Amphipods are generally laterally compressed crustaceans. The anterior part of the body is called "pereon" and the limbs of this part are "pereopods". There are 7 seven segments ("pereonites") in this part of the body. The coxae (innermost segment) of the pereiopods in most species form flattened lateral plates. The first two pairs of pereopods usually have sub-chelate claws and are called "gnathopods". The remaining periopods are used for locomotion (swimming and walking). The abdomen consists of 3 segments called "pleonites" and 3 segments forming an urosome (tail). The first 3 pairs of abdominal limbs are "pleopods" (swimming limbs) and the 3 pairs of uropods are rather reduced in size. Pleopods are absent in some species with reduced abdomen. The head usually has one pair of sessile compound eyes or eyes may be absent. The size, shape and colour of eyes are important for identification. The antennae (two pairs) are composed of a stalk (peduncle) of a few segments and a flagellum of numerous segments. The relative length and shape of the two pairs, the peduncle and flagellae are important for identification. Spines and setae may also be important for identification at species level. Some species build tubes.
The amphipod families containing invasive species in Nordic waters are: Gammaridae, Corophiidae, Caprellidae and Talitridae.
Drawings of the 4 amphipod families:
A: Family Gammaridae (Gammarus locusta)
B: Family Talitridae (Orchestia gammarellus)
C: Family Corophidae (Corophium volutator)
D: Family Caprellidae (Caprella linearis)
Drawings from Stephensen 1928.