Description: We conducted two experiments to assess the predation of female H. takanoi (a non-native species in the Baltic) on a native gammarid (Gammarus duebeni) and a non-native analogous gammarid (Gammarus tigrinus). The experiments were conducted at the GEOMAR Helmholtz Centre for Ocean Research Kiel. Crabs were sampled in the innermost part of the Kiel Fjord, Germany (59°19'44N, 10°08'55.5E) during the summer of 2021. This experiment covered a prey-switching trial where both prey species were offered simultaneously in varying densities (2:14, 4:12, 6:10, 8:8, 10:6, 12:4, 14:2; n = 6 replicates per ratio); once again, the amount of consumed prey per species was assessed after 6 hours of feeding by H. takanoi. All remaining parameters were the same as described for Experiment No. 1.

Description: We conducted two experiments to assess the predation of female H. takanoi (a non-native species in the Baltic) on a native gammarid (Gammarus duebeni) and a non-native analogous gammarid (Gammarus tigrinus). In the first experiment, we examined the functional response of female H. takanoi to G. duebeni and G. tigrinus. The experiments were conducted at the GEOMAR Helmholtz Centre for Ocean Research Kiel. Crabs were sampled in the innermost part of the Kiel Fjord, Germany (59°19'44N, 10°08'55.5E) during the summer of 2021. The experimental design utilized a fully factorial approach. One temperature (16°C) and one salinity (10) across five prey densities (1, 2, 4, 8, 16) were used. Each combination was replicated three times, alongside three replicates of predator-free controls at each prey density to quantify background prey mortality. The crabs were starved for 48 hours ahead of the experiment and were exposed to the respective number of gammarids for a 6-hour feeding period. The number of gammarids consumed was recorded.

Description: Globally, the number of invasive non-indigenous species is continually rising, representing a major driver of biodiversity declines and a growing socio-economic burden. Hemigrapsus takanoi, the Japanese brush-clawed shore crab, is a highly successful invader in European seas. However, the ecological consequences of this invasion have remained unexamined under environmental changes—such as climatic warming and desalination, which are projected in the Baltic Sea—impeding impact prediction and management. Recently, the comparative functional response (resource use across resource densities) has been pioneered as a reliable approach to quantify and predict the ecological impacts of invasive non-indigenous species under environmental contexts. This study investigated the functional response of H. takanoi factorially between different crab sexes and under environmental conditions predicted for the Baltic Sea in the contexts of climate warming (16 and 22 °C) and desalination (15 and 10), towards blue mussel Mytilus edulis prey provided at different densities. Hemigrapsus takanoi displayed a potentially population-destabilising Type II functional response (i.e. inversely-density dependent) towards mussel prey under all environmental conditions, characterised by high feeding rates at low prey densities that could extirpate prey populations—notwithstanding high in-field abundances of M. edulis. Males exhibited higher feeding rates than females under all environmental conditions. Higher temperatures reduced the feeding rate of male H. takanoi, but did not affect the feeding rate of females. Salinity did not have a clear effect on feeding rates for either sex. These results provide insights into interactions between biological invasions and climate change, with future warming potentially lessening the impacts of this rapidly spreading marine invader, depending on the underlying population demographics and abundances.

Description: Predatory non-indigenous species (NIS) have profound impacts on global ecosystems, potentially leading to native prey extinction and reshaping community dynamics. Among mechanisms potentially mediating predator impacts and prey invasion success are predator preferences between native vs. non-indigenous prey, a topic still underexplored. Using functional response and prey preference experiments, this study focused on the predation by the non-indigenous Japanese brush-clawed shore crab, Hemigrapsus takanoi, between the native gammarid Gammarus duebeni and the analogous non-indigenous Gammarus tigrinus. Although H. takanoi showed subtle differences in its functional response type between the two prey species, its preferences across their environmental frequencies were not strongly influenced by the prey invasion scenario. The findings highlight the need for a comprehensive understanding of interactions in ecosystems with multiple NIS, offering fresh insights into complex feeding interactions within marine environments.