The ocean is full of surprises. Sometimes, as happened several years ago in Canada, mysteries appear floating in the form of drifting human feet. However, in other cases, most of them, one must descend to the depths to try to solve the mysteries. That is precisely what a group of researchers set out to do. It all began by dropping a cow carcass.
A cow at 1,600 meters. In one of the most unusual marine experiments of recent years, a group of scientists threw a dead cow to a depth of 1,629 meters on a continental slope in the South China Sea, off the Chinese island of Hainan, with the aim of simulating a whale fall and studying the behavior of deep-sea scavengers.
What they found surprised even the most experienced researchers: eight Pacific sleeper sharks (Somniosus pacificus) appeared at the site, marking the first documented observation of this species in the region. The finding not only unexpectedly expands the distribution map of this elusive shark but also provides valuable information about its behavioral patterns, feeding hierarchies, physiological adaptations, and possible geographic expansion.
An unexpected visitor. Although the Pacific sleeper shark has a wide distribution in the northern Pacific Ocean (from Japan to Alaska and south to Baja California), its detection in the waters of southern China was not only unexpected but also raises questions about the true extent of its habitat, its possible displacement due to climate change, or even the existence of a stable, yet unrecorded population in that region.
Mealtime etiquette. The images captured by the underwater cameras not only confirmed their presence but also revealed unusual behavior for large predators: a kind of turn-taking system, in which the sharks lined up to feed on the carcass, yielding to other individuals approaching from behind.
This type of "feeding etiquette," rarely observed in predatory species, suggests that the feeding order may be determined by the competitive intensity of each individual, rather than a chaotic struggle for resources, indicating a level of social organization more complex than previously suspected in these animals.
New clues. The study also documented variations in behavior according to body size. Specimens exceeding 2.7 meters in length showed much more aggressive and direct attacks on the carrion, while smaller sharks opted for cautious movements, circling the carcass before approaching.
The pattern suggests that even in an environment where food is scarce and opportunities are random, sleeper sharks may have developed a coexistence strategy with hierarchical ranks that minimizes direct conflict.
One more thing. Another notable finding was an eye retraction behavior observed during feeding. Since this species lacks a nictitating membrane (the protective "third eyelid" found in other vertebrates like cats or certain reptiles), researchers believe this retraction reflects an evolutionary adaptation to protect the eyes during bites or struggles, providing new insight into the defensive physiology of these sharks in their natural environment.
The unknown. And more, as the recordings also revealed other revealing aspects. Namely: several sharks carried visible parasites on their eyes, identified as copepods, although the species could not be precisely classified. This detail reinforces the biological parallel between Pacific sleeper sharks and their better-known relatives, Greenland sharks, which also often host parasites on their visual organs.
Apart from the sharks, the experiment attracted a surprising variety of abyssal fauna, such as snailfish and numerous amphipods, all drawn to the source of decomposing organic matter. These records confirm that the deep zones of the South China Sea not only harbor biodiversity still poorly documented but could be more productive than previously thought, contradicting the idea that tropical depths are biologically poorer than their polar counterparts.
The big question. Underlying this, the presence of these sharks raises a crucial question: is it a recent range expansion due to global warming, or has it always been part of their habitat and simply never been observed? The species is known to have appeared occasionally in regions as far away as Palau or the Solomon Islands, suggesting there may be more southern populations than the scientific literature indicates.
However, the "frequent appearance" in the southwestern South China Sea, according to the research team led by Han Tian, suggests more of a structural lack of data in a poorly explored region than a recent change in distribution pattern. In that sense, the experiment with the cow carcass has not only provided a point observation but has opened a path to revise key concepts about the marine biogeography of abyssal species.
The new finding. In February 2026, a shark (from the sleeper shark family, possibly a southern sleeper shark, a very close relative of the Pacific sleeper shark) was documented for the first time in history off the South Shetland Islands in Antarctic waters, at a depth of 490 meters.
This is the first time an elasmobranch has been filmed in the Southern Ocean, and several experts point out that warming waters may be facilitating movements of this shark family towards areas where they were not previously expected, although they also warn that they may have been there for a long time without being detected.
Knowing the depths. All these findings underscore the usefulness of simple but carefully designed experiments to obtain data on remote, inaccessible, and often poorly understood environments. The idea of simulating a whale fall with a cow not only proved effective but also demonstrated its power as an ecological magnet capable of revealing complex biological interactions.
In a context where climate change and human activity are altering ecosystems even at great depths, this type of research is crucial for understanding the invisible functioning of the deep ocean. The appearance of eight sleeper sharks where no one expected them, behaving with order, measured aggression, and sophisticated adaptive mechanisms, is further proof that the deep sea holds secrets that we are only just beginning to understand.
A version of this article was published in July 2025.
Image | Ocean-Land-Atmosphere Research (2025)
