Seisonacea (for order) Seisonidae is a family of rotifers, found on the gills of Nebalia, a marine crustacean.

It is considered to have diverged from the other rotifers early on, and in one treatment is placed in a separate class Seisonoidea.

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.

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.