Seisonidae is a family of

rotifer The rotifers (, from Latin 'wheel' and 'bearing'), sometimes called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic Coelom#Pseudocoelomates, pseudocoelomate animals. They were first describ ...

s, found on the gills of '' Nebalia'', a marine

crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

. Peculiar among rotifers, they are gonochoric; males and females are both present and are equal in size. Both genders are similar with paired

gonads A gonad, sex gland, or reproductive gland is a mixed gland and sex organ that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, ...

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

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

Seisonoidea. They have a large and elongate body with reduced corona. 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. 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.

Species

Two genera with total four species belong to Seisonidae: * '' Paraseison'' Plate, 1887 ** '' Paraseison annulatus'' (Claus, 1876) — ectoparasite of ''Nebalia'' ** ''Paraseison kisfaludyi''A new species of Paraseison (Rotifera: Seisonacea) from the coast of California, USA
/ref> * '' Seison'' Grube, 1861 ** '' Seison nebaliae'' Grube, 1861 –

commensal Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit f ...

of ''Nebalia'' ** '' Seison africanus'' Sorensen, Segers & Funch, 2005 — host is unknown.

References

Pararotatoria Rotifer families {{Rotifer-stub