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| Amphipoda | |
|---|---|
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| Gammarus roeselii | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Malacostraca |
| Subclass: | Eumalacostraca |
| Superorder: | Peracarida |
| Order: | Amphipoda Latreille, 1816 |
| Suborders | |
Amphipoda (/æmˈfɪpədə/) is anorder ofmalacostracancrustaceans with nocarapace and generally with laterally compressed bodies. Amphipods (/ˈæmfɪpɒdz/) range in size from 1 to 340 millimetres (0.039 to 13 in) and are mostlydetritivores orscavengers. 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 terrestrialsandhoppers, such asTalitrus saltator andArcitalitrus sylvaticus.
Amphipods arecharacterized by sessile eyes (withouteyestalks), nocarapace, and the presence ofgills on thecoxae.[1] Their bodies are divided into 13 segments, which can begrouped into the head,thorax andabdomen.[2]
The head is fused to the thorax, and bears two pairs ofantennae and one pair ofcompound eyes.[3] It also carries themouthparts, but these are mostly concealed.[4] Amphipods are one of the few groups to possessgnathopods, which are uniquely modified feeding legs.[5]
The thorax and abdomen are usually quite distinct and bear different kinds of legs; they are typically laterally compressed.[3] The thorax bears eight pairs ofuniramousappendages, the first of which are used as accessory mouthparts; the next four pairs are directed forwards, and the last three pairs are directed backwards.[3] Gills are present on the thoracic segments, and there is anopen circulatory system with aheart, usinghaemocyanin to carryoxygen in thehaemolymph to the tissues. The uptake andexcretion ofsalts is controlled by specialglands on the antennae.[2]
The abdomen is divided into two sections comprising three segments each:[1] the pleosome which bearsswimming legs, and the urosome, which comprises atelson and three pairs ofuropods which do not form a tail fan as they do in animals such astrue shrimp.[3] Furthermore, amphipods are the only malacostracan crustaceans that possess more than one pair of uropods.[6] Both of these limb types are usually well-developed.Epimera are present on the abdomen.[1]
Some amphipods possess aberrant body plans, such as the elongateskeleton shrimp (Caprellidae) andMacrohectopus (Macrohectopidae).
Amphipods are typically less than 10 millimetres (0.4 in) long, but the largest known amphipods are much larger;specimens ofAlicella gigantea collected from 5,300 metres (17,400 ft) deep are 27–28 cm (10.6–11.0 in) long,[7] reconstructed gut contents from ablack-footed albatross indicates 34 centimetres (13 in),[8] and photographed animals from theKermadec Trench indicate lengths of up to 34.9 centimetres (13.7 in).[9] The smallest known amphipods are less than 1 millimetre (0.04 in) long.[10] The size of amphipods is limited by the availability ofdissolved oxygen, such that the amphipods inLake 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) inLake Baikal at 455 metres (1,500 ft).[11]
Some amphipods exhibitsexual dimorphism. In dimorphic species, males are usually larger than females, although this is reversed in the genusCrangonyx.[12]
Amphipods engage inamplexus, a precopulatory guarding behavior in which males will grasp a female with their gnathopods and carry the female held against their ventral surface. 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.[12]
Mature females bear amarsupium, orbrood pouch, which holds hereggs while they arefertilised,[2] and until the young are ready to hatch.[3] As a female ages, she produces more eggs in each brood. Mortality is around 25–50% for the eggs. There are nolarval stages; the eggs hatch directly into ajuvenile form, andsexual maturity is generally reached after sixmoults. Some species have been known to eat their ownexuviae after moulting.[2]
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Amphipods are found in almost all aquatic environments, fromfresh water to water with twice thesalinity ofsea water[2] and even in theChallenger Deep, the deepest known point in the ocean.[14] They are almost always an important component of aquatic ecosystems,[15] often acting as mesograzers;[16] a monograph calls them ″integral members of global marine ecosystems″.[13]
Most species in the suborder Gammaridea areepibenthic, although they are often collected inplankton samples. Members of the Hyperiidea are all planktonic and marine;[3] many aresymbionts of gelatinous animals, includingsalps,medusae,siphonophores, colonialradiolarians andctenophores, and most hyperiids are associated with gelatinous animals during some part of their life cycle.[17]
Some 1,900 species, or 20% of the total amphipod diversity, live in fresh water or other non-marine waters. Notably richendemic amphipod faunas are found in the ancientLake Baikal and waters of theCaspian Sea basin,[18] with over 350 species inhabiting Lake Baikal.[19]
Thelandhoppers of the familyTalitridae (which also includes semi-terrestrial and marine animals) areterrestrial, living in damp environments such asleaf litter.[20] Landhoppers have a wide distribution in areas that were formerly part ofGondwana, but have colonised parts ofEurope andNorth America in recent times.
