Encystment – Protozoan

Encystment is characteristic of the life cycle of many protozoan. The protozoan secretes a thickened envelope (cyst) around itself and becomes inactive with almost a complete shutdown of metabolic activity. This protective cyst is resistant to either desiccation or low temperatures and enables the animal to pass through unfavorable environmental conditions like food deficiency, desiccation, decreased oxygen concentration, pH, etc.

Reproductive phases such as fission, budding and syngamy may occur inside the cysts of some species Encystment is however, not found or is very rare in marine species. Protozoa may be dispersed long distances in either the motile or encysted stages. Water currents, wind, mud, debris, the bodies of birds and animals are common agents of dispersal.

Cysts of some soil inhabiting and freshwater protozoa show amazing durability. The soil ciliate Colpoda has been known to survive in dried soil for up to 38 years On the other hand not all cysts are as durable. Cysts of Entamoeba histolytica can tolerate the acidic medium of the stomach but do not survive desiccation, temperature above 50°C or sunlight. A return of favorable conditions initiates excystment (escape from cysts) in those protozoa in which the cysts are a resistant stage.