Pacific Sleeper Shark (Somniosus pacificus)

The Pacific sleeper shark, Somniosus pacificus, is one of the largest sharks in the North Pacific, with adults reaching up to 700 cm in length and 889 kg. Formally described in 1944 by Henry B. Bigelow and William C. Schroeder, it inhabits depths from the surface to at least 2,205 m across a range stretching from Japan and the Siberian coast to Baja California, Mexico. Its liver chemistry, tooth architecture, and reproductive biology differ markedly from those of shallow-water sharks, and it remains poorly understood: no reproductively mature Pacific sleeper shark has ever been captured in Alaskan waters. The IUCN Red List classifies it as Near Threatened, with a decreasing population trend.

Key facts

  • Scientific name: Somniosus pacificus
  • Kingdom: Animalia
  • Phylum: Chordata
  • Class: Elasmobranchii
  • Order: Squaliformes
  • Family: Somniosidae
  • Genus: Somniosus
  • Taxonomic rank: species

Taxonomy and Evolutionary History

Somniosus pacificus belongs to the family Somniosidae and the order Squaliformes, within the class Elasmobranchii. Its formal description was published in 1944 by Henry B. Bigelow and William C. Schroeder in the Proceedings of the New England Zoological Club (volume 23, pages 21–36). The holotype specimen, catalogued as MCZ: 1103-S, is held at the Museum of Comparative Zoology.

Within the genus Somniosus, the species belongs to the subgenus Somniosus (Somniosus), which comprises three large-bodied species: the Greenland shark (S. microcephalus), the Pacific sleeper shark (S. pacificus), and the southern sleeper shark (S. antarcticus). Despite their shared body plan, genetic studies using combined mitochondrial and nuclear DNA markers can distinguish S. pacificus from the Greenland shark, its morphologically cryptic sister species, which diverged from ancestral sleeper sharks during the early Quaternary.

The boundary between the two species is not sharp. Possible hybrids between S. pacificus and the Greenland shark have been documented near Baffin Bay, eastern Canada, and juvenile Greenland sharks from the Canadian Arctic have been found carrying Pacific sleeper shark mitochondrial DNA haplotypes, indicating ongoing interspecific gene flow where the two species’ ranges overlap.

Physical Characteristics

Adult Pacific sleeper sharks range from 370 to 700 cm in length, with an average of 456 cm, and reach masses of 318 to 889 kg. The body is heavily cylindrical. The snout is short and rounded. Two equal-sized dorsal fins carry no spines, and the first dorsal fin origin sits closer to the pelvic fins than to the pectoral fins. The caudal fin is asymmetrical with a well-developed ventral lobe, and the precaudal fins are small.

Coloration in life is uniformly greyish-pink with bluish-black fins; live specimens may bear white spots on the dorsal surface. The upper teeth are lanceolate, spear-like in form, while the lower teeth have short, low, strongly oblique cusps with high narrow roots. This arrangement lets the shark grasp prey with the upper jaw and saw or cut with the lower jaw.

Unlike typical shallow-water sharks, the Pacific sleeper shark’s liver oil contains no squalene. Instead, the liver contains diacylglyceryl ethers (DAGE) and triacylglycerol (TAG), both low-density compounds that maintain their fluidity at the coldest deep-sea temperatures, providing buoyancy without the squalene-based oil common to other sharks. The flesh carries high concentrations of N-trimethylamine oxide (TMAO), which the shark produces to counteract the osmotic effects of concentrated urea; when humans consume the flesh, TMAO causes effects similar to intoxication.

Sensory Biology

Because Pacific sleeper sharks typically inhabit deep or murky water, they rely less on sight and more heavily on electrical and physical stimuli. They possess ampullae of Lorenzini for detecting bioelectric fields emitted by prey, and a lateral line system for detecting vibrations propagating through the water.

Vision is further compromised in most individuals by the parasitic copepod Ommatokoita elongata, which permanently attaches to the corneas and causes infections, edema, corneal tears, and other lesions. A 2002 study found a prevalence of infection of 97% in sharks sampled from Prince William Sound, Alaska.