Around 750 species in 160 genera and 30 families aretroglobitic andlive in caves, and are found in almost all suitable habitats, but with theircentres of diversity in theMediterranean Basin, southeasternNorth America and theCaribbean.[21]
Amphipods are a potential means of mitigatingeutrophication inaquaculture facilities.[22]
Compared to other crustacean groups, such as theIsopoda,Rhizocephala orCopepoda, relatively few amphipods areparasitic on other animals. The most notable example of parasitic amphipods are thewhale lice (family Cyamidae). Unlike other amphipods, these are dorso-ventrally flattened, and have large, strong claws, with which they attach themselves tobaleen whales. They are the only parasitic crustaceans which cannot swim during any part of theirlife cycle.[23]
Most amphipods aredetritivores orscavengers,[2] with some beinggrazers ofalgae,omnivores orpredators[3] of smallinsects andcrustaceans.[2] Food is grasped with the front two pairs of legs, which are armed with large claws.[2] More immobile species of amphipods eat higher quantities of less nutritious food rather than actively seeking more nutritious food.[24] This is a type of compensatory feeding.[24] This behaviour may have evolved to minimise predation risk when searching for other foods.[24]Ampithoe longimana, for example, is more sedentary than other species and have been observed to remain on host plants longer.[24] In fact, when presented with both high- and low-nutrition food options, the sedentary speciesAmpithoe longimana does not distinguish between the two options.[24] Other amphipod species, such asGammarus mucronatus andElasmopus levis, which have superiorpredator avoidance and are more mobile, are better able to pursue different food sources.[24] In species without the compensatory feeding ability, survivorship, fertility, and growth can be strongly negatively affected in the absence of high-quality food.[24] Compensatory feeding may also explain the year-round presence ofA. longimana in certain waters.[25] Because algal presence changes throughout the year in certain communities, the evolution of flexible feeding techniques such as compensatory feeding may have beenbeneficial to survival.[25]
Ampithoe longimana has been observed to avoid certain compounds when foraging for food.[26] In response to this avoidance, species of seaweed such asDictyopteris membranacea orDictyopteris hoytii have evolved to produce C11 sulfur compounds and C-9oxo-acids in their bodies as defence mechanisms that specifically deter amphipods instead of deterrence to consumption by other predators.[26]
In populations found inbenthic ecosystems, amphipods play an essential role in controllingbrown algae growth.[16] The mesograzer behaviour of amphipods greatly contributes to the suppression of brown algal dominance in the absence of amphipod predators.[16] 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.[16]
The incidence ofcannibalism andintraguild predation is relatively high in some species,[27] although adults may decrease cannibalistic behaviour directed at juveniles when they are likely to encounter their own offspring.[28] In addition to age, sex may affect cannibalistic behaviour as males cannibalised newly moulted females less than males.[27]
They have, rarely, been identified as feeding on humans; inMelbourne 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 notvenomous and do not cause lasting damage.[29]
The nameAmphipoda comes, viaNeo-Latinamphipoda, from theGreekrootsἀμφί 'on both/all sides' andπούς 'foot'. This contrasts with the relatedIsopoda, which have a single kind of thoracic leg.[30] Particularly amonganglers, amphipods are known asfreshwater shrimp,scuds, orsideswimmers.[2][31] The common names relate to their swimming habits, described as "scudding" and often having the animal tilt to one side.[32]
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:[6]
A previous classification comprised the four subordersGammaridea,Caprellidea,Hyperiidea, andIngolfiellidea,[33] of which Gammaridea contained the majority of taxa, including all the freshwater andterrestrial species.[4] Gammaridea was recognised as a phylogenetically problematic group, and a new classification was developed byJames K. Lowry and Alan Myers in a series of works over 2003–2017, usingcladistic analysis of morphological characters.[34][35][1] 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.[6] At the same time, Ingolfiellidea was split from Amphipoda and reclassified as orderIngolfiellida.[1] 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.[36]
| New Amphipoda classification of Lowry and Myers[35][1] | |||
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Amphipods are thought to have originated in theLower Carboniferous. Despite the group's age, however, thefossil record of the order Amphipoda is meagre, comprising specimens of one species from theLower Cretaceous (Hauterivian)Weald Clay (United Kingdom)[37] and 12 species dating back only as far as theUpper Eocene, where they have been found inBaltic amber.[38][39]
