Somniosus pacificusPacific Sleeper Shark(Also: Sleeper shark)

Geographic Range

Pacific sleeper sharks (Somniosus pacificus) are native to the Nearctic, Palearctic, and Oriental regions. They inhabit coastal areas near the Hawaiian Islands, the west coast of North America, and coastal northeast Asia. In North America, their range extends along the coast as far south as the tip of the Baja California peninsula and as far north as northern Alaska. In Asia, their range includes coasts along parts of Japan, Russia, and Mongolia, predominately in the Sea of Okhotsk. (Rigby, et al., 2021)


Pacific sleeper sharks are an oceanic species inhabiting pelagic, benthic, and reef habitats. They live along the continental shelf and slopes, changing depths depending on oceanic temperature changes. At low latitudes, Pacific sleeper sharks live as deep as 2,205 m. They are epibenthic, meaning they spend most of their time along the seafloor, just above oceanic substrates. In areas at higher latitudes, with colder waters, Pacific sleeper sharks inhabit littoral regions and spend more time close to the surface of the water. (Courtney and Sigler, 2007; Ebert, et al., 2013; Hulburt, et al., 2006; Mecklenburg, et al., 2018; Rigby, et al., 2021)

  • Range depth
    0 to 2205 m
    0.00 to 7234.25 ft

Physical Description

Pacific sleeper sharks range from 370 to 700 cm in length as adults, with an average length of 456 cm. Adults reach masses of 318 to 889 kg. Females typically have shorter bodies, but are heavier than males. Male sleeper sharks also have paired external reproductive organs, called claspers. Pacific sleeper sharks have wide midsections, which remain wide toward their anterior side. Their snouts are narrow with rounded tips and their caudal regions are narrow. Their caudal fins are heterocercal, with the dorsal section being longer than the ventral section.

Pacific sleeper sharks have rough skin due to their placoid scales. These scales are also called denticles and do not grow as sharks age; instead, new denticles replace the old ones as Pacific sleeper sharks grow. Pacific sleeper sharks have eyes that are circular in shape and are often found with ectoparasites.

Juvenile Pacific sleeper sharks vary in color from light gray to dark grey, but are typically lighter in color than adults. At birth, they are estimated to be less than 43 cm long. (Benz, et al., 2004; Ebert, et al., 2013; Mecklenburg, et al., 2018; Orlov and Moiseev, 1999)

  • Sexual Dimorphism
  • female larger
  • sexes shaped differently
  • Range mass
    318 to 889 kg
    700.44 to 1958.15 lb
  • Range length
    370 to 700 cm
    145.67 to 275.59 in
  • Average length
    456 cm
    179.53 in


Pacific sleeper sharks are ovoviviparous, which means young develop from eggs, which hatch internally and are birthed live. Most female sharks carry around 300 eggs, but only around 10% of these develop into embryos. Unfertilized eggs reportedly range from 45 to 58 mm. Based on anecdotal reports, Pacific sleeper sharks are estimated to be 42 cm or less at birth. One individual captured in fishing nets was 74 cm long and was likely a young juvenile, as it still had a visible umbilical scar.

The size at which Pacific sleeper sharks reach maturity is unknown, but one mature female was 430 cm long upon capture. Pacific sleeper sharks exhibit indeterminate growth, meaning they continue to grow throughout their life, although growth rates typically slow down when individuals reach adulthood. The largest reported Pacific sleeper sharks were between 5 and 8 m long, although these measurements were estimates and not quantitatively confirmed. (Cox and Francis, 1997; Ebert, et al., 2013; Yano, et al., 2007; Yopak, et al., 2019)


There is limited information regarding the mating systems of Pacific sleeper sharks, but they likely employ internal fertilization. Males possess paired external reproductive organs, called claspers, which they likely insert into the cloaca of a female with which they are mating. It is currently unknown if Pacific sleeper sharks are monogamous within a breeding season. (Lea, 1987; Yano, et al., 2007)

