Amphipoda () is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods () range in size from 1 to 340 millimetres (0.039 to 13 in) and are mostly detritivores or scavengers. More than 10,700 amphipod species are currently recognized. They are mostly marine animals but are found in almost all aquatic environments. Some 2,250 species live in fresh water, and the order also includes the terrestrial sandhoppers, such as Talitrus saltator and Arcitalitrus sylvaticus.
Description
Anatomy
thumb|Anatomical diagram of the [[gammaridean amphipod Leucothoe incisa]]
Amphipods are characterized by sessile eyes (without eyestalks), no carapace, and the presence of gills on the coxae.
The head is fused to the thorax, and bears two pairs of antennae and one pair of compound eyes. It also carries the mouthparts, but these are mostly concealed.
The thorax and abdomen are usually quite distinct and bear different kinds of legs; they are typically laterally compressed. reconstructed gut contents from a black-footed albatross indicates , and photographed animals from the Kermadec Trench indicate lengths of up to . The smallest known amphipods are less than long. The size of amphipods is limited by the availability of dissolved oxygen, such that the amphipods in Lake Titicaca at an altitude of can only grow up to , compared to lengths of in Lake Baikal at .
Some amphipods exhibit sexual dimorphism. In dimorphic species, males are usually larger than females, although this is reversed in the genus Crangonyx.
Mature females bear a marsupium, or brood pouch, which holds her eggs while they are fertilised, They are almost always an important component of aquatic ecosystems, often acting as mesograzers; a monograph calls them ″integral members of global marine 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;
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, with over 350 species inhabiting Lake Baikal.
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 live in caves, and are found in almost all suitable habitats, but with their centres of diversity in the Mediterranean Basin, southeastern North America and the Caribbean.
Amphipods are a potential means of mitigating eutrophication in aquaculture facilities.
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.
Foraging behaviour
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.
In populations found in benthic ecosystems, amphipods play an essential role in controlling brown algae growth. although adults may decrease cannibalistic behaviour directed at juveniles when they are likely to encounter their own offspring. In addition to age, sex may affect cannibalistic behaviour as males cannibalised newly moulted females less than males.
Nomenclature
The name Amphipoda comes, via Neo-Latin ', from the Greek roots 'on both/all sides' and '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. The common names relate to their swimming habits, described as "scudding" and often having the animal tilt to one side.
Taxonomy
Over 10,700 species of amphipods are currently recognised. The higher taxonomy of Amphipoda has been thoroughly rearranged in the 21st century, and currently comprises six suborders:
- Amphilochidea (89 families; 700 genera; 4,308 species)
- Colomastigidea (2 families; 3 genera; 60 species)
- Hyperiidea (35 families; 76 genera; 284 species)
- Hyperiopsidea (3 families; 5 genera; 15 species)
- Pseudingolfiellidea (1 families; 1 genera; 4 species)
- Senticaudata (109 families; 983 genera; 6,007 species)
<gallery widths=200 heights=160>
File:Lepidepecreum longicornis.jpg|Lepidepecreum longicorne (Amphilochidea: Lysianassidae)
File:Hyperia galba.jpg|Hyperia galba (Hyperiidea: Hyperiidae)
File:Pariambus typicus.jpg|Pariambus typicus (Senticaudata: Caprellidae)
</gallery>
A previous classification comprised the four suborders Gammaridea, Caprellidea, Hyperiidea, and Ingolfiellidea, of which Gammaridea contained the majority of taxa, including all the freshwater and terrestrial species. Gammaridea was recognised as a phylogenetically problematic group, and a new classification was developed by James K. Lowry and Alan Myers in a series of works over 2003–2017, using cladistic analysis of morphological characters. It started with breaking up and replacing Gammaridea. The largest of the new suborders, Senticaudata, comprises over half of the known amphipod species, including practically all freshwater taxa.
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! style="background:#F0F2F5" | New Amphipoda classification of Lowry and Myers and 12 species dating back only as far as the Upper Eocene, where they have been found in Baltic amber.