Author: LeSueur, 1818
Field Marks:
Short to moderately long snout, no fin spines on dorsals, no anal fin, slender-cusped teeth without cusplets in upper jaw, bladelike, oblique and relatively short-cusped teeth in lower jaw, denticles with narrow, hooked, cuspidate crowns, lips not fringed and pleated, first dorsal fin on middle of back and usually behind pectoral fins, but well ahead of pelvic fins, second dorsal fin slightly smaller than first, caudal fin somewhat paddle-shaped, with a long lower lobe, size moderate large to very large.
Diagnostic Features:
Anterior nasal flaps short, not expanded into barbels; snout short to moderately long, broadly rounded to pointed and somewhat flattened, length 2/5 to less than 1/3 of head length and 2/3 to less than 2/5 of distance from mouth to pectoral origins; gill openings moderately wide, last one about as long as first four; lips thin, not fringed, pleated or suctorial; teeth strongly different in upper and lower jaws, upper small, with narrow, acute, erect cusps and no cusplets, not bladelike, lowers much larger, bladelike, interlocked, with a low to moderately high, oblique or semierect cusps and distal blade, edges serrated or not; tooth rows 35 to 63/34 to 68. Both dorsal fins spineless; first dorsal fin on middle of back, with origin sometimes extended forward as a low ridge over pectoral bases but usually well behind pectorals, insertion far in front of pelvic origins but slightly closer to pelvic bases than pectorals; second dorsal slightly smaller than first and with base 3/4 length of first dorsal base or less; origin of second dorsal varying from over anterior half of pelvic bases to somewhat posterior to pelvic free rear tips; pectoral fins with short, narrowly to broadly rounded free rear tips and inner margins, not expanded and acute or lobate; caudal fin semisymmetrical and paddle-shaped, with a relatively short upper lobe and long lower lobe, and a strong subterminal notch. No precaudal pits, or lateral keels, or midventral keels on caudal peduncle. Dermal denticles with oblique to erect, ridged hooked, cuspidate narrow crowns, not flat, depressed and blocklike. Cloaca normal, not expanded as a luminous gland. Colour medium grey to blackish, without conspicuous light fin edges.
Remarks:
There have been reviews of this genus in Bigelow and Schroeder (1957), Bass, d'Aubrey and Kistnasamy (1976), and Que'ro (1976). As noted by Que'ro (1976), the species of Somniosus fall in two very welldefined groups, which can be ranked as subgenera: Somniosus, for the two large species S. microcephalus and S. pacificus; and a group for the small species S. rostratus (and its synonyms) which Que'ro named as a new subgenus, Brevisomniosus. However, Brevisomniosus is long antedated by Rhinoscymnus Gill, 1864 (and also Heteroscymnus Tanaka, 1912), which has as its type species Scymnus rostratus Risso, 1826. Heteroscymnus was often recognized as a distinct genus before Bigelow and Schroeder (1957) synonymized it with the genus Somniosus, and it is herein ranked as a junior synonym of the subgenus Rhinoscymnus Gill, 1864.
Another species of Somniosus may be present in the eastern Atlantic but has yet to-be named. Bigelow and Schroeder (1957) and Quero (1976) noted that a Somniosus specimen in the Bocage Museum in Lisbon, Portugal, captured off Portugal, differed from all other species in having an extremely long head, long, pointed snout, and a long, slender caudal peduncle. Unfortunately this specimen was lost in the fire that recently destroyed the Bocage Museum, and no other specimens like it are in existence in other museum collections. A figure of this specimen is given here.
The treatment of species used here differs from Bigelow and Schroeder (1957), Quero (1976) and Cadenat and Blache (1981) in recognizing only Somniosus microcephalus, S. pacificus, and S. rostratus; Fulgosi and Gandolfi (1983) independently came to similar conclusions on the number of valid species in this genus.