Current information on the reproductive behavior of Pacific sleeper sharks is based entirely on anecdotal evidence. Breeding interval, breeding season, and gestation periods are unreported for Pacific sleeper sharks. They are known to be ovoviviparous, and young are suspected to be independent at birth. Pacific sleeper sharks are estimated to be 42 cm or less at birth, and birth mass is unknown. Age at maturity is unknown for Pacific sleeper sharks, but closely related Greenland sharks (Somniosus microcephalus) may not reach sexual maturity until they are as much as 134 years of age. Ebert et al. One study estimated size at maturity for female Pacific sleeper sharks to be around 370 cm, based on 15 captured individuals. Although males may reach maturity around the same lengths, only one adult close to this length, at 397 cm long, has been reported. In a study along coastal Alaska, all 12 males captured were juveniles measuring 162 to 333 cm in length. (Ebert, et al., 2013; Nielson, et al., 2020; Smith, et al., 2022; Yano, et al., 2007)

There is limited information regarding parental investment in Pacific sleeper sharks. Females carry developing eggs internally and give birth to live young, but likely provide no further care once their young are born. Males exhibit no parental investment beyond the act of mating. However, all information on Pacific sleeper shark reproduction is based on anecdotal evidence.


There is limited information regarding Pacific sleeper shark longevity, but other in the genus (Somniosus) are reported to live 392 +/- 120 years in the wild. Based on the estimated age of maturity, it is possible that Pacific sleeper sharks live over 200 years. There are no records of Pacific sleeper sharks being kept in captivity. (Bishop and Horning, 2019; Nielsen, 2016)


Sleeper sharks are nocturnal and thought to be opportunistic predators. They are often described as sluggish, with rates of movement reported to be around 0.25 m/s. However, their ability to feed on fast-moving fish species suggest that they can move faster if necessary. In general, Pacific sleeper sharks are solitary except for mating interactions. (Carroll, 2005; Fujiwara, et al., 2021)

Home Range

Pacific sleeper sharks conduct daily movements vertically through the water column. They remain in deep waters during the day and become more active closer to the surface or in shallower waters at night. There are no reports describing the size of home ranges or territories for Pacific sleeper sharks. (Carroll, 2005; Yano, et al., 2007)

Communication and Perception

Pacific sleeper sharks perceive their environment using visual and mechanical cues. However, because they often live in deep or murky waters, they rely less on sight and more heavily on physical and electrical stimuli to detect prey, predators, and potential mates. Pacific sleeper sharks have organs called ampullae of Lorenzini, which detect electrical signals that aquatic organisms emit. They also have a lateral line system, which detects vibrations propagating through the water from nearby movement. Pacific sleeper sharks communicate using physical stimuli when mating. During copulation, males bite their female mate and insert their claspers into the cloaca of the female. (Ebert, et al., 2013)

Food Habits

Pacific sleeper sharks are carnivorous and adults likely consume a variety of bony fish, crustaceans, mollusks, and other sharks. There is limited information regarding specific prey items of Pacific sleeper sharks, with data from only a few publications that involved nearly all juveniles. Presumably, larger individuals consume larger prey items.

In the western Bering Sea, juvenile Pacific sleeper sharks most commonly ate red squid (Berryteuthis magister), though they also ate giant grenadiers (Albatrossia pectoralis) and popeye grenadiers (Coryphaenoides cinereus). To a lesser degree, juveniles in this area also ate sponges, amphipods, snails, cephalopods, carrion from pinnipeds, and numerous types of bony fish. Specific Pacific sleeper shark diets are reported to differ slightly based on sex, geographic location, and the depth at which they spend their time.

A study on the stomach contents of 11 juvenile Pacific sleeper sharks in Alaska found giant pacific octopuses (Enteroctopus dofleini) in 72% of individuals, arrowtooth flounder (Atheresthes stomias) in 63%, unknown bony fish species in 45%, and various squid species in 36%. This study also found various flatfish species (order Pleuronectiformes) in 18% of individuals and trace evidence of flathead soles (Hippoglossoides elassodon), walleye pollocks (Theragra chalcogramma), rockfish (genus Sebastes), salmon (genus Oncorhynchus), cods (family Gadidae), shrimp in the family Crangonidae, hermit crabs in the family Pagurides, and various snail species. Other studies on Pacific sleeper sharks have mentioned direct predation on harbor seals (Phoca vitulina). (Orlov and Moiseev, 1999; Yang and Page, 1999)

  • Animal Foods
  • mammals
  • fish
  • carrion
  • mollusks
  • aquatic crustaceans
  • other marine invertebrates


Pacific sleeper sharks likely serve as prey for orcas (Orcinus orca), are known predators of most species in the genus Somniosus. Humans (Homo sapiens) also catch Pacific sleeper sharks as part of the fishing industry, although the majority of caught individuals are juveniles. (Ford, et al., 2011)

Ecosystem Roles

Pacific sleeper sharks feed on a wide range of prey including bony fish, cephalopods, and pinnipeds. Predators of these sharks include orcas (Orcinus orca) and humans (Homo sapiens).

