Seisonidae is a family of rotifers, found on the gills of Nebalia, a marine crustacean. Peculiar among rotifers, males and females are both present and equal in size. Males and females are similar with paired gonads. It is considered to have diverged from the other rotifers early on, and in one treatment is placed in a separate class Seisonoidea. They have a large and elongate body with reduced corona.