Temporal range: Eocene–Recent
Latreille, 1816 
Amphipoda is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. The name amphipoda refers to the different forms of appendages, unlike isopods, where all the thoracic legs are alike. Of the 7,000 species, 5,500 are classified into one suborder, Gammaridea. The remainder are divided into two or three further suborders. Amphipods range in size from 1 to 340 millimetres (0.039 to 13 in) and are mostly detritivores or scavengers. They live in almost all aquatic environments; 750 species live in caves and the order also includes terrestrial animals and sandhoppers such as Talitrus saltator.
Etymology and names
The name Amphipoda comes, via the New Latin amphipoda, from the Greek roots ἀμφί ("different") and πούς ("foot"), in reference to the two kinds of legs that amphipods possess. 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.
No panegyrist of the Amphipoda has yet been able to evoke anything like popular enthusiasm in their favour. To the generality of observers they are only not repelled because the glance which falls upon them is unarrested, ignores them, is unconscious of their presence.
The thorax and abdomen are usually quite distinct and bear different kinds of legs; they are typically laterally compressed, and there is no carapace. The thorax bears eight pairs of uniramous appendages, the first of which are used as accessory mouthparts; the next four pairs are directed forwards, and the last three pairs are directed backwards. 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. The uptake and excretion of salts is controlled by special glands on the antennae.
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.
Amphipods are typically less than 10 millimetres (0.39 in) long, but the largest recorded living amphipods were 28 centimetres (11 in) long, and were photographed at a depth of 5,300 metres (17,400 ft) in the Pacific Ocean. Samples from the Atlantic Ocean with a reconstructed length of 34 centimetres (13 in) have been assigned to the same species, Alicella gigantea. The smallest known amphipods are less than 1 millimetre (0.04 in) long. The size of amphipods is limited by the availability of dissolved oxygen, such that the amphipods in Lake Titicaca at an altitude of 3,800 metres (12,500 ft) can only grow up to 22 millimetres (0.87 in), compared to lengths of 90 millimetres (3.5 in) in Lake Baikal at 455 metres (1,500 ft).
Reproduction and life cycle
Mature females bear a marsupium, or brood pouch, which holds her eggs while they are fertilised, and until the young are ready to hatch. As a female ages, she produces more eggs in each brood. Mortality is around 25%–50% for the eggs. There are no larval stages; the eggs hatch directly into a juvenile form, and sexual maturity is generally reached after 6 moults. Some species have been known to eat their own exuviae after moulting.
Diversity and classification
Amphipods are difficult to identify, due to their typically small size, subtle differences in structures, such as mouthparts or limbs, and the fact that they sometimes require dissection to identify those differences. As a result, ecological studies and environmental surveys often lump all amphipods together. Carolus Linnaeus described two species of amphipods in the tenth edition of his Systema Naturae, which is defined as the starting point for zoological nomenclature. His descriptions (such as that for Gammarus pulex: "Cancer macrourus articularis, manibus adactylis, cauda attenuata spinis bifidis") were, however, "very poor", and could apply to "nearly every species of amphipod".
Around 7,000 species of amphipods have so far been described, and placed in three or four suborders. One suborder, Gammaridea, contains more than 5,500 species, including all the freshwater and terrestrial species. Suborder Ingolfiellidea contains around 40 species in 2 families, and the group is sometimes treated among the Gammaridea, rather than as a suborder in their own right.
The classification of the Amphipoda is not yet settled, with the relationships within the suborder Gammaridea suffering the most confusion. The classification given here, from the rank of suborder down to superfamily, follows that of Martin & Davis, except that superfamilies are recognised here within the Gammaridea. An alternative classification has been developed in the works of Myers & Lowry (2003, 2013) where a suborder Senticaudata is being split off the Gammaridea based on the occurrence of strong apical setae on the uropods.
Amphipods are thought to have originated in the Lower Carboniferous. Despite the group's age, however, the fossil record of the order Amphipoda is meagre, comprising specimens of 11 species dating back only as far as the Upper Eocene, where they have been found in Baltic amber.
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. 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. Many are symbionts of gelatinous animals, including salps, medusae, siphonophores, colonial radiolarians and ctenophores, and most hyperiids are associated with gelatinous animals during some part of their life cycle.
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 Gondwanaland, 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.
Compared to other crustacean groups, such as the Isopoda, Rhizocephala or Copepoda, relatively few amphipods are parasitic on other animals. The most notable example of parasitic amphipods are the whale lice (family Cyamidae); unlike other amphipods, these are dorso-ventrally flattened, and have large, strong claws, with which they attach themselves to baleen whales. They are the only parasitic crustaceans which cannot swim during any part of their life cycle.
Most amphipods are detritivores or scavengers, with some being grazers of algae, omnivores or predators on small insects and crustaceans. Food is grasped with the front two pairs of legs which are armed with large claws. The incidence of cannibalism and intraguild predation is relatively high in some species. There is evidence that adults may decrease cannibalistic behavior directed at juveniles when it is likely they could encounter their own offspring.
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