G.O. Sars, 1864
— 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 of M. gibbosa is robust and compact, much smaller than M. didelphys. 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 short, anterior margin transverse, scarcely produced in the middle line into a very short, pointed rostrum which only just covers the proximal end of the first segment of the antennular peduncles; antero-lateral angles rounded; posterior border emarginate, short, leaving the whole of the last two, and part of the sixth, thoracic somites exposed in dorsal view. On the dorsal surface of the carapace, in the median line, there are two nodules which are very noticeable in lateral view, one just behind the cervical sulcus and the other near the posterior end of the carapace.
Antennal scale oval, setose all round; outer margin straight; inner margin convex; extending for about one-third of its length beyond the antennular peduncle; three times as long as broad at its widest part; a small distal suture present.
Eyes large, set somewhat apart and extending considerably beyond the lateral margins of the carapace; distal dorsal margin of the eyestalk produced in the median line into a well-marked papilla.
Thoracic limbs first and second thoracic limbs as described for the genus. Third thoracic limb short and robust with the merus longer than the tarsus. Pleopods as described for the genus.
Telson shorter and broader than in M. didelphys, especially at the apex; hollowed from above with lateral margins upturned; lateral margins armed, according to age, with eight to eighteen evenly spaced spines; apex with outer corners rounded and without spines at the angles; two, rarely three, small spines in the centre; no median cleft. The characteristic pair of black spots on the base of the telson is well marked.
Uropods short and broad; exopod four times as long as broad; outer margin straight, inner margin convex; endopod shorter, tapering distally; posterior to the large statocyst it is about half as broad as the exopod; inner margin armed near the statocyst with a row of five small spines.
Pigment in the form of minute dots and curved lines as in M. didelphys but arranged in bands of more or less density. The animals are usually of a very dark grey colour but they may be of all colour varieties from almost black to an opaque white with brown flecks. There is a branching chromatophore on each eyestalk, on the basal joint of the antennular peduncle and on each side of the marsupium.
Length 6-7 mm.
Hyperbenthic to planktonic; surface to 60 m.
This species, as the name implies, can readily be recognised by the two large nodules on the carapace. It is the smallest British species of the genus and its dark colour and sigmoid curve of the abdomen make it easy to recognise at first sight. The form of the telson varies to some extent with age. In young specimens there are fewer spines on the lateral margins and the apex is more evenly rounded than in older animals.
Mysidopsis gibbosa is a more southern form than Mysidopsis didelphys and inhabits much shallower water. Though it is known from area some miles out to sea, at or lose to the bottom. In depths of as much as 50 m, it is most abundant in coastal waters quite close inshore in depths to 20 m. Here it lives in swarms either on the bottom or swimming just clear of the bottom among seaweeds. It has been taken at the surface and appears to move upward from the bottom during the hours of darkness.
Distribution in the North Sea
Northeastern North Sea, Skagerrak, off W Norway.
E North Atlantic: 30? - 62°N; Baltic, Mediterranean; coastal to shelf.
[After Tattersall and Tattersall, 1951]