Description: 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.

Description: Predicting range expansion of invasive species is one of the key challenges in ecology. We modelled the phenological window for successful larval release and development (WLR) in order to predict poleward expansion of the invasive crab Hemigrapsus sanguineus along the Atlantic coast of North America and north Europe. WLR quantifies the number of opportunities (in days) when larval release leads to a successful completion of the larval phase; WLR depends on the effects of temperature on the duration of larval development and survival. Successful larval development is a necessary requirement for the establishment of self‐persistent local populations. WLR was computed from a mechanistic model, based on in situ temperature time series and a laboratory–calibrated curve predicting duration of larval development from temperature. As a validation step, we checked that model predictions of the time of larval settlement matched observations from the field for our local population (Helgoland, North Sea). We then applied our model to the North American shores because larvae from our European population showed, in the laboratory, similar responses to temperature to those of a North American population. WLR correctly predicted the northern distribution limit in North American shores, where the poleward expansion of H. sanguineus appear to have stalled (as of 2015). For north Europe, where H. sanguineus is a recent invader, WLR predicted ample room for poleward expansion towards NE England and S Norway. We also explored the importance of year‐to‐year variation in temperature for WLR and potential expansion: variations in WLR highlighted the role of heat waves as likely promoters of recruitment subsidising sink populations located at the distribution limits. Overall, phenological windows may be used as a part of a warning system enabling more targeted programs for monitoring.