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(Van Beneden, 1861)

? 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 long and slender; cephalothorax compressed laterally and tapering slightly towards the anterior end; shorter than the last five abdominal somites; abdomen cylindrical at anterior end but tapering and compressed laterally towards the posterior end of the fifth somite so that it narrows considerably in dorsal view; last segment less compressed and broadening out in dorsal view.
Carapace short, anterior margin produced into a very short blunt rostrum between the bases of the eyestalks, sides well developed, completely covering the sides of the body. On the dorsal surface, posterior to the cervical groove, there is a deep depression in the mid-dorsal line. Posterior margin deeply emarginate exposing the last two thoracic somites; The sides of this emargination are not regular but "normally" show a peculiarity which is very rare among mysids; on each side of the cleft a deep notch separates off a broad lanceolate lobe whose tip is prolonged into a flap projecting forwards and upwards. The surface of the carapace appears to be regularly mottled and Sars (1877, p. 66) states that in the living animal the carapace is so transparent that in its lateral portions the circulation of the blood can be clearly seen and that in spirit specimens the coagulation of the blood shows up the pattern of the arrangement of the vessels, giving the carapace the appearance of being ornamented with a regular polygonal network.
Antennular peduncle long and stout, second segment short and armed on outer margin with three blunt spines, third segment nearly as long as the first, bearing a curved, finger-shaped process on the dorsal surface at the anterior end; outer flagellum swollen at the base and bearing long, twisted, flattened setae; the usual hirsute male lobe present.
Antennal peduncle long, extending to the middle of the last segment of the antennular peduncle; second segment three times as long as the third.
Antennal scale: scale shorter than the peduncle, outer margin straight, naked, terminating in a strong spine which extends considerably beyond the obliquely truncate apex with its small distal suture.
Eyes small, cylindrical and thick, cornea occupying about one-third of the eye, slightly narrower than the eyestalk.
Thoracic limbs 3-8 becoming progressively stronger and longer posteriorly, slenderly built, armed with groups of long setae on the inner margin. The number of subsegments into which the fused carpopropodus is divided increases posteriorly as follows: In the third limb, seven; in the fourth, eight; in the fifth, nine; in the sixth, eleven; in the seventh, thirteen; in the eighth, fourteen. Each subsegment bears a small brush of setae on the inner angle and a small spine on the outer angle. The terminal segment bears two or three bristles at the tip but no nail. Large genital appendage at the base of eighth limb in the male subcylindrical with irregularly lobate apex and armed with a row of setae. Pleopods in the male; sympod of first pair swollen, armed with ten to twelve long plumose setae on inner margin; exopod small, unjointed; third pair with exopod very long and slender, three times as long as the normal endopod, three small segments close to the proximal end followed by four long segments; apex armed with two unequal, strong, barbed setae and a small bristle; fourth and fifth pairs small with the exopod reduced to a single segment. Pleopods in the female very reduced, first pair with long arcuate sympod bearing one group of very long plumose setae at the proximal end and another at the distal end; exopod unsegmented, oblong with a few plumose setae; endopod triangular, tapering to a sharp point which bears a single long plumose seta. Remaining female pleopods reduced to small simple plates with a few setae.
Telson narrowing slightly distally, shorter than last abdominal somite, lateral margins armed on each side with six large, irregularly spaced spines, of which the one at the apex is the longest, apex cleft to about one-ninth of the length of the telson, cleft armed with about twelve spines on each side.
Uropods slightly longer than telson; endopod slenderer than exopod, tapering distally, armed with six long irregularly-spaced spines among the setae of the inner margin; outer margin of exopod bearing a close-set, graduated row of large spines on the distal two-thirds of its length. These spines consist of two portions, the proximal of which is armed along one margin with a close row of very fine hairs while the distal portion is tapering, non-plumose and more strongly chitinised so that it appears to be solid.

Very transparent with diffuse, faint, yellowish-red pigment at the edges of the segments, along the middle of the telson and around the mouthparts. On each side on the brood pouch there is a large branching, star-shaped pigment spot of the same diffuse colour.

Length adult female 13 mm, of male 15 mm.

Shallow waters to hyperbenthic and euryhaline; surface down to 60 metres; common at surface at night.

This species can readily be recognised by the lateral compression of the fifth abdominal somite and the less compressed last somite; the length of the spine on the outer margin of the antennal scale; the six spines on the lateral margins of the telson; the armature of the uropods and the form of the pleopods of the male. For many years the lobes on the posterior margin of the carapace were regarded as the most conclusive character whereby this species could be identified. It has now been found that, not only are they sometimes absent, but that they are present occasionally in G. normani, so that this character cannot be relied on to distinguish the species.

This species is adapting itself to waters of less salinity, that it originated from waters of higher salinity and that it is morphologically the same as the specimens from waters of lower salinity. Populations of the Black Sea differ physiologically in having an external medium of optimum salinity of 18ä as against 35ä in the Atlantic.

Distribution in the North Sea:
All North Sea; widely distributed along the coasts of Denmark, The Netherlands and Belgium, France and in the southern portion of the North Sea. The species is much less northern in its distribution than Gastrosaccus spinifer.

World distribution:
Atlantic <26-56°N, Mediterranean, Black Sea, Sea of Azov, Suez Canal; coastal to shelf.

Gastrosaccus sanctus