The power of diet – the effect of different types of diet on the plastic body size response to temperature

The temperature-size rule (TSR) describes the growth pattern of ectotherms, according to which they grow smaller at higher temperatures. While the general mechanisms of TSR are well studied, the role of diet in shaping these responses remains less understood. In this study, we examined how different diets influence the strength of TSR (= slope of the response) and the conditions under which it occurs. We used three clones of Lecane inermis rotifers of known thermal preferences: warm-preferring specialist, cold-preferring specialist and a generalist. The rotifers were cultivated at four temperatures, selected specifically for each clone, two lower, between which the rotifers switch from the negative to the positive growth rate (T min ;8/10°C and 10/12°C), and two higher, around the expected optimal temperature at which fitness is the highest (T opt ; 28 °C and 35 °C). The clone-specific thresholds were established in previous studies. The three laboratory diets were: a fluid with a known microorganismal content (Bio-Trakt bioproduct), a fluid that acts as a food source for bacterial biofilm (Molasses), and a powder that acts as a food source for bacterial biofilm (patented NOVO). We showed that the diets differently affected T min , T opt and the strength of TSR performance (measured as a comparison of body size within the range between T min and T opt ). Of these three TSR response descriptors, T min was the most sensitive to diet, while T opt was the least. Our results show that different diets can affect the phenotypic plasticity patterns of organisms in different ways, and that different aspects of phenotypic plasticity (such as lower thermal limit, higher thermal limit, or strength of response) are sensitive to nutritional conditions in different ways.