Some 1,900 species live in fresh water, and the order also includes the terrestrial sandhoppers such as Talitrus saltator and Arcitalitrus sylvaticus.

[6] Gills are present on the thoracic segments, and there is an open circulatory system with a heart, using haemocyanin to carry oxygen in the haemolymph to the tissues.

[4] The abdomen is divided into two parts: the pleosome which bears swimming legs; and the urosome, which comprises a telson and three pairs of uropods which do not form a tail fan as they do in animals such as true shrimp.

[8] Samples retrieved from the stomach of a black-footed albatross had a reconstructed length of 34 centimetres (13 in); it was assigned to the same species, Alicella gigantea.

Amplexus can last from two to over fifteen days, depending on water temperature, and ends when the female molts, at which point her eggs are ready for fertilisation.

[13] Mature females bear a marsupium, or brood pouch, which holds her eggs while they are fertilised,[4] and until the young are ready to hatch.

[4] There are no larval stages; the eggs hatch directly into a juvenile form, and sexual maturity is generally reached after 6 moults.

[19] The more recent work of Copilaş-Ciocianu et al. (2020) using analysis of molecular data (including 18S and 28S rRNA sequences and the protein coding COI and H3 sequences) found general support for three major groups corresponding to suborders Amphilochidea, Hyperiidea and Senticaudata, but suggests some groups need to move between Amphilochidea and Senticaudata in a taxonomic revision.

Pseudingolfielloidea Lanceoloidea Scinoidea Vibilioidea Phronimoidea Platysceloidea Colomastigoidea Pagetinoidea Podosiroidea Hyperiopsoidea Carangoliopsoidea Biancolinoidea Caspicoloidea Kurioidea Talitroidea Calliopioidea Hadzioidea Aoroidea Cheluroidea Chevalioidea Corophioidea Aetiopedesoidea Isaeoidea Microprotopoidea Neomegamphoidea Photoidea Rakirooidea Caprelloidea Bogidielloidea Gammaroidea Allocrangonyctoidea Crangonyctoidea Maxillipioidea Oedicerotoidea Liljeborgioidea Eusiroidea Amphilochoidea Leucothoidea Iphimedioidea Dexaminoidea Synopioidea Haustorioidea Alicelloidea Stegocephaloidea Lysianassoidea Aristioidea Amphipods are thought to have originated in the Lower Carboniferous.

Notably rich endemic amphipod faunas are found in the ancient Lake Baikal and waters of the Caspian Sea basin.

[29] The landhoppers of the family Talitridae (which also includes semi-terrestrial and marine animals) are terrestrial, living in damp environments such as leaf litter.

Around 750 species in 160 genera and 30 families are troglobitic, and are found in almost all suitable habitats, but with their centres of diversity in the Mediterranean Basin, southeastern North America and the Caribbean.

[33] Other amphipod species, such as Gammarus mucronatus and Elasmopus levis, which have superior predator avoidance and are more mobile, are better able to pursue different food sources.

[33] In species without the compensatory feeding ability, survivorship, fertility, and growth can be strongly negatively affected in the absence of high-quality food.

[35] The incidence of cannibalism and intraguild predation is relatively high in some species,[36] although adults may decrease cannibalistic behaviour directed at juveniles when they are likely to encounter their own offspring.

[36] They have, rarely, been identified as feeding on humans; in Melbourne in 2017 a boy who stood in the sea for about half an hour had severe bleeding from wounds on his legs that did not coagulate easily.