Description: Under climate change, increased temperatures combined with food limitation may be critical for species with complex life cycles, if high growth rates characterise the larval development. We studied the effect of increased temperature and food limitation on larval survival and on functional traits (developmental time, body mass, elemental composition, growth) at moulting and metamorphosis in the crab Carcinus maenas collected in the North Sea (Helgoland, Germany). We followed the approach of models of metamorphosis integrating responses of body mass and developmental time to increased temperature and food limitation. We also evaluated if body mass decreased with temperature (according to the temperature-size rule) and if developmental time followed an inverse exponential reduction (expected from some metabolic theories), as both trends are relevant for modelling effects of climate change on fitness and population connectivity. Larvae produced by four females during the reproductive period (i.e. spring-summer 2016) were reared separately from hatching to metamorphosis to the megalopa at two food conditions (ad libitum and low food availability), and at four temperatures covering the range experienced in the field (〈20°C) and those expected from climate change (〉20°C). Survival and developmental rates were obtained by daily monitoring of the experiments. Biomass data (body mass and elemental composition) were obtained by sampling larvae at the zoea IV and megalopa stages and further processed with standard methods (see Torres & Giménez 2020 for details). 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: Pelagic Sargassum and floating marine debris were collected in the Sargasso Sea (subtropical NW Atlantic) during the cruise MSM41 of the German research vessel Maria S. Merian in April 2015. Flotsam items were collected with a dip net (mesh size: 1 mm) from the rescue boat of the research vessel at 19 stations distributed in coordination with the other sampling activities of the cruise. Few, mostly larger, debris samples were taken from aboard the research vessel using a dip net with a mesh size of 9 cm. The exact sampling position of each floating item was recorded by a handheld GPS. All mobile epifaunal organisms were carefully washed down from the flotsam items with freshwater and retrieved in a sieve with a mesh size of 250 µm. The species were identified and the individuals counted. Each Sargassum thallus was inspected under a stereo microscope to identify the attached sessile epifaunal species. Sessile species were not quantified. The marine debris items or sub-samples thereof (in case of large bulky items) were preserved in buffered formalin-seawater solution. The sessile epifauna on the debris items was analyzed after the cruise in the laboratories of the Alfred Wegener Institute in Bremerhaven, Germany. Each Sargassum clump was blotted on tissue paper to remove adherent seawater and weighed. The sizes of the debris items were measured to calculate the surface area.