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We investigated the interaction of intergenerational temperature and food quality change on the fitness of two species of Daphnia, a freshwater keystone herbivore. The effect of a change in temperature (15uC vs. 20uC) and food quality (Cryptomonas sp. as high-quality food vs. Chlamydomonas sp. as relatively low-quality food) on juvenile growth rate and clutch size of Daphnia magna and Daphnia pulex was measured in 16 combinations of maternal and offspring environments in standardized growth assays. Both species showed similar responses. The positive effect of food quality on growth rate and clutch size was stronger if the maternal diet was of low quality. High maternal diet quality generally increased growth rate and reproduction, especially if the offspring diet was of low quality. The maximum fitness was attained by animals acclimated at 15uC and growing at 20uC, combining the faster growth rate at warm assay temperature and bigger clutch size of cold-acclimated animals. Intergenerational temperature regime had a strong influence on the effects of maternal and offspring food quality. Good maternal food quality was more important for fitness under less favorable temperature conditions. Low offspring food quality was more detrimental to fitness in animals experiencing temperature change between maternal acclimation and growth assay and less so under constant temperature conditions. Intergenerational changes in temperature and the maternal effects of food quality strongly affect the dietary requirements of these organisms. Hence, extrapolation from studies conducted under constant temperature conditions may seriously underestimate dietary constraints in natural environments.

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Species distribution boundaries are often associated with aspects of climate and modified by competitive interactions. In the tropical region, Diaphanosoma is the most diverse genus of Cladocera. Diaphanosoma dubium is widely distributed in China, whereas the tropical Diaphanosoma excisum is restricted to the islands off the south China coast. The mean temperature of the coldest month at the northern limit of its distribution is 15°C. We studied the effect of temperature (10–40°C) on life history traits of clones from 16 populations of D. dubium and D. excisum in life‐table experiments in order to determine if their thermal performance curves (TPCs) explain their distribution and to estimate the extent and mode of genetic variation in their TPCs. Our results show that both species have higher optimum temperatures (To ∼ 33°C), critical thermal minimum (CTmin = 10–15°C), and maximum (CTmax = 39–40°C) than temperate cladocerans. Clones of D. excisum had narrower TPCs and higher To than D. dubium clones, in accordance with predictions based on changes of temperature distributions with latitude. Significant levels of genetic variation in TPCs were present in both species. D. excisum had higher fitness than D. dubium at all temperatures above 15°C. However, at 15°C and below, D. dubium fitness was higher. Negative r at 15°C and below in combination with competition with the more cold‐adapted D. dubium exclude D. excisum from mainland China. TPCs of interacting species may be the basis of predicting future species distributions as temperature increases change the balance of biotic interactions.