Isopoda is an order of crustaceans that includes woodlice and their relatives. Most isopods are small greyish or whitish animals with rigid, segmented exoskeletons. They have two pairs of antennae, seven pairs of jointed limbs on the thorax, and five pairs of branching appendages on the abdomen for respiration. Aquatic species live in marine or freshwater habitats, mostly on the bottom, but some can swim for a short distance. Terrestrial forms tend to be found in cool, moist places. Around 4,500 species dwell in salt water, 500 in fresh water and another 5,000 on land. Some isopods eat dead or decaying plant and animal matter, others are grazers or strain food particles from the water around them, a few are predators, and some are parasitic, mostly on fish. Some species are able to roll themselves into a ball to conserve moisture or as a defence mechanism. The fossil record of isopods dates back to the Carboniferous period (in the Pennsylvanian epoch), at least 300 million years ago, when they lived in shallow seas. (Full article...)
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