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Scientists have studied the predatory behavior of sharks in deep-sea ecosystems. This research provides insights into the behavior and distribution range of Pacific sleeper sharks. The research is in the journal Ocean-Land-Atmosphere Research on June 1, 2025.

For a long time, detailed observations of the Pacific sleeper shark have been scarce because these sharks are capable of diving to depths exceeding 1,000 meters. To investigate natural whale fall processes in the deep sea, researchers deployed a cow carcass at a depth of 1,629 meters on the continental slope southeast of Hainan Island in the northern South China Sea. Unexpectedly, video recordings from the deployment documented the first known occurrence of the Pacific sleeper sharks in the region, providing new insights into the species' distribution range.

The video footage has given researchers new insights into the Pacific sleeper sharks' predatory behavior. The footage shows multiple sharks attacking the cow carcass and displaying queuing behavior while they are feeding.

Scientists noted wide differences between the feeding strategies of the larger sharks (greater than 2.7 meters long) and smaller sharks (less than 2.7 meters long). The larger sharks aggressively attacked the carcass while the smaller sharks circled cautiously. The 2.7-meter threshold was used solely for distinguishing large and small individuals in this study and does not carry biological significance.

Scientists also noted that the sharks displayed distinct eye retraction movements while they were feeding. Members of the genus Somniosus, like the Pacific sleeper shark, lack a nictitating membrane on the eye. Scientists believe the eye retraction movements are likely a protective adaptation that these sharks use as a defensive mechanism during feeding.

Another behavior scientists noted on the video footage was that sharks yielded their positions to individuals approaching from behind. Scientists are not sure of the causes for this polite queuing behavior. They observe that it closely resembles the hierarchical feeding patterns observed when sharks are scavenging from large floating whale carcasses at the water's surface. However, it is also possible that the first shark departed to avoid competition with the approaching shark.

"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," said Han Tian from Sun Yat-sen University and Southern Marine Science and Engineering Guangdong Laboratory in China.

Historically, Pacific sleeper sharks have been documented in the North Pacific Ocean, ranging from Japan and the Bering Sea to the Gulf of Alaska and Baja California. These observations of the Pacific sleeper shark in the South China Sea reflect the southernmost known occurrence of this species in the Pacific Ocean Basin. This helps scientists to better understand its distribution but also raises new questions about the environmental and ecological factors that might be causing this shift.

Looking ahead, the researchers want to continue to explore how deep-sea ecosystems can sustain such large-bodied organisms. "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.

The researchers look forward to seeing comparative studies from other marine regions to collectively shed light on deep-sea biodiversity and energy flow, providing substantial biological evidence. In this particular study, all Pacific sleeper sharks they identified were female. Because the females of other species, such as the megamouth shark, are also relatively common in the South China Sea, scientists wonder whether this region might serve as a nursery ground for large sharks.

"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.