According to a study, eight Pacific sleeper sharks (Somniosus pacificus) appeared during an experiment in the South China Sea, marking the first known occurrence of the species in this region. The sharks were drawn to a cow carcass dropped 1,629 meters (5,344 feet) deep into the sea as part of a study on the processes that unfold when large animals fall to the ocean floor.
A Deep-sea Investigation With Unexpected Results
The experiment, which aimed to simulate the arrival of a whale carcass, took place near Hainan Island, where scientists positioned the carcass on the continental slope. The unexpected visitors appeared on the video footage recorded as part of the research.
This was the first documented instance of the species in the South China Sea, and it offered intriguing insights into the sharks’ feeding behavior. The video revealed not only their predation habits but also a surprising display of queueing behavior. Sharks positioned at the front of the group gave way to those approaching from behind, suggesting that feeding priority was governed by individual competitive intensity.
Han Tian, a researcher at Sun Yat-sen University and the Southern Marine Science and Engineering Guangdong Laboratory in China, explained, “This behavior 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.”
Unbelievable Shark Behavior
The study also uncovered fascinating variations in behavior based on size. Sharks measuring over 2.7 meters (8.9 feet) were notably more aggressive when attacking the carcass, while smaller individuals showed caution and often circled the carcass deliberately. The team found that the larger sharks had a more dominant approach to feeding.
“The highly aggressive behavior of sharks observed in the South China Sea suggests that this region still harbors abundant food sources in the deep sea. But what exactly are they? This question is intriguing for both animal distribution and behavioral research,” said Tian.
In addition to these predatory behaviors, the video footage revealed a unique eye retraction behavior, believed to be a protective adaptation. Unlike many animals with a “third eyelid” or nictitating membrane, which shields the eyes during feeding, the Pacific sleeper sharks retract their eyes during such encounters.
Pacific Sleeper Sharks’ New Range
While the Pacific sleeper shark is known to inhabit waters from Japan to Alaska and even down to the Tonga Trench, this sighting raises questions about its range. The species is generally associated with colder waters, and previous sightings in the Solomon Islands and Palau were thought to represent the furthest southward extension of their distribution. The recent discovery in the South China Sea adds an additional layer of mystery.
“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,” said Tian.
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