Temperature effects on a marine herbivore depend strongly on diet across multiple generations.

Increasing sea surface temperatures are predicted to alter marine plant-herbivore interactions and, thus, the structure and function of algal and seagrass communities. Given the fundamental role of host plant quality in determining herbivore fitness, predicting the effects of increased temperatures requires an understanding of how temperature may interact with diet quality. We used an herbivorous marine amphipod, Sunamphitoe parmerong, to test how temperature and diet interact to alter herbivore growth, feeding rates, survival, and fecundity in short- and long-term assays. In short-term thermal stress assays, S. parmerong was tolerant to the range of temperatures that it currently experiences in nature (20-26 °C), with mortality at temperatures > 27 °C. In longer term experiments, two generations of S. parmerong were reared in nine combinations of temperature (ambient, + 2, + 4 °C) and diet (two high- and one low-quality algal species) treatments. Temperature and diet interacted to determine total numbers of amphipods in the F generation and the potential F population size (sum of brooded eggs and newly hatched juveniles). The size and development rate of F individuals were affected by diet, but not temperature. Consumption rates per capita were highest at intermediate temperatures but could not explain the observed differences in survival. Our results show that predicting the effects of increasing temperature on marine herbivores will be complicated by variation in host plant quality, and that climate-driven changes to plant availability will affect herbivore performance, and thus the strength of plant-herbivore interactions.