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Author: (Risso, 1826)

Field Marks:
Short, rounded snout, two spineless, equal-sized dorsal fins, no anal fin, long ventral caudal lobe, first dorsal fin on back closer to pectorals than pelvic fins, short keels on base of caudal fin.

Diagnostic Features:
Head short, length from snout to pectoral fins from 20 to 23% total length in specimens above 70 cm total length; snout short and broadly rounded; cusps of most lower teeth large, high, and semioblique, roots low; total tooth rows 57 to 63/33 to 36. Insertion of first dorsal fin closer to pectoral bases than pelvic bases; interdorsal space equal or greater than distance from snout tip to second gill slits. A short lateral keel present on base of caudal fin. Caudal peduncle short, distance from second dorsal insertion to upper caudal origin 2 times second dorsal base or less, distance from pelvic insertions to lower caudal origin subequal to dorsal caudal margin. Lateral trunk denticles with flat, wide, crowns and horizontal cusps, giving skin a smooth texture. Vertebral column with well-defined centra. Size small, not exceeding 1.4 m.

Geographical Distribution:
Eastern North Atlantic: Madeira, France; western Mediterranean. Western Pacific: Japan.

Habitat and Biology:
A rare to sporadically common, little-known small sleeper shark of the outer continental shelves and upper slopes, occurring on or near the bottom at depths of 200 to 1000 m. Development ovoviviparous. Probably eats deepwater bottom fishes and invertebrates. This species was long B.ought to have luminous organs, but Fulgosi and Gandolfi (1983) recently showed that the structures in question are really pit organs. Hence this species agrees with the large species of Somniosus (subgenus Somniosus) in lacking light organs.

Size:
Maximum total length about 140 cm; adult males 71 cm; adult females 82 to 134 cm; size at birth between 21 and 28 cm.

Interest to Fisheries:
Minimal, caught on longlines and with bottom trawls in the eastern Atlantic and used for fishmeal and possibly for human consumption.

Remarks:
I tentatively include the Japanese Heteroscymnus longus and the Mediterranean S. bauchotae in synonymy of S. rostratus. I examined a specimen of the Japanese longus and agree with Bigelow and Schroeder (1957) and Quero (1976) that it is difficult to separate from S. rostratus. S. bauchotae was separated from S. rostratus by its rounded, truncated first dorsal free rear tip, more angular, posteriorly concave pectoral fins (less angular in S. rostratus), presence of a lateral keel on the caudal base (supposedly absent in S. rostratus), distance from second dorsal origin to lower caudal origin about twice second dorsal base (about equal to it in S. rostratus), and in having narrower lower teeth relative to their base heights (bases slightly higher than wide in S. bauchotae, vice versa in S. rostratus). S. bauchotae was described from a single specimen, 130 cm long (Quero, 1976). I suspect that the first dorsal rear tip of the holotype of S. bauchotae is abnormally truncated, possibly as the result of damage and subsequent regrowth of the fin tip. The pectoral fin shape, tooth shape, and postdorsal space length characters may be indicative of individual variation rather than species distinction. A caudal keel was pictured on a Mediterranean S.rostratus with an elongated first dorsal rear tip, broadly rounded pectoral apices, postdorsal space slightly longer than the second dorsal base, but with tooth bases higher than wide (Tortonese, 1956, fig. 100). A Japanese specimen of rostratus on hand with elongated first dorsal rear tip, postdorsal space 1.3 times second dorsal base, nearly straight posterior margins but narrowly rounded apices on its pectoral fins, and lower teeth with bases slightly higher than wide has well developed caudal keels. I suspect that the caudal keels are characteristic of S. rostratus but may have been overlooked in some accounts of the species (such as Maul, 1955). A specimen of S. pacificus on hand lacks the keels.

After the above remarks were written the writer received the paper of Fulgosi and Gandolfi (1983), which essentially duplicated his conclusions on the validity of S. bauchotae and S. longus, but based on a number of specimens of S. rostratus from the Mediterranean Sea. All of these specimens have the small caudal keels.

Type material:
Holotype: Unknown. Type Locality: Mediterranean Sea, deep water.

Little sleeper shark (Somniosus rostratus)