TEMPERATURE MODULATES COMPENSATORY RESPONSES TO FOOD LIMITATION AT METAMORPHOSIS IN A MARINE INVERTEBRATE

1. In species with complex life cycles, increased temperatures combined with food limitation may be critical, if high growth rates characterise the larval development. 2. We used the crab Carcinus maenas as a model species in order to determine how temperature modifies the effect of food limitation on larval survival and on functional traits at metamorphosis (developmental time, body mass, growth rates, carbon and nitrogen content). 3. We followed the approach of models of metamorphosis integrating responses of body mass and developmental time. We also evaluated if increased temperature would lead to (1) decreased body mass (as expected from the so-called temperature-size rule) and (2) exponential reductions in developmental time (as expected from metabolic theories of ecology). 4. Larvae produced by four females were reared separately from hatching to metamorphosis to the megalopa at two food conditions (ad libitum and food limitation), and at four temperatures covering the range experienced in the field (<20°C) and those expected from climate change (>20°C). 5. Under ad libitum food conditions, responses in larvae from most females were not consistent with the temperature-size rule nor with expectations from the metabolic theory of ecology. 6. At low temperatures (<20°C), body mass and nitrogen content at metamorphosis were little affected by food limitation while effects on carbon content were small. Increased developmental time partially or fully compensated for reduced growth rates. We interpreted this response as adaptive, as minimising fitness costs associated to reduced body mass. In larvae from three females food limitation resulted in small reductions in larval survival. 7. High temperatures (>20°C) exacerbated the effect of food limitation on mortality in larvae from three females. Developmental time was longer and larvae metamorphosed with reduced body mass, carbon and nitrogen content. Thus, compensatory responses failed and multiple fitness costs should be expected in individuals facing food limitation at increased temperatures. 8. We propose that integrative studies of traits at metamorphosis could be a basis to develop a mechanistic understanding of how species with complex life cycles will respond to climate change. Such models could eventually include hormonal and metabolic regulation of development as drivers of responses to environmental change.