Ultrastructural variability in the locomotor cortex of the ciliated protozoa, Mytilophilus pacificae

Mytilophilus pacificae is an endocommensal ciliate found in the mantle cavity of the Pacific Coast mussel Mytilus californianus. In this paper we report our findings on pellicular organization of this species. Transmission and scanning electron microscope examination of the somatic cortex revealed that a number of different types of kinetids, i.e. monokinetids, dikinetids, and polykinetids are found in the locomotor cortex. The type and distribution of the kinetids are described. Surprisingly, the locomotor region was found to be highly variable among individuals with respect to its kinetid distribution; each cell appears to have its own characteristic kinetid pattern. Some cells have mostly monokinetids and dikinetids in their locomotor cortex, while others may have dikinetids and polykinetids but very few monokinetids. In contrast to the locomotor region, the thigmotactic field (a region specialized for attachment) is exclusively composed of dikinetids and shows no heterogeneity. The finding of ultrastructural variability in the locomotor cortex was unexpected since, in the view of the structural conservatism hypothesis, the somatic cortex is seen as a 'stable' element. These observations raise new questions with regard to cortical pattern formation in this organism.