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(Norman, 1863)

Description
Generic features. Carapace more or less produced between the eyes into a pointed rostrum; antero-lateral angles rounded. Antennal scale lanceolate, setose all round, a small distal articulation usually present. Eyes large and well developed; somewhat flattened dorso-ventrally so that they are oval in lateral view; pigment black or brownish black.
Endopod of the first thoracic limb only six-segmented, having the third and fourth segments fused. Second thoracic limb with the normal seven segments in the endopod. Pleopods of the female rather broad with the lateral basal dilation small; those of the male well developed, biramous, natatory; exopod of fourth pair longer than the endopod, terminating in a single very strong spinous seta. Uropod broad, with the exopod considerably broader and longer than the endopod, terminating in a single very strong spinous seta.
Telson hollowed above so that it is has the shape of a trowel; entire or with incipient apical cleft; short spines along full margins fewer than 20, not arranged in series; apex entire (unarmed or with 2 small spines) or with a few small unarmed median notch.
Marsupium composed of three pairs of in incubatory lamellae of which the anterior pair is small and rudimentary.
— Species. General form robust, cephalothorax narrowing anteriorly.
Lateral margins of telson unarmed or with a few small spines; apex entire (unarmed or with 2 small spines) or with a few small unarmed median notch.
Carapace with anterior margin produced into a broad triangular rostrum with an acute apex which extends to more than half-way along the first segment of the antennular peduncle; antero-lateral angles smoothly rounded; posterior border emarginate leaving the last two, and part of the sixth, thoracic somites exposed in dorsal view.
Antennular peduncle only slightly longer than the eyes; much more robust in the male than in the female, male lobe large and very hirsute.
Antennal scale narrowly ovate, setose all round, about one-third longer than the antennular peduncle; breadth at the widest part from one-third to one-half of the length of the scale; a small distal suture present.
Eyes large, set somewhat apart, globular but slightly flattened dorsoventrally, extending considerably beyond the lateral margins of the carapace; a small finger-like papilla present on the inner dorsal side of the eyestalk; pigment black or blackish brown.
First thoracic limb robust, epipod furnished with one strong basal seta and another situated about one-fourth of the length from the base; endopod as described for the genus; nail well developed. Second thoracic limb not much longer than the first; merus long and stouter than the carpus; nail strong and curved. Remaining thoracic limbs with the endopods strong and subequal; merus considerably longer and thicker than the fused carpopropodus; on the inner side of the merus, towards the distal end, there are two groups of extremely long setae. A similar group arises on the inner distal corner of the second subsegment of the carpopropodus, dactylus very small; nail very long and slender. Genital appendage of the male short and cylindrical, armed distally with seven to eight curved bristles. Pleopods of the female broader than in Leptomysis but of the same general form; of the male well developed; exopod of the fourth pair longer than the endopod with a single, long, barbed seta at its tip.
Telson longer than the last abdominal somite, lateral margins roundly convex at the base, concave at about one-third of their length from the proximal end and then straight and converging to a very narrow truncate apex; armed with eight to twelve unevenly spaced spines, which are more crowded near the base and are not symmetrical; apex armed at the outer corners with two strong spines; no median setae. The whole telson is hollowed from above as in Leptomysis.
Uropods with exopod particularly broad outer margin straight, inner margin convex, endopod less than half as broad as the exopod and about two-thirds as long, armed on the inner margin near the statocyst with a single long spine.

Colour
Pigment in the form of small dots and the general colour varies, according to their expansion from a greyish brown to almost purple. The deepest colour is along the ventral surface, particularly in the abdomen and around the mouth. There is a branching chromatophore on each side of the brood pouch in the female and another on the base of the antennule in both sexes. On the base of the telson are two well-marked black spots which are characteristic of so many of the species of this genus. The colour varies with the depth at which the animal lives, those from shallow watcr being very dark and those from deep water very pale.

Size
Adult up to 16 mm long.

Depth range
Hyperbenthic. The species has been recorded from the bottom at depths of 20-550 m, being most abundant at 60-115 m.
It is possible that it carries out certain migratory movements away from the bottom, such as have been recorded of other mysids, but the species is somewhat slow moving and does not appear to move far from the bottom.

Remarks
This species may at once be distinguished by the form of the rostrum and by the shape and armature of the telson. Among British species of the genus the colour is a very useful guide.

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

World distribution
E North Atlantic: 44 - 68°N, Mediterranean; coastal to shelf.

[After Tattersall and Tattersall, 1951]

Mysidopsis didelphys