Growth Rates of Marine Ciliates on Diverse Organisms Reveal Ecological Specializations within Morphospecies

Abstract The growth rate of 31 stocks of Euplotes, a cosmopolitan, marine, unicellular protist, on six food species representing two different food types, microalgae and bacteria, has been determined. The 31 stocks represented nine reproductively isolated groups (biological species?) based on breeding relationships. Three morphospecies, E. vannus, E. crassus, and E. minuta, each with both autogamous and cross-breeding breeding groups, were included. The mean number of fissions completed in 5 days of a breeding group growing on one of the six food species varied from zero to 17.06. There is a strong interaction between morphospecies and food type. The largest morphospecies, E. vannus, translates the nutritional content of algae into growth better than it translates that of bacteria, while the reverse is true for E. minuta, the smallest morphospecies. Autogamous breeding groups grow more rapidly on algae than on bacteria when compared to cross-breeding groups in the same morphospecies. Two breeding groups cannot grow on Escherichia coli. ANOVA of fissions completed in 5 days revealed significant main effects and interactions between many hierarchical levels of stocks and food species. These significant interactions indicate that genetically determined ecologically important information is present at all taxonomic levels—morphospecies, breeding system, breeding group, and stock. As all these levels are biologically meaningful, measuring biodiversity in the E. vannus–crassus–minuta complex solely on morphospecies will inadequately represent the ecological diversity present in the organisms and their environment. Received: 11 May 1998; Accepted: 31 July 1998