The non-native crab Hemigrapsus takanoi in the south-western Baltic Sea: Population structure and sensitivity to salinity and temperature shifts

The Asian brush clawed shore crab Hemigrapsus takanoi is native to the north-western Pacific Ocean. Since the 1990s, H. takanoi is known to invade the intertidal zones of the European Atlantic and the North Sea coasts, where this species shares habitats with for example the native European green crab Carcinus maenas, and competition in habitat and food sources between both species was observed. H. takanoi was first detected in the Baltic Sea in summer 2014. Numerous juveniles and adults including ovigerous females were observed from its south-western region (most inner part of the Kiel Fjord). Gaining knowledge about population structure of a recently introduced species and its interactions with the biodiversity in its recipient habitat is crucial to assess the effects on the ecosystem. In addition, information of factors promoting its invasion and establishment success is important to predict future invasion potential and extension ranges. Thus, the main scope of this thesis is 1) to evaluate the current status of H. takanoi in south-western Baltic Sea Kiel Fjord, 2) to gain insights into the tolerance of different life history stages to salinity stress, 3) to assess larval responses to climate driven environmental variables, more specifically, on how tolerance to low salinity regimes is affected by increasing seawater temperature. Up to 596 H. takanoi adult specimens were found from the inner Kiel Fjord during monitoring in 2017. Although males reached larger sizes than females, with sizes up to 29 mm compared to 25.5 mm for females, females were more abundant than males. Reproductive season started in June and lasted into August. The current study investigated that, the relatively large claws of males comparing to females, helped them to open mussels of Mytilus edulis up to a size of 18.1‒21.0 mm, while the largest size opened by females was up to 12.1‒15.0 mm only. In addition, males consumed twice as much as mussel biomass compared to females of similar sizes. Consumption rates for both sexes increased by increasing temperatures over seasonal changes. Performance of H. takanoi towards a wide salinity gradient was different among the different life history stages. Early life history stages were more vulnerable than juveniles and adults. Fitness parameters, represented by consumption rates on M. edulis, of juveniles and adults, showed a significant positive correlation with increasing salinity. For adults, feeding was highest at a salinity of 25. However, larvae showed low resistance to lower salinities, and failure in development to megalopa at salinity below 16, which represents the current Kiel Fjord mean salinity conditions over the year. In general, survival to megalopa increased with increasing salinity under the examined salinity treatments (salinities of 0‒35 and 10‒25 during the 2017 and 2018 experiments, respectively). The highest numbers of megalopa were recorded at salinity 25. The results of this thesis show that H. takanoi is able to complete its entire life cycle in the brackish Kiel Fjord, however, its sensitivity to low salinity might be a barrier for their further spread towards the Baltic Proper. In addition, the result revealed that intrapopulation variability, seen in the offspring of only one out of five females that succeeded to reach the megalopa at a salinity of 16, might potentially be the reason for H. takanoi population persistence in Kiel Fjord. H. takanoi early life history stages have been shown to suffer predicted near-future climate change and were affected by the tested multiple stressors (salinity and temperature). There were no interaction effects of temperature and salinity on larval survival and their time to reach the megalopa. Nevertheless, increased temperature accelerates their development and raised larval mortality at all salinity levels. At higher temperature, no larvae reach megalopa below salinity of 19. In conclusion, H. takanoi successfully established a population in the south-western Baltic Sea (inner Kiel Fjord) and will most likely add more pressure on M. edulis compared to conditions before their arrival. The species showed an ontogenetic shift in response to salinity with development. Tolerance to low salinity is likely a key determinant for H. takanoi’s further spread throughout the whole Baltic Sea today and in future conditions.