Intraspecific variation in sensitivity to food availability and temperature-induced phenotypic plasticity in the rotiferKeratella cochlearis

Organisms with wide environmentally-induced morphological plasticity and cosmopolitan distribution, e.g., the common freshwater rotifer Keratella cochlearis, are ideal models to study the evolution of plastic polymorphisms and the capacity of zooplankton to adapt to local selection conditions. We investigated population-level differences (population-by-environment interaction) in sensitivity to food availability and temperature-induced phenotypic plasticity between two clones of K. cochlearis (Gosse) isolated from neighboring populations in Ruidera Natural Park (Spain) with different trophic statuses: Tinaja (TIN) lake (mesotrophic) and Cueva Morenilla (MOR) lake (eutrophic). Using common-garden experiments, each clone proved to have a different sensitivity to food availability, with substantial phenotypic differences between them. When rotifers grew at moderate temperature (15.6°C), low food levels were more efficiently used by the TIN versus MOR clone, whereas high food levels were more efficiently used by the MOR versus TIN clone. The posterior spine was much longer and the lorica wider in the TIN versus MOR clone, with no difference in lorica length. Phylogenetic analysis based on COI sequences showed that both populations have the same haplotype. This is the first study to show possible local adaptation by a rotifer species to habitats that consistently differ in food availability. We also detected an intriguing deviation from the expected negative relationship between posterior spine length and temperature. Our experimental results indicate that intermediate temperatures may activate the gene responsible for spine elongation in K. cochlearis. This suggests that rotifers in nature could use water temperature as proxy signal of a change in predation risk before defense is needed.