Thermal phenotypic plasticity of the second limb in Daphnia

Thermal plasticity is expected to affect the responses of freshwater zooplankton to climate perturbations. We tested the hypothesis of thermal plasticity in the rigid setae of the second limb of Daphnia galeata. We also aimed to test the role of allometry and the ability to induce a rapid plastic response within an individual lifetime (i.e. postnatal). In all, 300 specimens of Daphnia were dissected from four sets of clonal thermal transfers and seasonal variants from nature. Specimens of D. galeata were cultured under identical conditions save temperature (10 and 20°C). Clonal neonates were reciprocally transferred across temperature regimes to determine the degree of postnatal plasticity. Setae length ratios supported thermal plasticity of rigid seta 2, but a thermal effect was not significant for the length ratios of the soft setae. Simple linear regressions of body length (mm) and setae lengths (mm) under warm and cold conditions revealed significant slope differences (i.e. allometry) among temperatures for the three setae examined. The rigid seta had the largest thermal effect size (slope difference), growing larger at colder conditions for a given body size compared to the soft seta. Rapid thermal plasticity in non-filtering limbs is more important than previously appreciated for Daphnia.