Scientists were left gobsmacked by what they saw after dropping a cow carcass deep into the ocean. Despite having mapped merely a quarter of our planet’s ocean floor, the vast and mysterious deep sea is teeming with extraordinary life.
This was precisely what researchers discovered during a daring experiment in the depths of the South China Sea, stretching from Singapore all the way to the Strait of Taiwan. The team submerged a cow carcass 1,629 metres down near Hainan Island to observe the ensuing underwater activity. To their bewilderment, a band of elusive Pacific sleeper sharks, previously unrecorded in these parts, made an unforeseen appearance to dine on the sunken offering.
What added to the researchers’ surprise was the observation of an orderly predatory practice that seemed akin to queuing. Detailed in the published Ocean-Land-Atmosphere Research study, scientists noticed that the front-liners among the sharks would make way for newcomers approaching the carcass from behind.
Han Tian from Sun Yat-sen University and Southern Marine Science and Engineering Guangdong Laboratory in China, reflected: “This behaviour suggests that feeding priority is determined by individual competitive intensity, even in deep-water environments, reflecting a survival strategy suitable for non-solitary foraging among Pacific sleeper sharks.”
The elusive Pacific sleeper shark was believed to inhabit the North Pacific from Japan to Mexico, lurking at depths of up to 2,000m near continental slopes and shelves.
Their diet is diverse, feasting on both surface and bottom-dwelling creatures like crabs, salmon, octopus, rockfish, and squid, though it remains a mystery whether they hunt seals or scavenge them.
Tagging studies in the Northeast Pacific have revealed that some sleeper sharks exhibit vertical movements, ascending and descending over 200 meters per hour.
During daylight hours, these sharks retreat below the photic zone, where sunlight penetrates the ocean, only to rise to the surface under the cover of darkness.
Scientists noted that larger sharks, those exceeding 8.9 feet, showed more aggressive behaviour towards the carcass than their smaller counterparts, which displayed circling behaviour.
Researcher Han suggests this aggression might hint at plentiful food sources in the area, yet what exactly constitutes their diet remains an “intriguing” puzzle.
The study also observed that while feeding, the sharks retract their eyes, likely as a “protective adaptation” since they lack the nictitating membrane found in other species.
Additionally, it was noted that some of these sharks, relatives of the Greenland shark, host parasites similar to copepods, although the exact species wasn’t identified.
Discussing the habitat of the elusive sharks, Han remarked: “Although Pacific sleeper sharks have also been found in the deep waters of their typical distribution range in the North Pacific, their frequent occurrence in the southwestern region of the South China Sea suggests that our understanding of this population remains significantly limited.”