Pronounced ecophysiological clonal differences of two common freshwater ciliates, Coleps spetai (Prostomatida) and Rimostrombidium lacustris (Oligotrichida), challenge the morphospecies concept

Cell volume, growth and production rates of two common planktonic freshwater ciliates, the prostomatid Coleps spetai and the oligotrich Rimostrombidium lacustris, were investigated in clonal laboratory cultures. The clones were isolated from oligo-mesotrophic alpine Lake Mondsee, Austria, during summer and kept in culture with the small cryptophyte Cryptomonas sp. as food. All parameters investigated revealed significant clonal differences among both species. The extent of the clonal differences was comparable to differences observed earlier between similar planktonic ciliate species. The ecological relevance of varying clonal growth rates was evaluated using a simple numerical model. The model outcome suggests that differences in growth rates by 10% may significantly alter the clonal composition in the course of a ciliate peak in temperate lakes. The experimental results and the model outcome are discussed in the context of the functional diversity of freshwater ciliates. It is concluded that the morphospecies concept, which is the most widely used concept by both ciliate taxonomists and ecologists, may severely underestimate the ecological plasticity and the functional diversity of aquatic ciliates.