ALBERT

Description: The shore crab Hemigrapsus takanoi Asakura and Watanabe, 2005, native to the Northwest Pacific, was recorded in European waters about 25 years ago and it was first found in the Baltic Sea in 2014. Information on population structure of invaders and their new niche is needed in order to understand their biological impact. Over one year, we assessed temporal changes in relative abundance, size-class and sex ratio, as well as breeding season of H. takanoi in the Kiel Fjord (Western Baltic Sea). In addition, prey size preference and consumption rates on mussels (Mytilus edulis Linnaeus, 1758) were experimentally assessed in spring, summer and autumn. A total of 596 individuals were collected with highest and lowest abundances in June and February, respectively. Females were dominant over males (sex ratio 1.4:1), but males grew to larger sizes. H. takanoi reproduced between June and August with ovigerous females representing 30% of the entire female abundance registered over the entire year. Males were able to open larger mussels (due to larger claws) and consumed twice as many mussels when compared to females of similar size. Consumption rates for males were 6 and 2 times higher in summer (seawater temperature of 19 °C) compared to spring (8 °C) and autumn (13 °C), respectively. Females consumed 3 times more mussels in autumn than in spring. H. takanoi is an active predator, capable of reproduction in stressful brackish water conditions. Due to large abundances and high feeding pressure, this recently introduced species could play a key role in structuring post-settlement population dynamics of the dominant habitat builder M. edulis.

Description: 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.

Description: Salinity is a common stressor restricting the distribution of various decapod crustaceans. The interactive effects of such regional stressors with global climate change drivers are important to be considered when aiming to realistically predict the potential of a species’ dispersal and further spread into new habitats. Within species, their larval stages commonly determine a species tolerance and with this their potential to invade and successfully develop a sustaining population. This laboratory study investigated the combined effect of salinity (6 levels, 10–25) and temperature (19 and 23 °C) on larval survival, development to megalopa, and feeding (in Zoea I, III, and V) of the decapod Hemigrapsus takanoi. Larval development and survival to megalopa were generally favored by increasing salinity. While no larva developed to the megalopa stage at 23 °C and a salinity of 16, in 19 °C some larvae could successfully develop under a salinity as low as 16. All larval stages fed generally more with increasing salinity and temperature, but there was no interaction between the two factors. The results revealed that the H. takanoi population from Kiel Fjord (southwestern Baltic Sea) is capable of completing its larval development under the current Kiel Fjord environmental conditions. The geographical spread of this H. takanoi population into the wider Baltic Proper may, however, be restricted mainly due to the inability to establish and maintain a self-sustaining population under lower salinity conditions. Furthermore, the projected desalination of the Baltic Sea together with rising temperatures due to global warming and heat waves in summer may likely exert additional stress to this existing population, unless H. takanoi adapts at appropriate rates.

Description: The Asian shore crab Hemigrapsus takanoi, native to the northwest Pacific Ocean, was recently discovered in Kiel Fjord (southwestern Baltic Sea). In laboratory experiments, we tested the salinity tolerance of H. takanoi across 8 levels (0 to 35) and across 3 life history stages (larvae, juveniles and adults) to assess its potential to invade the brackish Baltic Sea. Larval development at different salinities was monitored from hatching to the megalopa stage, while survival and feeding of juveniles and adults were assessed over 17 d. Larvae of H. takanoi were able to complete their development to megalopa at salinities 〉= 20 and the time needed after hatch to reach this stage did not differ between salinities of 20, 25, 30 and 35. At a salinity of 15, larvae still reached the last zoea stage (zoea V), but development to the megalopa stage was not completed. All juveniles and adults survived at salinities from 5 to 35. Feeding rates of juveniles increased with increasing salinity across the entire salinity range. However, feeding rates of adults reached their maximum between salinities of 15 and 35. Our results indicate that both juveniles and adults of H. takanoi are euryhaline and can tolerate a wide range of salinities, at least for the time period tested (2 wk). However, larval development was impaired at salinities lower than 20, which may prevent the spread of H. takanoi into the Baltic Proper.