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S.I. Smith, 1874

Description:
? Generic features: antennal scale short, oval, apex round, setose all round. Third thoracic endopods large, carpus and propodus fused, swollen, undivided, with a powerful subchela. Labrum normal (= not drawn out into a spine).
Males with the pleopods rudimentary and a long, tubular genital organ. Sternal processes present on thoracic somites of male. Telson without long apical spines and a small cleft; margins convex.
Female with two pairs of incubatory lamellae arising from the last two pairs of thoracic limbs.
? Species. General form somewhat robust. Carapace short and broad in dorsal view; anterior end not produced into a definite rostrum, bluntly rounded; deeply emarginate posteriorly, leaving the last two thoracic somites exposed in dorsal view.
Antennular peduncle strong and extending forward to the distal end of the antennal scale; inner flagellum short, outer flagellum much thicker and longer than the inner; setose lobe in the male rudimentary, consisting of a small ridge, bearing a few setae. The peduncle is more robust in the male than in the female.
Antennal peduncle well developed, equal in length to the scale.
Antennal scale short, elliptical, about three and a half times as long as broad, setose all round, a small distal suture present; a prominent spine present on the outer distal corner of the sympod.
First thoracic limb moderately long with very well developed lobe on the second segment; epipod long and narrow. Second thoracic limb slender, second segment produced on the inner margin; terminal segment somewhat swollen, armed with nine or ten strong spinous spines and many long slender setae, of which a long straight row runs obliquely across the segment, no nail. Third thoracic limb with the endopod very strong and stout. Carpus and propodus fused and undivided, nearly equal in length to the merus, armed on the distal inner margin with three pairs of strong spines, dactylus small and almost square in lateral view, bearing a large strong nail which bends down between the spines of the distal end of the carpopropodus to form a prehensile " claw". Fourth to the eighth thoracic limbs slightly longer but much more slender than the third, carpopropodus six- to eight-segmented, nail small and bristle-like. Pleopods rudimentary and unmodified in both sexes.
Telson subequal in length to the last abdominal somite but much narrower, lateral margins convex distally, armed along the posterior half of their length with 11-18 equidistant spines which increase regularly in size distally to the terminal spine at the apex. This spine has no smaller spine on its inner side, such as is found in most of the other species of the genus. Cleft broadly rounded at the proximal end, lateral margins very slightly convex with 16-20 teeth extending round the whole length of the margins in adult specimens.
Uropods with endopod about one-third longer than the telson, armed with 17-19 evenly-spaced spines on the inner margin extending from the region of the statocyst to the distal extremity; exopod slightly longer than endopod.

Colour:
Males are semitranslucent and colourless; in females, the antennulae, the flagella of the antennae, the eyestalks, the thorax, the marsupial pouch and the articulations of the caudal appendages are a beautiful rose colour. Nouvel (1940, p. 4) describes the general colour as greenish yellow, the thickenings of the chitin at the joints violet, the antennae white, and the ovaries and eggs a clear green.

Size:
Adult up to 8 mm long.

Habitat:
Normally in shallow waters from surface to 60 metres. It has been taken at the surface on rare occasions. In American waters the species is found at much greater depth than those from European waters, as deep as 280 metres.

Distribution in the North Sea:
NW North Sea, off W Norway, probably absent from the southern part of the North Sea.

World distribution:
North Atlantic ?30- 61°N; Baltic; Mediterranean; coastal to ?slope.

Heteromysis formosa