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(Goës, 1864)

Description:
? Generic features: general form slender and transparent, somewhat laterally compressed. Eyes small. Carapace strongly emarginate in the posterior median line, carpal and propodal segments of the endopod of the third to the eighth thoracic limbs fused and the fused segment divided into many small subsegments. Outer margin of exopod with strong spines; inner margin of endopod with ca. eight spines. Telson long with quadrangular, with apical cleft armed with spines. Male pleopods biramous, natatory, exopod of third pair very large. Pleural plates of first abdominal somite of female very enlarged, forming part of the marsupium (brood pouch).
? Species. General form very similar to G. sanctus. Body form generally slender. Carapace with deeply emarginate posterior margin bearing eight or nine delicate prolongations, forming a kind of fringe. On either side of this " fringe " the margin is cut into a small slit of which the posterior edge overlaps the anterior forming a small lobe.
Antennal scale short, outer margin without setae and terminating in a strong spine.
Eyes small.
Thoracic limbs: first three abdominal somites almost cylindrical, fourth somite laterally compressed, with a distinct ridge running along the mid-dorsal line. In the fifth somite this lateral compression is much more marked and the dorsal ridge forms a sharp keel which ends posteriorly in an upturned membranous process which, in dorsal view, is broad at its base and tapers to a point. The sixth somite is not quite so compressed laterally as the fifth so that in dorsal view the abdomen widens posteriorly before the insertion of the telson. The large epimeral plates of the first abdominal somite in the female are not so rounded as in G. sanctus and G. normani. They are attached to the anterior end of the somite, leaving free a greater portion of the dorsal margin than in the other species. Ventrally these plates are bent under the body somewhat sharply to cover the brood pouch, giving them a definite rhomboid shape in lateral view. The anterior edge of these plates is sometimes microscopically serrated and does not overlap the hinder margin of the carapace. The boundaries of the parts are extremely difficult to define owing to their great delicacy and transparency.
Telson equal in length to the last abdominal somite, less than three times as long as broad at the base; lateral margins armed with about eight spines which increase in size distally but the last two on each side are not so markedly larger than the others as they are in G. normani; deeply cleft to more than one-fourth of its length the cleft armed with about twenty graduated spines on each side.
Uropods with endopod extending almost to the tips of the long terminal spines of the telson, inner margin armed with about 10 very long, slender spines set somewhat irregularly among the setae; exopod slightly shorter than the endopod, with about a dozen strong spines arming the distal two-thirds of the outer margin. These spines are very delicately plumose along their inner edge and their tips are much less tapering than in G. sanctus and G. normani. On the upper surface of the endopod there is a strong, posteriorly directed spine in the region of the statocyst.

Colour:
Living specimens extremely transparent with only diffuse pigment arranged as in G. sanctus. Specimens preserved in spirit show the same polygonal markings on the carapace as have been described above in G. sanctus. Stebbing mentions that certain parts, such as the sides of the marsupial pouch and the first and third abdominal somites, show " dendritic markings " and further states that the outer lamina of the uropod displays semitransparent fine honeycomb markings.

Size:
Adult up to 21 mm long.

Habitat:
Hyperbenthic. It has been found very often in swarms near the sea bottom from close inshore out to the open sea in depths up to over 275 metres.

Remarks:
This species can readily be recognised by the peculiar fringe on the middle portion of the posterior margin of the carapace; the marked lateral compression of the fourth and fifth abdominal somites; the sharp longitudinal keel in the mid-dorsal line and the peculiar membranous spine-like prolongation of this keel at the posterior margin of the fifth abdominal somite, from which the species gets its name.
Analysis of stomach contents indicated that the species must live entirely on bottom detritus and animal food such as copepods and small amphipods. It is itself preyed upon by a great variety of fishes and has been found in quantities in the stomachs of haddock, cod, whiting, plaice, thornback, rays, mackerel, herring and other fishes.

Distribution in the North Sea:
All North Sea; widespread off the coasts of Norway, Denmark, The Netherlands and Belgium, the English Channel and the north and west coasts of France.

World distribution:
E Atlantic <26-58°N; Baltic; Mediterranean; Black Sea; coastal to upper slope.

Gastrosaccus spinifer