Similar to Gammarus salinus except that the body and appendages are typically more densely setose. Urosome segments 1 to 3 with weakly rounded dorsal humps, spine groups with several long setae. Epimeral plates 2 and 3 acute distally, posterior margin with several short setules. Head with the lateral lobes broadly truncated, post-antennal sinus deep. Eyes moderately large, elongate kidney-shaped. Male antenna 1 densely setose, peduncle article 1 with five to eight ventral setal groups, article 2 with six to ten setal groups, article 3 with three to six setal groups (excluding the apical setae); accessory flagellum a little shorter than the peduncle article 2, 4 to 7-articulate; small male specimens with fewer setal groups. Male antenna 2 peduncle with numerous groups of long setae, flagellum setose, calceoli present in male, present or absent in female. Female antenna 2 peduncle article 4 with five or six setal groups.
Mandible palp article 1 with one to five apical setae, article 3 with three to five setal groups on the outer surface, ventral setae irregular. Male gnathopod 1 propodus with the palm weakly sinuous, median palmar spine well separated from the large group of angle spines, proximal posterior margin with about six setal groups. Male gnathopod 2 propodus broader than 1 and the palm less strongly oblique, median palmar spine well separated from the angle spines. Female gnathopod 1 propodus oval, palm convex, oblique; gnathopod 2 propodus subrectangular, palm almost transverse. Pereiopods 3 and 4 densely setose, setae much longer than the associated spines. Pereiopod 5 basis with the posterodistal angle freely produced; pereiopods 6 and 7 basis elongate, posterior margin with long setae, distal angle not produced but with a pair of spines and a few long setae, merus and carpus with some setae distinctly longer than the associated spines; density of setae rather variable. Uropod 3 inner ramus about three-quarters of the length of the proximal article of the outer ramus, margins with spines and long plumose setae. Telson lobes with two or three apical spines, without or with one subapical spine, one or two lateral spines, each group with several long setae.
Up to 22 mm in male, 15 mm in female.
Light greyish green or yellow with darker transverse markings, often with red spots on the lateral pleosome segments.
A common estuarine species preferring areas of very low salinity, found most commonly just below low water mark but also found intertidally. May penetrate considerable distances upstream in suitable habitats, often reaching to the limits of tidal influence.
North-East Atlantic extending into the Arctic Ocean; widespread in north-western Europe and ranging from the White Sea, northern Norway and Iceland to a southerly limit along the coast of Brittany.