Class Seisonidea consists of one marine genus, Seison, with two species (S. nebaliae and S. annulatus) which are epizoic on the crustacean Nebalia. Seisonidea are the only rotifers whose reproduction is strictly bisexual. They also possess the unusual feature of having well-developed males. In this paper we review the literature concerning biogeographical distribution, relationships with their host, and morphology, and present new information obtained with SEM and optical microscopy of both living animals and serial sections.
Some seisonid characters, such as the paired retrocerebral organ, nervous system, and trophi, suggest that they hold a primitive position within phylum Rotifera. However, they do share features with other rotifers: paired gonads and an unusual locomotory behavior with Bdelloidea; lateral antennae, cellular stomach wall, arrangement of intracytoplasmic lamina, and other characters with Monogononta. Some features are peculiar to Seison: obligatory amphimixis, endolecythal eggs, encysted spermatozoa, unique mastax, and absence of copulatory organ in males and of vitellaria in females. Reduction of the corona and absence of resistant stages in Seisonidea may be related to their habitat and to their life style. We propose a closer relationship of Seisonidea to Monogononta than previously asserted.