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(G.O. Sars, 1864)

Description
— Generic features. The Genus Leptomysis is easily distinguished from most other genera by the form of the evenly rounded apex of the telson with no trace of an apical cleft. The large number of spines arming the lateral margins of the telson are arranged in groups of smaller spines between larger ones.
Eyes well developed, almost globular, with strong, triangular eyestalks and extending well beyond the lateral margins of the body.
Pleopods of the female rudimentary, in the form of slender unjointed, setose plates; those of the male well developed, biramous and natatory. Second and third segments of endopod of first thoracic limbs separate and distinct. Uropod long and slender, with very long narrow exopod without suture. Endopod of uropod with spines on inner margin only.
Marsupium consisting of three pairs of incubatory lamellae.
— Species. General form, slender and graceful. Carapace produced anteriorly into a more or less acutely pointed, triangular rostrum; posterior margin emarginate.
Carapace short, very little wider than the abdomen; emarginate posteriorly leaving the last three thoracic somites exposed in dorsal view; anterior margin produced forward into a triangular rostrum, acutely pointed and extending almost to the distal margin of the first segment of the antennular peduncle margins of the rostrum slightly convex for the anterior third of their length, then somewhat concave; well marked notch present on each side at the base of the rostrum just above the insertion of the eyestalks, anterolateral angles produced forward into short acute points.
Integument of the whole body, including the appendages and eyestalks, covered with minute oblong scales which tilt upward distally and appear in profile as small spinules so that the animal presents a very characteristic hispid appearance.
Antennal peduncle comparatively slender; first segment hollowed dorsally with the outer distal corner produced and armed with a brush of two to five plumose setae; second segment very short with the distal margin produced dorsally into a blunt rounded cone which is armed with an obliquely-set row of long curved bristles; third segment cylindrical, nearly as long as the first.
Antennal scale very long, subulate, terminal segment about one-third of length of whole scale and armed with five setae on each side and a terminal seta; peduncle slender; strong spine present on the outer distal angle of the sympod.
Eyes pyriform, large, extending to the distal margin of the second segment of the antennular peduncle and projecting considerably beyond the lateral margins of the carapace; pigment black or black with a brownish tinge.
First thoracic limb with endopod slender, lobe from the basis well developed; terminal segment broader than long and armed with strong, barbed setae; epipod slender, armed with a single very long plumose seta close to its base. Second thoracic limb longer than the first and somewhat more slender; basis expanded on the inner margin; terminal segment armed with about ten strong barbed, regular setae and a very fine bristle-like nail. Remaining thoracic limbs very slender; merus equal in length to the three-segmented carpopropodus; dactylus very small with a very long slender nail. Genital appendage of the eighth thoracic limb of the male small, conical and armed with a few curved bristles. Pleopods of the female rudimentary, in the form of simple, unsegmented plates; of the male well developed, biramous, natatory: exopod of the fourth pair longer than the endopod, with the distal three segments armed.
Telson long and narrow, linguiform, equal in length to the last abdominal somite; constricted near the base, distal to the constriction the lateral margins converge very slightly to the narrowly rounded apex; armed throughout with crowded spines which, especially distally, are arranged in series of large spines, the spaces between them being occupied with one to six smaller spines; apex armed with four spines of which the outer ones are about twice as long as the inner. In the region of the insertion of the last long marginal spine there is, as a rule, a definite constriction of the telson.
Uropods very narrow: exopod very long and slightly curved outward; endopod with large statocyst, about three-fourths of the length of the exopod, armed along the whole of the inner margin with a row of irregularly arranged unequal spines.

Colour
Body transparent and nearly colourless with only very sparse pigment, which appears as a diffused yellowish red colour and is chiefly confined to the abdomen. There is a small pigment spot on the ventral surface of each abdominal somite and on the sixth somite this is diffused upward and round the sides of the body along the middle of the telson and partly along the uropods as a reddish yellow shading. On the sides of the thorax there are a few more or less branching pigment spots of a reddish colour. Both sexes are of the same colour.

Size
Adult up to 13 mm long.

Depth range
Littoral to hyperbenthic. This temperate, neritic species is abundant in water of from 10-500 m. around North Sea coasts and the northern coasts of Europe. Though normally a bottom-living form it may be taken in mid-water or at the surface in suitable conditions. When found at the bottom it is usually taken in considerable numbers and evidently lives in swarms.

Distribution in the North Sea
All North Sea, off W Norway, Skagerrak.

World distribution
E North Atlantic 33?-68°N; Baltic; Mediterranean; coastal to slope.

[After Tattersall and Tattersall, 1951]

Leptomysis gracilis