The seisonids are unusual rotifers in many ways. The order consists of one genus, Seison with two species- S. nebaliae and S. annulatus. These species are marine and epizoic on the crustacean Nebalia (Leptostraca: Crustacea). These rotifers are also unusual in their reproductive features. They reproduce only bisexually. The males are fully developed. Some other unique characeristics of seisonids include no resting stage, encysted spermatozoa, unique mastax, and the absence of vitellaria in females. Seisonids have reduced coronas that are not used in feeding or locomotion and they have fulcrate trophi. Their phylogenetic position is not agreed upon-some argue they are the most primitive rotifers, others assert bdelloids hold this position.
For a detailed review of the biology of seisonid rotifers see Ricci et al. (1993).
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| Trophi of Seisonidea | General morphology of Seison nebaliae |
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