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Sexton, 1939

Urosome segments 1 to 3 without dorsal humps; spine groups small with a few short setae. Epimeral plates 2 and 3 quadrate with the distal angle slightly produced, posterior margin with a few long setae. Head with the lateral lobes broadly truncated, post-antennal sinus deep; eyes moderately large, kidney-shaped. Antenna 1 shorter than antenna 2 in male but rather variable, in some females antenna 1 may be as long as, or longer than, antenna 2; peduncle sparsely setose, flagellum slender; accessory flagellum shorter than the peduncle article 2, up to 6-articulate. Male antenna 2 peduncle with numerous fan-like groups of long setae, flagellum also densely setose, inner margin of peduncle and proximal part of the flagellum in male in summer with long fine curled setae; in winter these are straight; calceoli absent. Antenna 2 in female much smaller and more slender, setose but without curled setae; peduncle article 4 with two to four setal groups.
Mandible palp article 1 devoid of setae, article 3 with two setal groups on the outer surface (occasionally only one in female), ventral setae irregular. Male gnathopod 1 propodus oval, palm oblique and weakly sinuous, a large truncated median spine well separated from the group of angle spines, proximal posterior margin with about four setal groups. Male gnathopod 2 propodus broader than in gnathopod 1, palm generally similar, posterior margin with seven or eight setal groups. Female gnathopod 1 propodus oval, palm oblique; gnathopod 2 propodus subrectangular, palm almost transverse. Pereiopods 3 to 4 and 6 and 7 in male, merus, carpus and propodus with long fine curled setae on the inner surface, as well as long marginal setae and stout spines. Pereiopod 5 basis with the distal angle freely produced; pereiopods 6 and 7 basis elongate, posterior margin with many long setae, distal angle not produced but with a pair of slender spines. Pereiopods in female without fine curled setae. Male uropod 3 inner ramus slender and about three-quarters of the length of the outer ramus, margins spinose and setose, including many plumose setae. Female uropod 3 with spines elongate, setae much less dense than in male. Telson lobes with two apical spines with long setae, one or two subapical setae, one lateral spine with one or two setules.

Up to 12 mm in male, 10 mm in female.

In living specimens, the pattern is very distinctive, pale green or yellowish in male, rather more blue in female, with marked transverse bands of pale green with deep blue or black with gold.

Found in coastal brackish waters, and fresh water of high ion content, in lakes and rivers.

Introduced into Europe from North America. Baltic, Germany, The Netherlands, Britain and Ireland.

Gammarus tigrinus