Illoricate. Dioecious, sexes of similar morphology and size. Head, trunk and foot clearly defined. Toes absent. Two ovaries without vitellarium. Males completely developed, testes paired, penis lacking. Gut functional in both sexes. Corona rudimentary. Trophi fulcrate. Epibiotic and/or ectoparasitic on Nebalia (Crustacea, Leptostraca). Exclusively marine.
Phylogenetically the Seisonidae plays a key role in the understanding of the Rotifera and related phyla. Its putative relation with the Bdelloidea in the Digononta has repeatedly been refuted in cladistic analyses. These, to the contrary, suggest a sister-group relationship between the Monogononta and Bdelloidea, as Eurotatoria. The latter hypothesis is further supported by other studies, but these additionally suggest that the Seisonidea and Acanthocephala are sister groups, or place Acanthocephala close to Eurotatoria. The recent discovery of Micrognathozoa, yet another related taxon of minute worms having a trophi-like jaw apparatus further adds to the confusion. Thus, even though little is certain about the phylogenetic position of the seisonids, it is clear that they are pivotal to understanding the evolution of rotifers.
