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Hunchback amphipod
(Iphimedia gibba)
Louse amphipod
Temnophlias capensis

 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.

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

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