Slaters Pillbugs and Woodlice: Isopoda

Isopods come in a variety of shapes and sizes. Most species have long bodies and are somewhat flat from top to bottom. They are mostly small, ranging from 0.02 to 0.6 inches (0.5 to 15 millimeters) in length. However, the largest species, Bathynomus giganteus, is an ocean bottom-dwelling creature that measures up to 19.7 inches (500 millimeters).

All isopods have three major body regions: head, thorax, and abdomen. Both pairs of antennae are unbranched, or uniramous (YU-neh-RAY-mus). The first pair is usually small, while the second is long and well developed. Unlike most crustaceans, the compound eyes, if present, are not set on stalks. Each compound eye has multiple lenses. The jaws, or mandibles, are uniramous and quite variable. Depending on the species, they are used to grind plant tissues, slice flesh, or pierce the tissues of living animals.

The first thoracic segment is tightly joined, or fused, to the head. Its uniramous limbs are called maxillipeds (mack-SIH-leh-pehds). Maxillipeds are thoracic limbs that work together with the mouthparts. Isopods do not have a shieldlike carapace that covers the head and thorax. Most species have seven pairs of walking legs called pereopods (PAIR-ee-oh-pawds). The pereopods are usually short, but in some species they are long and spiderlike. Plates on the underside of the thorax form a brood pouch, or marsupium (mar-SOUP-ee-uhm). The marsupium is where the eggs are kept and young hatch.

The six-segmented abdomen of terrestrial (te-REH-stree-uhl) species, or isopods that live on land, have two or more pairs of appendages called pleopods (PLEE-oh-pawds). The pleopods are white, egg-shaped, and are used for breathing. These species also take oxygen into their bodies directly through their external skeleton, or exoskeleton. The abdomen of all species ends with a pair of biramous (BY-ray-mus), or branched, uropods (YUR-oh-pawds), or tail appendages. Between the uropods is a taillike segment called the telson. The telson is tightly joined, or fused, to one or more abdominal segments.

Parasitic isopods, species that spend most or all of their lives feeding and living on fish and other crustaceans in the ocean, look different from other species. The females have wide thoracic segments, while segments that make up the abdomen tend to be much narrower. Males have body segments of similar width and are more egg-shaped in outline, similar to pillbugs. Both pairs of antennae of parasitic isopods are usually very small.