G.O. Sars, 1877
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 that of G. sanctus but somewhat smaller and more slender; abdomen tapering posteriorly but not markedly compressed laterally in the neighbourhood of the fifth segment.
Carapace small, narrowing anteriorly, anterior margin forming a very short, blunt rostrum between the bases of the eyestalks, sides of carapace well developed, completely covering the body and the basal segments of the legs in lateral view; posterior margin of carapace deeply arcuate, usually simple but may rarely bear lobes as in G. sanctus.
Antennular peduncle with outer margin of second segment with two small blunt spines, inner margin with one long slender spine, third segment of peduncle shorter than first, armed with five spines on the inner margin; base of outer flagellum swollen in both sexes.
Antennal peduncle with second and third segments each with a single very long plumose seta on its inner distal angle; scale small, only slightly more than half as long as the antennular peduncle, outer margin naked, terminating in a small spine beyond which the rounded apex extends considerably, small distal suture present.
Antennal scale short, outer margin without setae and terminating in a strong spine.
Eyes small.
Thoracic limbs 3-8 almost precisely as in G. sanctus. Second pleopod very distinctive and characteristic for the species, the rami not developed for swimming; exopod eight-segmented, each of the four distal segments bears a single, very delicately plumose seta on the outer distal margin and there is a tiny spine on the inner margin of the fourth segment from the tip and another on the tip of the distal segment, endopod shorter than exopod, six-segmented and very characteristically sigmoid, first segment broad, bearing four or five slightly plumose setae on its outer margin, distal three segments very slender, curved and unarmed except for two very long slender plumose setae on the tip of the terminal joint. Third pleopod in the male, exopod similar to that in G. sanctus except that the distal end of the penultimate segment is swollen; endopod reduced to a single segment bearing a few small setae; on the outer side of the distal margin of the sympod there is a peculiar, almost axe-shaped process which bears no setae. Fourth and fifth pleopods in the male very small, endopod reduced to a single segment, subtriangular, the square distal end bearing a long plumose seta at each angle.
Telson somewhat longer and more slender than in G. sanctus longer than last thoracic somite, lateral margins armed with ten to eleven spines of which the distal two on each side are much longer than the others.
Uropods slightly longer than telson, endopod tapering distally, equal in length to exopod, inner margin with eight or nine long slender spines set irregularly among the setae; exopod with proximal fourth of outer margin bare, then armed with a close set row of graduated spines to the tip. These spines are very slender and delicately plumose on their inner margins whilst the tip appears to be solid and tapers to a very fine point.
Colour:
Transparent with pigment arranged as in G. sanctus.
Size:
Adult up to 11 mm long.
Habitat:
Hyperbenthic; ca. 200 metres.
Remarks:
This species is a bottom-living form and appears to live normally swimming just clear of the bottom. It is caught in such numbers in townets fishing at the bottom, but not attached to dredges which disturb the bottom, that one can infer that it does not bury itself in the same way as Gastrosaccus sanctus does. It frequents waters from fairly close inshore to open water. It is no so euryhaline as Gastrosaccus sanctus and it is not found in harbour or estuarine waters.
Distribution in the North Sea:
Maybe SW North Sea, entering from the E Channel; western North Sea.
World distribution:
E North Atlantic <26 - 61°N; Mediterranean; Black Sea; coastal, shelf.