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(Norman, 1860)

? Generic features. Carapace with anterior margin either broadly rounded or produced forward into a very short, obtusely triangular rostrum.
Antennal scale oval with distal spine and naked outer margin.
Proximal subsegment of carpo-propodus not swollen, longer than the succeeding sub-segment and separated from the second by a transverse articulation. Endopod of the first thoracic limb with lobes from second, third and fourth segments well developed; no nail. Thoracic limbs 3-8 with tarsus 5-11-segmented, the first of which is the longest and marked by transverse articulation. Pleopods of the male, first and second pairs rudimentary; third pair biramous, with both exopod and endopod short, tapering and unsegmented; fourth pair with exopod composed of five to six elements of the type usually found in the tribe; endopod very small, two-segmented; fifth pair rudimentary as in the female.
Telson deeply cleft and exopod of uropod setose all round.
? Species. Carapace unusually narrow, the part anterior to the cervical sulcus narrower than the first four abdominal somites; anterior margin broadly rounded; anterolateral angles rounded, not forming " shoulders "; posterior border emarginate, leaving the whole of the last, and part of the seventh, thoracic somites exposed in dorsal view.
Antennular peduncle very long and slender, with the first segment longer than the third and fourth together; flagella, especially the inner, short.
Antennal peduncle slender, shorter than antennular peduncle.
Antennal scale long and narrow, extending slightly beyond the antennular peduncle; outer margin naked and terminating in a strong tooth; apex extending beyond this tooth for about one-third of the total length of the scale; a small distal suture present.
Eyes narrow, long and cylindrical; extending considerably beyond the lateral margins of the carapace; cornea occupying only the distal third of the whole organ; pigment black.
First thoracic limb with endopod limb with very well developed lobes from the second, third and fourth segments. Second thoracic limb slender, endopod short. Endopods of third to the seventh thoracic limbs very slender and rather short; carpopropodus longer than the merus and divided into nine subsegments of which the first is the largest; endopod of the eighth limb smaller than the others with only six or seven subsegments to the carpopropodus, no nail present. Pleopods of the male, first and second pairs rudimentary as in the female; third pair biramous with the unsegmented exopod shorter than the endopod and armed with one terminal plumose seta; fourth pair with extremely small two-segmented endopod; exopod long, extending to the distal end of the telson; penultimate and ultimate segments armed each with a very long barbed seta, forming a weak kind of pincers.
Telson long and slender with a marked constriction near the base, and another weaker constriction at the distal end near the apex; cleft broad and triangular, with slightly convex margins, armed with 24-28 teeth on each side; lateral margins armed with a regular row of 24-30 small spines which are somewhat inset in the region of the proximal constriction, and which cease a short distance from the tip of each apical lobe.
Uropods with the exopod long and slender; more than half as long again as the telson; endopod subequal in length to the telson, tapering and very slightly curved inward for the distal fourth of its length; inner margin armed with a very dense row of spines which show indications of being arranged in series of larger spines with graduated smaller spines between. Each of these series has its larger spines placed proximally, a reversal of the order which is usually found when series are present.

Clear and glossy with the branching chromatophores so delicate and faint that the animal appears quite colourless and transparent, except for the black eyes.

Length 18 mm.

Usually a littoral and neritic species; hyperbenthic; 0-110 metres.

The species is common around North Sea coasts and is recorded as occurring in enormous shoals both in the North Sea: waters and northern European seas. It has been taken in water of 115 m depth, but is more often found in shallow water close to the shore.

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

World distribution:
E North Atlantic: 28-63°N; Baltic; estuarine to shelf.

Schistomysis spiritus