Distribution and Habitat

The species inhabits the North Pacific from Japan and the Siberian coast through the Bering Sea and south to Baja California, Mexico. Its vertical range extends from the surface to at least 2,205 m. Depth use varies with latitude: at lower latitudes, Pacific sleeper sharks live as deep as 2,205 m, while at higher latitudes, where surface water is colder, individuals inhabit littoral zones and spend more time near the surface. One individual has been recorded trapped in a tide pool.

A tagging study published in June 2024 in the California Fish and Wildlife Journal fitted four Pacific sleeper sharks with satellite tags off California. Those sharks, measuring 1.5 to 2.1 m, spent more than 99% of their tracked time between 200 and 600 m depth, with an overall average depth of 424 ± 84 m, at ambient temperatures of 3.3 to 11.0 °C.

Diet and Feeding

Pacific sleeper sharks are opportunistic predators and scavengers. A study of 198 stomachs found that teleosts were the dominant prey in August and cephalopods the dominant prey in May, with marine mammal remains present in 15% of stomachs regardless of season. In the Gulf of Alaska specifically, arrowtooth flounder (Atheresthes stomias) was the most important prey by weight, representing 67% of total stomach content weight and occurring in 64% of the 13 stomachs examined. Giant Pacific Octopus (Octopus dofleini) was the most important invertebrate prey, occurring in 73% of those stomachs.

Molecular genetic analysis of stomach contents identified grey whale (Eschrichtius robustus) and harbour seal (Phoca vitulina) remains. At least 70% of the cetacean material was judged to have been scavenged rather than taken alive. The species also consumes fast-swimming prey such as Pacific salmon (Oncorhynchus sp.), which are most likely taken alive.

Reproduction and Age

The Pacific sleeper shark is ovoviviparous. Females may carry up to 300 eggs, though only around 10% of these are reported to develop into embryos. Pups are estimated to be 42 cm or less at birth.

Reaching reproductive maturity appears to take decades. No reproductively mature Pacific sleeper shark has ever been captured in Alaska, and no pregnant female has ever been retained by any fishing vessel. Radiocarbon dating of eye lenses from a 14-foot female caught in the Aleutians indicated she was at least 35 years old and still reproductively immature.

Standard vertebral aging techniques, which work for most sharks, fail entirely in this species. The vertebral column of the Pacific sleeper shark lacks the calcification needed for ring counting, a limitation described by an Alaska Department of Fish and Game biologist as resembling “a weird corrugated hose-like structure.” Researchers have consequently turned to bomb radiocarbon dating of eye lenses to estimate age.

Conservation Status

The IUCN Red List classifies the Pacific sleeper shark as Near Threatened, with a decreasing population trend. The status was upgraded from Data Deficient on account of the species’ vulnerability to fishing pressure and evidence of regional population declines.

In Alaskan waters, the majority of Pacific sleeper shark mortality results from fisheries bycatch, primarily from bottom trawls and longlines. The North Pacific Fishery Management Council has highlighted conservation concerns in its stock assessment reviews. The species is included in two regional management plans: the Bering Sea and Aleutian Islands Fishery Management Plan and the Gulf of Alaska Fishery Management Plan, though neither plan targets the species specifically.

Although large-scale commercial targeting has never occurred, anecdotal reports describe fishers in southeast Alaska targeting sleeper sharks to sell their livers during a brief historical shark-liver fishery that also targeted the Pacific spiny dogfish (Squalus suckleyi).

Did you know

  • Although Ommatokoita elongata infected 97% of Pacific sleeper sharks sampled in Prince William Sound, Alaska, severely compromising vision in most individuals, research indicates the sharks continue to capture active prey, including pinnipeds, without needing acute eyesight. (animaldiversity.org)
  • The Pacific sleeper shark is suspected of preying on Steller sea lions (Eumetopias jubatus) near Alaskan rookeries, but a 2006 Journal of Fish Biology diet study of 198 stomach samples found no Steller sea lion tissue at all. Harbour seal remains were present in some stomachs and may have been consumed alive. (nsuworks.nova.edu)
  • The standard vertebral aging technique used for most sharks fails entirely for Pacific sleeper sharks. An Alaska Department of Fish and Game biologist described the vertebral column as resembling “a weird corrugated hose-like structure,” lacking the calcification required for ring counting. Researchers now use bomb radiocarbon dating of eye lenses to estimate the age of individual sharks. (apps-afsc.fisheries.noaa.gov)

Data sources

Data updated: 2026-10-26