Diagnosis_Genus: Oodinium Chatton 1912. External parasite of tunicates, chaetognaths and possibly polychaetes. The trophont is sac-like and is attached to the host by a stout peduncle with a very large, flat attachment surface closely adhering to but not penetrating into the epithelial cells of the hosts. The cell membrane of the attachment surface has numerous thin and long invaginations serving for increased absorption of the nutrients by osmotrophy. There are no chloroplasts, the trophont's theca has plates filling its alveoli and constituting together the dinoflagellate armour of the cell. There are no starch grains. The nucleus contains stout coiled chromosomes only in young individuals. The dinospore is a gymnospore. (Lom 1981).
Diagnosis_Species: Horiguchi & Ohtsuka 2001. Ectoparasite on chaetognath, Sagitta crassa. Vegetative cell (trophont) oval or rod-shaped, 30-150 µm in length; cytoplasm yellow-brown, granular in appearance; peduncle bulbous to rod-shaped, 6-500 µm in length, almost transparent, with many longitudinal striations; nucleus in mature trophont non-dinokaryotic, hemispherical in shape, situated in the upper extremity of the cell; nucleus in immature trophont dinokaryotic, oval; thecal plates present; no chloroplasts and eyespot present. Trichocysts observed in trophont. The pusule is located near the peduncle. Sporogenesis not observed.
The tip of the peduncle is embedded in the host tissue, but penetration seems superficial, and mechanical damage caused by the dinoflagellate does not appear extensive. A heavily infested individual of S. crassa continued to feed normally on copepods and appendicularians, and its ovary seemed to be normal as well.

Body_trophont_length: 30-150 µm
Pigment: No