A new species in the genus Cyrtostrombidium (Ciliophora, Oligotrichia, Oligotrichida): its morphology, seasonal cycle and resting stage.

Cysts of an oligotrich ciliate were collected from natural sediment samples in Onagawa Bay, northeastern coast of Japan, and examined for their excystment capability. A high excystment ratio was obtained at lower temperatures of 10 or 15 degrees C; no excystment occurred at 20 degrees C. Excysted vegetative cells were observed after protargol staining and were identified as a new species, Cyrtostrombidium boreale n. sp. The seasonal changes in the vegetative population and sedimentation of newly formed cysts were also investigated in situ. Planktonic vegetative cells were abundant during the cold season from February to May, when the water temperature was lower than 10 degrees C. Mass encystment occurred abruptly just after the seasonal peak of the vegetative population in April. These results indicate that C. boreale is a cold-water species and aestivates during the longer, warm period from late spring to fall.