The copepod species (Ommatokoita elongata) is a known parasite of Pacific sleeper sharks. These copepods attach to the corneas of Pacific sleeper sharks and can cause infections, edema, corneal tears, and other lesions. These parasites are common in both sexes, although potentially at higher rates in females. (Benz, et al., 2002; Orlov and Moiseev, 1999; Yang and Page, 1999)

Commensal/Parasitic Species

Economic Importance for Humans: Positive

Fisheries profit from the sale of Pacific sleeper shark livers, jaws, and fins. Although there are no data reporting if sleeper sharks are sold commercially, they are reportedly sold in countries such as Taiwan, where shark meat is consumed and sometimes used as a substitution for other animal meat. Liver oil is another popular product that can be derived from the livers of Pacific sleeper sharks. (Orlov, 2017; Rigby, et al., 2021)

  • Positive Impacts
  • food
  • body parts are source of valuable material

Economic Importance for Humans: Negative

Pacific sleeper sharks have toxins in their flesh that is reported to induce effects in humans similar to that of alcohol. Pacific sleeper sharks rely on concentrated urea to maintain osmotic homeostasis in marine environments. To reduce the negative effects of concentrated urea on their bodies, Pacific sleeper sharks create a chemical called N-trimethylamine oxide, which is what causes a seemingly drunken state when humans consume meat from Pacific sleeper sharks. (Laxson, et al., 2011)

Conservation Status

Pacific sleeper sharks are listed as a "Near Threatened" on the IUCN Red List. They have no special status on other national or international conservation lists.

Overexploitation by fisheries in the last 80 to 120 years has been the most substantial threat to Pacific sleeper sharks and other species in the genus Somniosus. Juveniles appear most susceptible to being bycatch by fisheries, likely because they prey on bony fish that are also important in the fishing industry. Pacific sleeper sharks are frequently captured in long-line fishing. Mortality rates following incidental catches and subsequent releases are unknown. It is estimated that the number of Pacific sleeper sharks throughout their distribution has fallen 29% in the last two centuries. Relatively little is known about reproduction, development, and longevity of Pacific sleeper sharks. This lack of knowledge is itself a threat to the persistence of this species, as researchers do not have enough information to implement effective conservation strategies.

Although there are no management plans in place to protect Pacific sleeper sharks specifically, they are part of two regional shark management plans: the Bering Sea and Aleutian Islands (BSAI) Fishery Management Plan and the Gulf of Alaska (GOA) Fishery Management Plan. Both of these plans have been in effect since 2011. There are further protections for sharks in Alaska, where there is a recreational fishing bag limit of one shark (of any kind) per day and just two sharks per year. Some countries, like Taiwan, have blocked deep-water dredging, thereby protecting Pacific sleeper sharks. In Hawaii, fishing is prohibited in certain areas, which protects sharks and the coastal communities in which they live. Expanded knowledge of the basic natural history of Pacific sleeper sharks, including reproductive behavior and breeding season timing, is essential to conservation efforts. (Rigby, et al., 2021)


Josie Helpard (author), Radford University, Sierra Felty (editor), Radford University, Karen Powers (editor), Radford University, Galen Burrell (editor), Special Projects.


Arctic Ocean

the body of water between Europe, Asia, and North America which occurs mostly north of the Arctic circle.

Pacific Ocean

body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.

World Map


Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.


an animal that mainly eats meat


flesh of dead animals.


uses smells or other chemicals to communicate


animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature


union of egg and spermatozoan


A substance that provides both nutrients and energy to a living thing.

indeterminate growth

Animals with indeterminate growth continue to grow throughout their lives.

internal fertilization

fertilization takes place within the female's body

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.


eats mollusks, members of Phylum Mollusca


having the capacity to move from one place to another.


specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.


active during the night


reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.


An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).


an animal that mainly eats fish


an animal which has a substance capable of killing, injuring, or impairing other animals through its chemical action (for example, the skin of poison dart frogs).


structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.


an animal that mainly eats dead animals


reproduction that includes combining the genetic contribution of two individuals, a male and a female


lives alone


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.


movements of a hard surface that are produced by animals as signals to others


uses sight to communicate


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