Amphipoda

Amphipoda
	; A gammarid amphipod (Gammarus roeseli)
Scientific classification
Domain:	Eukarya;
Kingdom:	Animalia;
Phylum:	Arthropoda;
Subphylum:	Crustacea;
Class:	Malacostraca;
Superorder:	Peracarida;
Order:	Amphipoda; Latreille, 1816
Sub-orders
	Gammaridea; Corophiidea; Hyperiidea; Ingolfiellidea

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Amphipoda (amphipods, also sometimes known as scud) is an order of animals that includes over 7,000 described species of shrimp-like crustaceans ranging from 1 to 140 millimetres (0.039 to 5.5 in) in length. Amphipod means "feet [poda] on both sides [amphi]", in Greek.

Most amphipods are marine; although a small number of species are limnic or terrestrial. Marine amphipods may be pelagic (living in the water column) or benthic (living on the ocean bottom). Pelagic amphipods are eaten by seabirds, fish, and marine mammals. Terrestrial amphipods, such as sand fleas, can often be seen amongst sand and pebbles or on beaches.

[edit] Distribution and life

Many species of pelagic amphipods are mutualistic or (usually) parasitic, living in association with jellyfish and salps. Phronima is a relatively common genus of pelagic amphipod that kills and cleans out the barrel-shaped body of a salp to live inside and raise its young.

Of the relatively few species of free-living, planktonic amphipods, the most abundant of all is Themisto gaudichaudii. Living in the Southern Ocean, this amphipod congregates in dense swarms, where it is a voracious predator of copepods and other small members of the zooplankton.

After copepods, krill and salps, which are mostly herbivorous, the carnivorous Themisto is the most abundant member of the mesozooplankton in the Southern Ocean.

In cold seas, benthic amphipods are enormously diverse and abundant. In the Southern Ocean, amphipods are the most abundant benthic crustaceans. Some are grazers, many are omnivorous, some even act as piranha-like scavengers, quickly cleaning the carcasses of dead tissue. Amphipods are one of the few animal groups frequently seen when submarines venture to the deepest parts of the oceans. Other benthic amphipods are the primary food of Gray Whales. Certain species of pelagic amphipods make vertical migrations diurnally.

A ship hull fouling species of amphipod common to Atlantic and estuarine waters is Jassa falcata.

[edit] Anatomy

Amphipod anatomy

Amphipods typically have a shrimp-like body, flattened from side to side. Unlike many crustaceans, they have no carapace over the thorax, which is not visibly divided from the abdomen in most species. The head has two well-developed antennae, and, in most cases, a pair of compound eyes. However, a few cave-dwelling species are blind and eyeless, while some deepwater species have their eyes divided into upper and lower portions, so that they effectively have four eyes in total.

Most amphipods have eight pairs of thoracic limbs. The first pair are fused at the base, and modified to act as mouthparts. The second and third pairs, or gnathopods, are enlarged and include pincer-like structures used to help gather food, while the remaining pairs are essentially unmodified. While similar patterns of thoracic limbs are found in other crustaceans, the arrangement of the six pairs of abdominal limbs is unique to the group. The first three pairs are pleopods, adapted for swimming, while the other three are uropods.

However, exceptions to many of these generalisations exist, since there are a number of highly specialised species within the order.^[1]

[edit] Biology

Amphipods have adapted to a range of different habitats, from pelagic animals swimming in the deep sea, to burrowing bottom-dwellers, and even including some terrestrial species living in moist leaf litter. Most are scavengers or detritus feeders, although a few are predators or ectoparasites on fish. A number of species are filter feeders, using hairs on their antennae or limbs to strain detritus from the water.

All amphipods breathe through gills, usually located on the inner surfaces of their thoracic limbs. As a result, even the land-dwelling forms cannot survive in dry environments, and are typically nocturnal to avoid drying out in the sun. The heart lies above the gills in the thorax, and is connected to only a relatively simple arterial system.

The male deposits his sperm into a pouch-like marsupium on the underside of the female, which she then sweeps into a brood chamber containing the eggs. Amphipods are ovoviviparous, since the female carries the eggs inside her body until they hatch. Depending on species, a female may produce anything from 2 to 750 eggs in a single clutch. They eventually hatch into immature forms closely resembling the adults; there is no distinct larval stage.^[1]