Inter- and intraspecific relationships between performance and temperature in a cryptic species complex of the rotifer Brachionus plicatilis

The strategy of decreasing size with increasing temperature operates at regional and phenotypic scale and presents a puzzle to researchers. In this work, we studied two aspects of the temperature–performance relationship along a temperature gradient, (i) comparing the population growth rates of three cryptic Brachionus species differing in adult size, and (ii) assessing the phenotypic plasticity of adult size, in one clone per species. The working hypotheses were that (i) the bigger the species the lower its optimal temperature for population growth, and (ii) the higher the temperature the smaller the individual within each focal species. The results showed that (i) the optimal temperature for population growth is related to species size in a manner foreseen by Bergmanns’ rule for two of the three species (the third, biggest species, performed evenly well at all temperatures examined, what could be explained by its generally eurioic character), and that (ii) the strategy of body size adjustment to environmental temperature differs between species and may depend on the level of temperature specialization. This work demonstrated the usefulness of inter- and intraspecific comparisons for studying the role of growth strategies in adaptation to temperature.