Amphipods are malacostracan crustaceans belonging to the order Amphipoda. They are characterized by having no carapace and having compressed bodies in the lateral plane. There have been over 9,500 species of Amphipoda described, most of them living in the marine environment, with some 1,900 species living in fresh water or on land. The term amphipoda comes from new Latin form with Greek roots of “different” and “foot”, referring to the two types of thoracic legs these animals have.
Amphipoda species have their bodies composed of 13 segments, with major parts grouped into three areas with the head, thorax and abdomen. As with many other crustaceans, the head and thorax are fused into a cephalothorax which carries two pairs of antennae, the mostly concealed mouthparts, and a pair of immobile eyes.
There are many distinctive characteristics between the thorax and the abdomen of amphipods, including the legs, which are different from one part to the other. However, both body parts are laterally compressed, and they are not protected by the characteristic carapace of most crustaceans.
There are eight pairs of legs on the thorax, with the first serving as accessories that aid mouthparts in feeding, with the next four pairs being directed forward and the last 3 backwards. The abdomen of Amphipoda species is composed of two parts, with the pleosome having the swimming legs and the urosome. The urosome is composed of a telson which is forked and of three pairs of uropodes which, in contrast with most other crustacean such as shrimp, do not form a fan-shaped tail.
While most amphipods do not grow to more than 10 millimeters in size, there are documented Amphipoda species which grow to 28 centimeters in length. These specimens have been photographed in the Pacific Ocean at the depth of 5,300 meters. There have been samples assigned to Alicella Gigantea that have reconstructed the total length of the animal to 34 centimeters, with specimens deep within the ocean still to be discovered.
The smallest recorded amphipod is less than 1 millimeter in length. Apparently, the amount of dissolved oxygen in the aquatic environment in which these animals live is the limiting factor in terms of body length that can be reached. For example, amphipods that can be found in Lake Titicaca, 3,800 meters high, grow only to 22 millimeters, while in Lake Baikal, at an altitude of 455 meters, they reach 90 millimeters in length.