(G.O. Sars, 1864)
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 two 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. Body, especially in the anterior region, more slender than in either Mysidopsis didelphys or Mysidopsis gibbosa. 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 long and narrow and not inflated in the cephalic region; without dorsal nodules; very slender anteriorly but becoming wider in the region of the sixth to the eighth thoracic somites; anterior margin produced forward into a very short, obtuse, triangular rostrum which usually extends only to the bases of the eyestalks but may reach almost to the middle of the first segment of the antennular peduncle; antero-lateral angles produced and rounded, posterior border emarginate leaving the last two thoracic somites exposed in dorsal view.
Antennular peduncle more robust in the male than in the female; distal outer angle of first segment produced and tipped with a few plumose setae; second segment short, its distal margin produced dorsally into a lobe which is armed at its base with a strong spine and tipped with setae.
Antennal scale narrow, linear, 7-8 times as long as broad and twice as long as its own and the antennular peduncles; a very large spine on the outer distal corner of the sympod; a small distal suture present.
Eyes large, extending well beyond the lateral margins of the carapace; finger-like process present on the dorsal distal surface of the eyestalk.
Thoracic limbs 1-2 as described for the genus. Remaining thoracic limbs with the endopods shorter and less robust than in M. didelphys and M. gibbosa; merus longer and broader than the tarsus and armed with a tuft of particularly long setae on the inner margin close to the distal end; distal end of the carpopropodus and the small dactylus covered with a dense mass of very fine setae which completely cover and hide the long slender nail. Pleopods of the male well developed, biramous, natatory; exopod of fourth pair slightly longer than the endopod and terminating in a single long, strong, modified seta.
Telson small, similar in general shape to that of M. didelphys but with the small apex rounded and cleft in the middle by a deep, unarmed, wedge-shaped incision which is about as deep as the apex is wide; lateral margins almost straight, armed with 14-18 graduated spines which increase in size distally; each lobe at the apex armed with two large spines (the outer shorter than the inner) which form the natural culmination of the series arming the lateral margins; no median setae. The telson is hollowed from above into a trowel shape.
Uropods short and robust; exopod much wider and nearly half as long again as the endopod; endopod broad proximally with a very large statocyst, tapering rapidly and slender distally; armed on the inner distal end of the statocyst with a single strong spine.
Colour
This species differs from Mysidopsis didelphys and Mysidopsis gibbosa in being practically colourless in the dorsal part of the body. Ventrally, from the anterior end to the telson, the body is of a dark purple colour and this shows through the transparent upper part so that from above the animal appears very dark, sometimes almost black. The antennal scale and uropods are more or less decorated with the same colour but the setae and bristles are of a very striking yellowish red colour. On the first segment of the antennular peduncle, and on each side of the brood pouch in the female, there is a dark branching chromatophore. The pair of dark spots at the base of the telson, which is so characteristic a feature of the genus, is very distinct and sends out branches forward over the last abdominal somite and backward over the telson. Pigment of the eye, black or with a brownish tinge.
Size
Adult up to 8-9 mm long.
Depth range
Benthic and planktonic; 100 m.
Remarks
This species can easily be recognised in life by its very distinctive colouring. In preserved specimens the slender form of the body, especially at the anterior end, the long slender antennal scale and the form of the telson, with its distinct unarmed cleft, marks it off clearly from the other North Sea species of the genus.
Distribution in the North Sea
All North Sea Skagerrak, off W Norway.
World distribution
E North Atlantic, 40-70°N. Mediterranean; coastal to shelf.
[After Tattersall and Tattersall, 1951]