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Author: Bigelow and Schroeder, 1944

Field Marks:
Short, rounded snout, heavy cylindrical body and small precaudal fins, two spineless, equalsized dorsal fins, no anal fin, long ventral caudal lobe, first dorsal fin on back closer to pelvics than pectoral fins, interdorsal space less than distance from snout tip to first gill openings, no short keels on base of caudal fin, upper teeth lanceolate, lower teeth with short, low, strongly oblique cusps and high narrow roots.

Diagnostic Features:
Head moderately long, length from snout to pectoral fins from 25 to 30% total length in specimens from 106 to 430 cm total length; snout short and broadly rounded; cusps of lower teeth short and low, strongly oblique, roots very high; total tooth rows 35 to 45/53 to 58. Insertion of first dorsal fin closer to pelvic bases than pectoral bases; interdorsal space less than distance from snout tip to first gill slits. No lateral keels present on base of caudal fin. Caudal peduncle short, distance from second dorsal insertion to upper caudal origin 1.8 times second dorsal base or less, distance from pelvic insertions to lower caudal origin less than dorsal caudal margin. Lateral trunk denticles with erect, narrow-crowns and hooked cusps, giving skin a rough, bristly texture. Vertebral column without well-defined calcified centra, notochord secondarily expanded. Size large, exceeding 4.3 m.

Geographical Distribution:
North Pacific: Japan along Siberian coast (USSR) to Bering Sea and southward to southern California (USA), and Baja California (Mexico).

Habitat and Biology:
A common boreal and temperate shark of the North Pacific continental shelves and slopes. At high latitudes with cold surface waters it ranges into the littoral and even the intertidal (one large individual was found trapped in a tide pool) as well as the surface; however in lower latitudes with temperate water it becomes a deepwater epibenthic shark, never coming to the surface and ranging down to at least 2000 m in the extreme southern end of its range (off southern California and Baja California).

Development is probably ovoviviparous, but pregnant females have yet to be found, and for some reason (such as segregation of pregnant females beyond the usual fisheries gear that captures these sharks or extremely low fecundity with a small fraction of adult females pregnant at any one time) are rare as in the closely related Greenland shark (S. microcephalus). However, adult females with up to 300 large eggs have been occasionally taken.

These sharks feed on a wide variety of surface and bottom animals, including flatfishes, Pacific salmon, rockfishes, harbor seals, octopuses, squids, crabs, tritons, and carrion. It is not known if seals and fast-swimming pelagic fish such as salmon are captured alive by these lumbering, sluggish sharks or are picked up as carrion. The small mouths andlong heads and oral cavities of these sharks suggest that they are powerful suction feeders, but this has yet to be observed. Pacific sleeper sharks commonly are attracted to traps set at great depths for sablefishes (Anoplopoma), and get trapped themselves or else eat catch and bait-can and escape.

Size:
Maximum captured and measured an adult female of 430 cm total length, but larger individuals estimated at 7 or more metres long have been photographed in deep water; another adult female was 3.7 m long.

Interest to Fisheries: Unknown.

Type material:
Holotype: Presumably Museum of Comparative Zoology, Harvard, MCZ, 1910 mm, immature male. Type Locality: Sagami Sea, Japan.

Pacific sleeper shark (Somniosus pacificus)