Ultrastructural studies on the endogenous stages of Eimeria variabilis (Thélohan, 1893) Reichenow, 1921, from Coitus ( Taurulus ) bubalis Euphrasen (Teleostei: Cottidae)

. Ultrastructural studies on the endogenous stages of Eimeria variabilis from the marine fish Cottus bubalis showed macrogamonts that resembled those of other fish coccidia except for small and large, probably protcinaceous, intracytoplasmic dense bodies. Similar dense bodies occurred in oocysts, but were not identified as wall forming bodies since many extruded from the contracted oocyst contents during sporulation. The thin, two‐layered oocyst wall (20 nm across) may have derived from smooth endoplasmic reticulum and the plasma membrane of the late macrogamont. At sporogony, the oocyst wall frequently peeled inwards in scroll‐like formations to leave a membrane 3‐nm thick, possibly of host origin. Sporogony produced four uninucleate sporoblasts that developed with some asynchrony within a single oocyst. Sporoblasts were limited by a single membrane initially, and additions to this membrane, apparently from dense material and filaments outside the sporoblast, built up the sporocyst wall. An area of Stieda‐like body formation was identified on the sporocyst wall and no other exit for sporozoites was seen. Full description of sporozoites, microgamonts and meronts awaits further study but similarities with those of other coccidia were noted.