their rarity, we still know very little about how cookiecutter bites present on human bodies. And predicting where, when and why such incidents are likely to occur is extremely tricky.
Nearly 10,000 miles away from the beaches of Queensland, one forensic anthropologist at the University College London has been working to change that.
A forensic mystery
Agathe Ribereau-Gayon came to study the small, cigar-shaped cookiecutter shark in an unanticipated way: her research began after an immense tragedy.
On June 30, 2009, Yemenia Flight 626 plummeted into the Indian Ocean on its way from Yemen to the Comoros Islands. The accident claimed the lives of 152 people, and inquiries into the causes and circumstances of the crash would take years to conclude.
In 2016, a study of the injuries sustained by the victims took Ribereau-Gayon and her colleagues down a winding and unexpected path that eventually led to an elusive deep-sea fish.
Of the 113 bodies pulled from the water after the Yemenia accident, 62 were covered with an array of unidentified lesions. Initial reports conducted on the remains failed to determine the cause of those injuries, which varied in shape, size, position and depth. Were they the result of decay? Or had they been inflicted during the accident itself? A second examination later pointed to another plausible origin: bottom-dwelling invertebrates.
For Ribereau-Gayon, however, none of these possibilities seemed to fit. Calling on the expertise of marine biologists, experts in legal medicine and crime-science specialists from around the world, she began an investigation that would finally reveal the real cause: the marks were made by cookiecutter sharks.
Uncharted waters
The study of what happens to a body between the time of death and – conditions allowing – eventual fossilisation is known as "taphonomy". For experts tasked with unravelling postmortem mysteries, the fleeting interval before flesh wastes away holds particular significance, and scientists are just beginning to look at how marine taphonomy – that's decomposition in the ocean – differs from decomposition on land.
"Marine taphonomy is growing because there is an ever-increasing number of human corpses recovered from the sea, linked with aircraft crashes, migrants' boat accidents or natural disasters," says Ribereau-Gayon.
Yet despite that growth, the field remains poorly understood. "It is far less studied than decomposition on land. This raises a major issue," adds Ribereau-Gayon.
In many cases, the marks and wounds left on bodies by marine life can resemble criminal or accidental injuries, which means that being able to identify them accurately can be critical to a forensic investigation.
But in the case of the Yemenia victims, finding out that crucial information posed an unprecedented challenge.
The extensive damage caused during the crash made it difficult to reconstruct what had happened to the bodies once they were submerged. And because some of the bites found on the victims were superimposed over injuries from the crash, they actually concealed crucial elements during the investigation.
"Accurate identification of trauma can sometimes be highly challenging, specifically when facing injury patterns that have never been reported before," explains Ribereau-Gayon.
With only sparse historical reports of cookiecutter bites on humans in existence, the team had little reference for comparison. What's more, the sheer number of bites found on the Yemenia victims also masked their real cause. Most published studies on the subject have dealt with cases involving just one or two cookiecutter bites. In total, the bodies of the Yemenia victims presented 560 of them.
"The great number of bite marks [had an] important impact on the state of the skin," adds Ribereau-Gayon. It's very possible that degradation caused by cookiecutter sharks has been underestimated in past forensic-anthropology and pathology work.
A sea of clues
The team's first substantial clue came during a size analysis of the victims' wounds. Many of them were too large and clean-edged to have been inflicted by the preliminary suspects: invertebrate animals like crabs or crayfish. The ocean's top predators had to be ruled out, too: while some of the wounds showed evidence of tooth marks, almost all of them were smaller than ten centimetres in width.
What came next was a vast search through fisheries bulletins as well as medical and marine-biology literature – which finally pointed to the elusive cookiecutter.
With their source revealed, the myriad of lesions suddenly presented the team with a unique opportunity: they were able to characterise cookiecutter bite marks – and how they affect human tissues – in greater detail than ever before.
What they discovered was that cookiecutter-inflicted injuries on humans don't always appear like the circular wound seen on the leg of seven-year-old Jack Tolley.
The research compiled by the team reflected a variety of bite shapes. "The natural diversity in sizes and features of cookiecutter sharks could contribute to the varying bite patterns," explains the team. "And it is thought that mature females (the largest individuals) probably make the largest wounds." I. brasiliensis is known to inflict slightly more circular bites than its cousin, I. plutodus, whose bites are more elliptical.
The study has produced something akin to a shark-bite reference chart, one that can now help experts conduct more accurate autopsies on bodies found at sea, and assist medical practitioners in identifying bites that appear unusual.
Future forensics cases might also benefit from techniques similar to those used in the study: examining cookiecutter bites might help investigators to establish how long a body has been in the water before being discovered. This, in turn, could help them to unravel what happened to a John or Jane Doe.
For this reason, the researchers are deeply grateful that the victims' families and local law enforcement allowed the study to take place.
Cryptic cookiecutters
Agathe Ribereau-Gayon and her colleagues may have set out to investigate a forensic mystery, but along the way, they've also managed to illuminate the behaviour and habits of a deep-dwelling shark that is seldom seen in its natural habitat.
"Only very little is known about cookiecutter sharks, including among biologists," says Ribereau-Gayon. "These sharks have a peculiar biology and are present at great depths in warm waters, so their behaviour is therefore very difficult to study."
Interestingly, this isn't the first time these sharks have caused an investigative runaround. In the 1970s, strange holes found in the rubber-covered sonar domes of US Navy submarines were initially attributed to some mysterious enemy weapon. It was later discovered that the culprit was a small shark with a "watermelon-baller" for a mouth.
Cookiecutters, sometimes delightfully likened to demonic cigars, have the largest teeth-to-body ratio of any shark species. Their signature bites are made possible by a set of jaws that works like a mealtime multitool – and at depths of up to 3,500 metres, where food is scarce, their unfussy eating habits are crucial for survival.
A pair of fleshy, suction-cup-like lips helps the animals latch onto prey; once in place, razor-sharp bottom teeth sink in, the body twists, and a chunk of flesh is "unplugged" for the taking. The food is then secured during the getaway by a set of hooked upper teeth.
For fisheries ecologist Brit Finucci, a deep-sea shark, rays and chimaera specialist who was not involved with the Yemenia study, learning about these rarely seen animals is an exciting challenge. And while forensics work falls outside her area of expertise, she found the recent study intriguing.
"I'm not sure how useful these findings would be to [wildlife] biologists," she notes, "but these observations could provide greater knowledge in species distribution, if the sharks weren't already known from an area beforehand."
We know that cookiecutter sharks traverse the full scope of the water column, but detailing their dietary habits is a difficult task. Most of the bites scientists encounter are found on surface-visiting animals like whales, large fishes and seals. Deep-sea species – goblin and megamouth sharks, for example – have occasionally shown up with missing plugs of flesh, but because these animals are hard to find, we don't really know how often cookiecutters hunt in the lowest reaches of their range.
Preliminary analysis of the bite wounds on the Yemenia victims indicated that the sharks fed in both very deep and very shallow water (perhaps even on the seafloor and at the surface). This seems to corroborate current thinking that cookiecutters regularly feed at a variety of depths – not just during their vertical migrations.
Ribereau-Gayon and the team, meanwhile, hope to broaden the scope of their study by applying what they learned at this interface of forensics and marine biology to other cases.
"Deep-sea critters are all so unique, with fascinating adaptations to their environment that we're only beginning to understand," adds Finucci.
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