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Order of malacostracan crustaceans

Amphipoda (/æ//m/'/f//ɪ//p//ə//d//ə/) is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods ('/æ//m//f//ɪ//p//ɒ//d//z/) range in size from and are mostly detritivores or scavengers. There are more than 9,900 amphipod species so far described. They are mostly marine animals, but are found in almost all aquatic environments. Some 1,900 species live in fresh water, and the order also includes the terrestrial sandhoppers such as Talitrus saltator and Arcitalitrus sylvaticus.

The name Amphipoda comes, via Neo Latin amphipoda, from the Greek roots [[:wiktionary:ἀμφί 'on both/all sides' and [[:wiktionary:πούς 'foot'. This contrasts with the related Isopoda, which have a single kind of thoracic leg. Particularly among anglers, amphipods are known as freshwater shrimp, scuds, or sideswimmers.

Amphipods are found in almost all aquatic environments, from fresh water to water with twice the salinity of sea water They are almost always an important component of aquatic ecosystems, often acting as mesograzers. Most species in the suborder Gammaridea are epibenthic, although they are often collected in plankton samples. Members of the Hyperiidea are all planktonic and marine. Some 1,900 species, or 20% of the total amphipod diversity, live in fresh water or other non marine waters. Notably rich endemic amphipod faunas are found in the ancient Lake Baikal and waters of the Caspian Sea basin. The landhoppers of the family Talitridae (which also includes semi terrestrial and marine animals) are terrestrial, living in damp environments such as leaf litter. Landhoppers have a wide distribution in areas that were formerly part of Gondwana, but have colonised parts of Europe and North America in recent times. 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. In populations found in Benthic ecosystems, amphipods play an essential role in controlling brown algae growth.

Most amphipods are detritivores or scavengers, In species without the compensatory feeding ability, survivorship, fertility, and growth can be strongly negatively affected in the absence of high quality food. Because algal presence changes throughout the year in certain communities, the evolution of flexible feeding techniques such as compensatory feeding may have been beneficial to survival. The incidence of cannibalism and intraguild predation is relatively high in some species, although adults may decrease cannibalistic behaviour directed at juveniles when they are likely to encounter their own offspring. In addition to age, the sex seems to affect cannibalistic behaviour as males cannibalised newly moulted females less than males.