We examined the functional response of the Japanese brush-clawed shore crab ( Hemigrapsus takanoi ) towards blue mussels ( Mytilus sp.) across four seasons for an ambient and +6 °C future warming scenario in the Baltic Sea. The experiment was carried out as a laboratory experiment at the GEOMAR Helmholtz Centre for Ocean Research Kiel. Crabs were sampled in the innermost part of the Kiel Fjord, Germany (54°19′44.8″ N, 10°08′55.5″ E) between Summer 2021 and Spring 2023 during the respective season. The experimental design used a fully factorial approach, examining the functional response across two temperatures, two sexes, and eight prey densities (1, 2, 4, 6, 8, 16, 32, 64) across four seasons. Each crab was subjected to a pre-experiment 48-hour starvation period and then exposed to a fixed number of mussels for a 72-hour feeding trial. The number of mussels consumed was recorded, providing data on the predatory impact of H. takanoi under varying temperature scenarios and across seasons.
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.
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.
Salinity in Kiel Fjord was recorded on mostly-weekly cruises of the research vessel FK Polarfuchs from 2005 to 2018 using a CTD48M probe (Sea and Sun technologies, Trappenkamp, Germany) at a station in front of the GEOMAR Pier (“Wittlingskuhle”, Position: 54°19'69 N, 10°09'06 E). These measurements are important for better understanding the salinity as one of the important abiotic factor determine the reproduction, establishment and has an effect on many species in the Baltic Sea in general and the fjord specifically.
We assessed temporal variability in relative abundance, size-class, bimoass, and sex ratio, as well as breeding season of the Asian shore crab Hemigrapsus takanoi in the Kiel Fjord (Western Baltic Sea) during 2017. In addition, their prey size preference and consumption rates on mussels (Mytilus edulis) were experimentally assessed in spring, summer and autumn.