Laboratory study of the life cycle of a freshwater strombidiid ciliate

Abstract

Sediment samples containing resting cysts of the planktonic ciliate Pelagostrombidium sp. were collected in October/November 2000 in Lake Mondsee, Austria, with sediment traps and stored at ca. 4°C in the dark. Within 6 mo, concentrations of viable cysts in these samples decreased to less than 4% of initial numbers, probably due to the combined effects of predation, infection and excystment. Spontaneous excystment at low temperatures was first observed in March 2001, when the cysts were ca. 4 mo old. Excystment of 2 to 5 mo old cysts could be triggered by a temperature shift from ca. 4 to 18°C in the presence of food algae. White light slightly enhanced this response. From excysted individuals, 12 non-clonal strains were established and maintained with cryptomonads as food. All strains produced cysts, which were first observed ca. 3 wk after excystment. Experiments with these laboratory cysts revealed that excystment could be triggered by a temperature stimulus in cysts older than 1 mo and occurred spontaneously at low temperatures in cysts older than 3 mo. The results of the present study suggest that cyst age and temperature are the main factors controlling excystment of Pelagostrombidium sp., and that the period of dormancy of this ciliate in sediments of temperate lakes generally does not exceed 6 mo. Cysts produced at the end of the growing season will excyst in early spring, whereas cysts formed in spring are likely to excyst in summer.