Amphipoda () is an order of with no carapace and generally with laterally compressed bodies. Amphipods () range in size from and are mostly or . There are more than 10,700 amphipod species 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 Talitridae such as Talitrus saltator and Arcitalitrus sylvaticus.
The head is fused to the thorax, and bears two pairs of antennae and one pair of . It also carries the mouthparts, but these are mostly concealed. Amphipods are one of the few groups to posses , which are uniquely modified feeding legs.
The thorax and abdomen are usually quite distinct and bear different kinds of legs; they are typically laterally compressed. The thorax bears eight pairs of uniramous , 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 Hemocyanin to carry oxygen in the hemolymph to the tissues. The uptake and excretion of salts is controlled by special on the antennae.
The abdomen is divided into two sections comprising three segments each: the pleosome which bears pleopod, and the urosome, which comprises a telson and three pairs of which do not form a tail fan as they do in animals such as true shrimp. Furthermore, amphipods are the only malacostracan crustaceans that possess more than one pair of uropods. Both of these limb types are usually well-developed. are present on the abdomen.
Some amphipods possess aberrant body plans, such as the elongate skeleton shrimp (Caprellidae) and Macrohectopus (Macrohectopidae).
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 fertilisation, 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 stages; the eggs hatch directly into a juvenile form, and sexual maturity is generally reached after six ecdysis. Some species have been known to eat their own after moulting.
Most species in the suborder Gammaridea are epibenthos, although they are often collected in plankton samples. Members of the Hyperiidea are all planktonic and marine; many are symbiosis of gelatinous animals, including , medusae, Siphonophorae, colonial and Ctenophora, and most hyperiids are associated with gelatinous animals during some part of their life cycle.
Some 1,900 species, or 20% of the total amphipod diversity, live in fresh water or other non-marine waters. Notably rich Endemism amphipod faunas are found in the ancient Lake Baikal and waters of the Caspian Sea basin, with over 350 species inhabitting Lake Baikal.
The 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 troglobite and Troglobiont, 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 , relatively few amphipods are parasitism on other animals. The most notable example of parasitic amphipods are the whale louse (family Cyamidae). Unlike other amphipods, these are dorso-ventrally flattened, and have large, strong claws, with which they attach themselves to . They are the only parasitic crustaceans which cannot swim during any part of their life cycle.
Ampithoe longimana has been observed to avoid certain compounds when foraging for food. In response to this avoidance, species of seaweed such as Dictyopteris membranacea or Dictyopteris hoytii have evolved to produce C11 sulfur compounds and C-9 in their bodies as defense mechanisms that specifically deter amphipods instead of deterrence to consumption by other predators.
In populations found in benthic ecosystems, amphipods play an essential role in controlling brown algae growth. The mesograzer behaviour of amphipods greatly contributes to the suppression of brown algal dominance in the absence of amphipod predators. Amphipods display a strong preference for brown algae in benthic ecosystems, but due to removal of mesograzers by predators such as fish, brown algae is able to dominate these communities over green and red algae species.
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, sex may affect cannibalistic behaviour as males cannibalised newly moulted females less than males.
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. This was found to have been caused by "sea fleas" identified as lysianassid amphipods, possibly in a feeding group. Their bites are not and do not cause lasting damage.
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 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. At the same time, Ingolfiellidea was split from Amphipoda and reclassified as order Ingolfiellida. The more recent work of Copilaş-Ciocianu et al. (2020) using analysis of molecular data 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.
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