Strain-specific thermal acclimation, but little evidence of transgenerational plasticity, in an asexual crustacean

Transgenerational plasticity has been suggested as a means for species to succeed in rapidly changing environments, such as increased temperature brought on by climate change. However, the evidence for this phenomenon in animals is mixed. The freshwater crustacean Daphnia magna displays transgenerational plasticity in response to environmental cues such as the presence of predators or food, but studies on heat tolerance have been less conclusive. We acclimated three asexual strains of D. magna—originally from London, UK (“GB”), Fülöpszállás, Hungary (“HU”), and Jerusalem (“IL”)—to either 20°C (control) or 28°C (heat-acclimated) conditions; we refer to this as the parental generation. In contrast, their offspring were reared at 20°C regardless of parental treatment. We assessed acute heat tolerance (time to immobility, Timm), heart rate, body size, and age at first reproduction in the parental and offspring generations. In the parental generation, heat-acclimated individuals of all strains had higher Timm and reproduced earlier. The increase in heat tolerance was greatest in the southernmost strain (IL), and smallest in the northernmost strain (GB). After heat acclimation, mean heart rate increased in GB, decreased in HB, and did not change in IL. These results highlight the importance of local adaptation in thermal plasticity. Parental acclimation temperature did not affect Timm in offspring; however, there was some evidence that offspring of heat-acclimated mothers reproduced later, reversing the trend observed in parents. We did not find support for transgenerational plasticity in heat tolerance, perhaps because offspring re-acclimated to the benign temperature of 20°C.