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(G.O. Sars, 1877)

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. Carapace produced anteriorly into a more or less acutely pointed, triangular rostrum; posterior margin emarginate. Lateral margins of telson unarmed, except for one large spine marking the distal enbd of each; apex produced, rounded armed with a dense comb-like row of regular teeth. This species resembles Leptomysis gracilis so closely in its general form and the the appendages that only the following characters are given.
The integument is smooth without any trace of the scales which are so marked a feature of L. gracilis.
The rostrum is of about the same length as in L. gracilis but the sides are straight and not at all convex in the middle of their length; no notches at base of rostrum.
Antennal scale: is longer and narrower in proportion to its length than in L. gracilis. The distal segment is more than half as long as the proximal one and has from 11-16 setae on each side-the setae are unusually short.
Eyes with the eyestalks broader proximally than in L. gracilis.
Telson less constricted near the base than in L. gracilis; broader, especially towards the distal end, with no trace of the constriction which marks the insertion of the distal pair of long marginal spines, these are separated from the large apical spines by a shorter interval than in L. gracilis and there are only two or three small spines in that interval.
Uropods and thoracic limbs not so slender as in L. gracilis and statocyst not so large; spines arming inner margin of endopod of uropod very crowded proximally, extending from the proximal region of the statocyst to the tip of the endopod; more regular than in L. gracilis and increasing in length gradually to the large spine which stands just behind the apex.

Colour
The body is covered all over, even to the eyestalks, with very fine branches of brownish pigment. This network is so fine and close that the. whole body, though transparent, appears opaque and of a brown colour.
This species is less transparent than L. gracilis owing to the close network of brown pigment which colours the whole body. lt may be so dark as to appear almost black. There is no trace of distinct pigment stars on the carapace but on each side of the abdomen the branching network is particularly close giving the appearance of two blotches on each somite. Two prominent dark spots are present at the base of the telson. In the female there is a large branching spot, on each side of the marsupium. Pigment of eyes black or black with a brownish tinge.

Size
Adult up to 18 mm long, but much smaller animals may be sexually mature.

Depth range
Littoral; usually found in swarms near the coast in depths of from surface to 60 m.

Remarks
This species may readily be distinguished from L. gracilis, which it so closely resembles in general form, by the characters given above. The safest guides in distinguishing the species are the integument and the relative proportions of the antennal scale especially the armature of the distal segment, and the shape of the telson are.

Distribution in the North Sea
All North Sea, littoral, shallow waters.

World distribution
E North Atlantic, Mediterranean; coastal.

[After Tattersall and Tattersall, 1951]

Leptomysis mediterranea