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 classSeisonoidea.[1] They have a large and elongate body with reduced corona.[2] Their muscular system is similar to that of other rotifers: they have longitudinal muscles as well as open annular muscles. Being attached for most of their life, they are semi-sessile, but are capable of detaching and crawl short distances if required.[3]
Feeding has never been observed directly, but the stomach in Seison nebaliae contained bacteria, while a substance that probably represents hemolymph of the Nebalia host was found in the stomach of Paraseison annulatus. The latter prefer to settle beneath the carapace on the gills of the host's legs, and the former is usually found on the host's carapase, trunk or legs. Their host often lives in tidal puddles with decomposing algae for a limited period of time, with oxygen content so low it sometimes reaches anoxia, which doesn't seem to bother the Seisonidae.[4]
^ abcdSorensen, M. V., H. Segers & P. Funch (2005). On a new Seison Grube, 1861 from coastal waters of Kenya, with a reappraisal of the classification of the Seisonida (Rotifera). Zoological Studies44 (1): 34–43. Fulltext
