Growth rates of marine ciliates on diverse organisms reveal ecological specializations within morphospecies

The growth rate of 31 stocks of Euplotes, a cosmopolitan, marine, unicellul ar protist, on six food species representing two different food types, micr oalgae and bacteria, has been determined. The 31 stocks represented nine re productively isolated groups (biological species?) based on breeding relati onships. Three morphospecies, E. vannus, E. crassus, and E. minuta, each wi th both autogamous and cross-breeding breeding groups, were included. The m ean number of fissions completed in 5 days of a breeding group growing on o ne of the six food species varied from zero to 17.06. There is a strong int eraction between morphospecies and food type. The largest morphospecies, E. vannus, translates the nutritional content of algae into growth better tha n 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 o f fissions completed in 5 days revealed significant main effects and intera ctions between many hierarchical levels of stocks and food species. These s ignificant interactions indicate that genetically determined ecologically i mportant information is present at all taxonomic levels-morphospecies, bree ding system, breeding group, and stock. As all these levels are biologicall y meaningful, measuring biodiversity in the E, vannus-crassus-minuta comple x solely on morphospecies will inadequately represent the ecological divers ity present in the organisms and their environment.