Seasonal variation of excystment pattern of the planktonic oligotrich ciliate Strombidium conicum

Cysts of the planktonic oligotrich ciliate Strombidium conicum were isolated from sediment samples, collected monthly in Onagawa Bay on the northeastern Pacific coast of Japan, and incubated under laboratory conditions of 20 °C in light. The excystment ability changed seasonally in a regular manner, which was demonstrated by alternation of three characteristic seasonal patterns of the cumulative excystment curve, i.e., rapid, delayed and transitional patterns. While the transitional excystment pattern was characteristic during the period from spring to midsummer, the rapid pattern occurred during late summer to early winter. The pattern changed again to transitional in midwinter and finally returned to the delayed pattern in late winter or early spring. We suggest that mud temperature was the most determinative factor of this seasonality in excystment ability. Such synchronization of annual excystment helps this species to proliferate rapidly and maintain the vegetative part of the population in the upper water column for a longer period of time where it is subjected to the dispersion process due to water movement. Received: 17 September 1996 / Accepted: 18 November 1996