Seisonacea is a group of dioecious epizoic marine rotifers. The sexes are of similar morphology, with the body divided into four parts: head, neck, trunk, and foot. Both sexes have paired gonads and a functional gut. The trophi are fulcrate. To date, only four species in two genera are known for the group world-wide (Ricci et al., 1993; Sørensen et al., 2005; Leasi et al., 2012). The two genera, Seison and Paraseison, are distinguished by differences in feeding in conjunction with the way their trophi and mastax function. Paraseison has a pumping mastax with protrusible, piercing fulcrum, used to suck blood from Nebalia as an epizoic ectoparasite. Seison has a pumping mastax with non-protrusible fulcrum, and lives as a microphageous epizoic symbiont on Nebalia.