Aquatic sowbugs are isopod crustaceans that are strongly dorso-ventrally flattened. The one in the photograph is about 6mm long and the head is on the left.

The aquatic sowbugs we find are brownish with darker mottling on the back, but other species are medium to dark gray. There are two pairs of antennae (one long, one short) and seven pairs of walking legs. The last segments of the abdomen are fused into a short, broad, shield-like region with two flat, forked structures that stick out at the rear. The sowbugs respire with flat gills at the base of the abdominal appendages

Aquatic sowbugs roam the bottom while feeding by scavenging, mostly on detritus. They, in turn, are eaten by crayfish.

Female isopods hold a large number of eggs in a pouch (marsupium) on the underside of the thorax. The hatchlings are held for 2-3 weeks by the female and the fat-looking female is probably what gives them their name.