The fine structure of the phoront of Gymnodinioides pacifica, a ciliated protozoan (Ciliophora, Apostomatida) from euphausiids of the Northeastern Pacific

Phoretic stages of the exuviotrophic apostome Gymnodinioides pacifica were examined using transmission and scanning electron microscopy (TEM and SEM). TEM revealed that the mature cyst wall possesses 2 or 3 layers differing by the presence or absence of the third inner layer. This inner layer may represent a different form of the middle wall material. The inner cyst layer is approximately 0.15 μm thick and has striations with a periodicity of ∼19 nm. The middle cyst layer has a variable thickness and the outer dense layer is ∼0.1 μm thick. The 3 layered cyst wall had a thickness of 0.3–0.7 μm and averaged 0.5 μm. Advanced phoront stages were enclosed by fully formed cyst walls or by cyst walls thinned to ∼0.1 μm, as the phoronts prepared to excyst prior to host ecdysis. Additionally, we report the fine structure of the rosette, trichocysts, nuclei, food plaquettes, oral fiber, and other cytoplasmic inclusions. SEM revealed an outer cyst wall layer connected to the secreted peduncle material, which was observed to extend over a wide (15 μm) area on the host setae. Cysts were usually attached at their posterior ends or, less frequently, along their side.