Rates of population increase and mortality for sessile ciliates on artificial substrates in a stream riffle

Abstract

Ciliophorans are capable of very high fission rates, but it is not known whether these rates are typical of field populations. This study measures rates of colony size and population increase, along with colony and individual survival times, for sessile species inhabiting artificial substrates placed in a stream riffle. This was accomplished by bringing the substrates into the laboratory for a short period each day and counting and recording the positions of individuals and colonies. Abundant species on the artificial substrates included Metacineta mystacina, Opercularia coarctata, and Epistylis plicatilis. Artificial substrates were colonized by these sessile ciliates within a week of being placed in the stream. After colonization, there was usually a period of exponential population growth, eventually followed by a levelling off or decline. Slow or negative population growth was associated with high mortality. Population doubling times and mean individual lifespan fall in the range of 5 to 24 h, indicating that these field populations displayed near-maximal fission rates.