Effects of cyst age on excystment of the ciliate Histriculus cavicola (Kahl, 1935) (Ciliophora: stichotrichia).

Events of excystment were compared among 3 strains of the ciliate Histriculus cavicola (Kahl), and were divided into 4 stages by morphological characteristics detectable with a dissecting microscope. When cysts were younger than 4 days, observation at 15 min intervals inhibited excystment for as long as the observation continued; termination of this frequent observation eliminated the inhibition. When cysts of strains HS-3 and HS-3 S were formed and aged for longer than 4 days in 10 times concentrated Osterhout's solution (10 × OS), almost all cells excysted within 4 h after induction of excystment. The 21 day cysts of HS-3 S excysted synchronously, indicating the usefulness of this species for the study of ciliate excystment. In the KM-1 strain, cysts younger than 4 days showed low excystment rates even when cysts were formed and aged in 10 × OS. When encystment was induced in culture supernatant, the excystment rate was low in younger cysts than 4 days of all 3 strains examined but the rate became higher with increasing age of the cysts. These results suggested that excystment was influenced by the age of the cysts, frequent observation, strain differences, and the medium in which the cysts were formed or aged. Control factors for excystment are discussed.