ALBERT

Description: The Asian shore crab H. sanguineus first appeared at the French coast in the late 1990’s. It rapidly extended its range further north to the German Wadden Sea and recently to western Sweden. In the intertidal area, it co-occurs with the European green crab C. maenas. As the ecophysiology of H. sanguineus is virtually unknown, the physiological capacities of both species and their potential for intra-guild competition were investigated. The aim of this study was to specifically compare the energy deposition and dietary preferences of ovigerous females of both species. Females of H. sanguineus and C. maenas carrying immature or mature eggs were collected in April, June, August and October 2015 in an intertidal area of the Island of Helgoland, North Sea. Total lipid levels and fatty acid compositions were determined of both midgut glands and eggs. In H. sanguineus, total lipid levels of the midgut glands were clearly higher than those of C. maenas (40% vs. 10% dry mass, DM). Immature eggs were quite lipid-rich in both species with 30% and 25%DM, respectively, whereas in mature eggs, lipid levels decreased to ~15%DM each. A Principal Component Analysis of the fatty acid compositions of midgut glands and eggs revealed separate clusters for both species with C. maenas lipids more characterized by membrane fatty acids. In C. maenas fatty acids of midgut glands and eggs clustered together largely dominated by carnivory biomarkers. Fatty acids of midgut glands and all eggs of H. sanguineus formed separate clusters and trophic markers indicate a more herbivorous diet. Higher lipid levels and thus more pronounced energy deposition in H. sanguineus midgut glands indicate higher starvation tolerance for females, a potential competitive advantage over C. maenas. Direct food competition, however, seems negligible, as H. sanguineus prefers a more herbivorous diet than C. maenas. Deviating fatty acid compositions in H. sanguineus midgut glands and eggs suggest that this species may represent an income breeder, utilizing energy from both the midgut gland but also from dietary input. Most brachyuran crabs are capital breeders, which rely exclusively on internal reserves.

Description: Biological invasions can entail major threats to marine biodiversity. Non-indigenous species may induce changes in community structure and ecosystem functioning, thereby affecting ecosystem services and human economic interest and health. Decapod brachyuran crabs are among the most invasive marine animal taxa. The originally European species Carcinus maenas and the Asian shore crab Hemigrapsus sanguineus invaded different coastal areas around the world. While their invasion history and ecology has been thoroughly investigated, physiological properties of H. sanguineus are virtually unknown. The aim of this study was to compare the metabolic energy demand of both crab species and - based on these data - to assess their potential dietary impact on the ecosystem. Respiration measurements were conducted with a flow-through system covering a temperature range naturally experienced by these crabs (5, 10, 15 and 20 °C). Both species were analyzed on the island of Helgoland in April, June and August 2015. A general linear mixed-effects model (LMM) was applied to test for the effects of species, temperature, biomass and sex on respiration rates. Overall rates increased with temperature but decreased with the mass of the crabs. Respiration rates did not differ significantly between sexes in both species. From the full model, two separated LMMs were created for either species. They allowed establishing species-specific equations for the prediction of respiration rates y (nmol d-1 g-1) for a crab of any given mass xMass (g) at any given ambient temperature xTemp (°C): ln y = 10.39 + (-0.34 * ln xMass) + (0.06 * xTemp) for C. maenas and ln y = 10.42 + (-0.39 * ln xMass) + (0.08 * xTemp) for H. sanguineus. The mass-specific respiration rates of C. maenas and H. sanguineus were quite similar. By applying the diet-dependent respiratory quotient, oxygen uptake may be used to calculate carbon uptake and metabolic energy demand either for single crabs or for entire populations of a given area. On the population level, the metabolic energy demand and thus ecosystem impact of both species depend primarily on their abundance in the field and, less so on their dietary preferences.

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.