Jan Phillipp Geißel

• Global climate change and biological invasions are reshaping biodiversity worldwide, particularly in semi-enclosed seas like the North Sea, Baltic Sea, and the Arctic Ocean, where rapid warming drives poleward species range shifts. These shifts depend on complex ecological and evolutionary processes, with multiple environmental drivers often interacting unpredictably. This underscores the use of multi-driver studies to unravel the interactive effects of co-occurring environmental drivers and ecologically relevant intraspecific variability. Because sympatric species might react differently to such change, interspecific comparisons are important in predicting potential changes in community composition. In benthic marine invertebrates with complex life cycles, larvae are the main dispersive life stages that can reach and colonise new habitats. Typically, there is a substantial variation in the magnitude of environmental effects between life stages, with larvae forming an important bottleneck in a population's ability to persist in a habitat. Coastal crabs are key species that play critical roles in food webs, functioning as both predators and prey, as well as ecosystem engineers that shape their habitats. They are also among the most successful marine invasive species. Using larvae of ecologically relevant coastal decapod crustaceans as models, this thesis investigates inter- and intraspecific variability across environmental gradients and elucidates its role in climate-driven range shifts. Using two model systems, (1) the interactive effect of salinity and temperature on larval development in invasive populations of two Hemigrapsus species, H. takanoi and H. sanguineus, along a salinity gradient from the North Sea to the Baltic Sea, and (2) the variability of larval traits like temperature tolerance in C. maenas across a temperature gradient in its native range along the European Atlantic coast were investigated. Based on individual studies on intraspecific variation, comparisons could be drawn among the three species, the native C. maenas and the two non-native species. Larval rearing experiments in factorial designs assessed survival, duration of development, and growth across environmental treatments, with larvae sourced from multiple populations and females. In the first part (Manuscript 1 & 2), the thesis demonstrates that both Hemigrapsus spp. found in Europe exhibit increased survival, developmental and growth rates compared to the native species at combinations of high temperatures and low salinities. Comparing populations of H. takanoi, a surprising lack of adaptations to low salinities was found in populations from the Baltic Sea. Larvae from the higher salinity areas in the North Sea showed higher survival and faster development than the populations from the Baltic Sea, with the population from the range edge showing poor performance across treatments. Accompanying population genetics analysis revealed no significant genetic separation among populations, indicating persistent connectivity among populations. In combination with the very low survival in the population from the range edge and the low salinity of the area, a source-sink dynamic among populations or complex ontogenetic migration patterns appear likely. In the second part (Manuscript 3 & 4), variability in larval traits, such as temperature tolerance was studied along a large latitudinal gradient in C. maenas. a strong increase in the body mass of freshly hatched larvae with latitude was found across 35° latitude of the native distribution range. Further, considerable variations in thermal tolerance across the range of this species were detected. Some variations met expectations based on the biogeography of the species, like the slightly elevated tolerance to chronically high temperatures in populations from southern Spain and the strong increase in tolerance to cold exposure (6 °C) with latitude. However, unexpected features like high survival rates in populations from the northern edge of the distribution range under chronically high temperatures were also found. In temperature ramp experiments, a correlation between the performance in cold ramps and heat ramps was found, indicating that there is no trade-off between these two traits but that they could be a measure of larval quality. The performance in the acute exposure experiments did not correlate with performance under chronic exposure, indicating that each method assesses different aspects of thermal tolerance. This thesis emphasises the importance of recognising intraspecific variation in response to environmental drivers, as it shapes both the success of invasive species and the adaptability of native species in changing environments. By integrating macrophysiological and multi-driver approaches, this thesis advances the understanding of species responses to environmental change and informs predictions about range shifts under future climates.