New Research Reveals the Deep-Sea Behavior of Pacific Sleeper Sharks
In a groundbreaking study published on June 1, 2025, in Ocean-Land-Atmosphere Research, scientists have uncovered fascinating insights into the elusive Pacific sleeper shark (Somniosus pacificus). The study documents their predatory behavior, social feeding patterns, and an unexpected appearance in the deep waters of the South China Sea, marking the southernmost known occurrence of this mysterious species.
📍 A Deep Dive into the Unknown
Due to their ability to dive to depths over 1,000 meters, Pacific sleeper sharks are rarely observed in their natural habitat. To study deep-sea scavenging, researchers placed a cow carcass at 1,629 meters below the surface near Hainan Island on the continental slope of the northern South China Sea.
What followed was unexpected—the video footage captured by stationary deep-sea cameras revealed multiple Pacific sleeper sharks feeding on the carcass. This was the first recorded appearance of the species in this region, drastically expanding our understanding of their distribution range.
🍴 Predatory Behavior and Feeding Strategies
The recorded footage showcased queuing behavior, where sharks appeared to take turns feeding on the carcass. Larger sharks (over 2.7 meters) displayed aggressive attacks, while smaller sharks circled the site cautiously, suggesting a hierarchy in feeding.
Interestingly, the 2.7-meter mark was only used as a classification threshold for the study—it does not represent a biological distinction between the sharks.
Researchers also noted a unique eye retraction movement during feeding. Unlike many sharks, Pacific sleeper sharks lack a nictitating membrane (a protective eyelid), and this eye withdrawal reflex likely serves as a defensive adaptation to prevent injury during aggressive feeding interactions.
🤝 Queuing Behavior in Deep Water: A Rare Social Trait?
One of the most intriguing findings was a “polite” queuing behavior, where sharks gave way to others approaching from behind. This resembles the hierarchical feeding patterns seen in sharks scavenging on floating whale carcasses at the ocean surface.
“This behavior suggests that feeding priority is determined by individual competitive intensity, even in deep-water environments,” said Dr. Han Tian from Sun Yat-sen University and the Southern Marine Science and Engineering Guangdong Laboratory.
🌍 A Shift in Distribution: What Does It Mean?
Previously, Pacific sleeper sharks were believed to inhabit only the North Pacific Ocean, including waters near Japan, the Bering Sea, Gulf of Alaska, and Baja California. Their presence in the South China Sea indicates a significant shift in habitat, possibly influenced by climate change, ocean currents, or food availability.
This discovery prompts new questions:
- What ecological factors support these large sharks in such deep regions?
- Could the South China Sea serve as a nursery ground for large-bodied sharks?
Notably, all individuals observed during the study were female, echoing similar patterns seen in other species like the megamouth shark, which is also frequently recorded in the region.
🔬 What’s Next for Deep-Sea Shark Research?
The team aims to investigate how deep-sea ecosystems support such large predatory species. “The highly aggressive behavior we observed suggests that there are still rich food sources hidden in these deep waters,” Tian remarked. “But what exactly are they? That remains a mystery.”
Researchers are calling for comparative studies in other marine regions to better understand deep-sea biodiversity, energy flow, and species distribution patterns.
📖 Citation and Reference
Han Tian. (2025). Predating Behavior of the Pacific Sleeper Shark in the Deep Waters of South China Sea. Ocean-Land-Atmosphere Research. DOI: 10.34133/olar.0095
📌 Key Takeaways:
- First recorded sighting of Pacific sleeper sharks in the South China Sea.
- Sharks displayed structured queuing behavior while feeding.
- All individuals were female, raising questions about the area being a potential nursery ground.
- Discovery significantly expands the known habitat of this elusive deep-sea predator.