ALBERT

Description: Shape variability represents an important direct response of organisms to selective environments. Here, we use a combination of geometric morphometrics and generalised additive mixed models (GAMMs) to identify spatial patterns of natural shell shape variation in the North Atlantic and Arctic blue mussels, Mytilus edulis and M. trossulus, with environmental gradients of temperature, salinity and food availability across 3980 km of coastlines. New statistical methods and multiple study systems at various geographical scales allowed the uncoupling of the developmental and genetic contributions to shell shape and made it possible to identify general relationships between blue mussel shape variation and environment that are independent of age and species influences. We find salinity had the strongest effect on the latitudinal patterns of Mytilus shape, producing shells that were more elongated, narrower and with more parallel dorsoventral margins at lower salinities. Temperature and food supply, however, were the main drivers of mussel shape heterogeneity. Our findings revealed similar shell shape responses in Mytilus to less favourable environmental conditions across the different geographical scales analysed. Our results show how shell shape plasticity represents a powerful indicator to understand the alterations of blue mussel communities in rapidly changing environments.

Description: Although geographical patterns of species' sensitivity to environmental changes are defined by interacting multiple stressors, little is known about compensatory processes shaping regional differences in organismal vulnerability. Here, we examine large-scale spatial variations in biomineralization under heterogeneous environmental gradients of temperature, salinity and food availability across a 30° latitudinal range (3,334 km), to test whether plasticity in calcareous shell production and composition, from juveniles to large adults, mediates geographical patterns of resilience to climate change in critical foundation species, the mussels Mytilus edulis and M. trossulus. We find shell calcification decreased towards high latitude, with mussels producing thinner shells with a higher organic content in polar than temperate regions. Salinity was the best predictor of within-region differences in mussel shell deposition, mineral and organic composition. In polar, subpolar, and Baltic low-salinity environments, mussels produced thin shells with a thicker external organic layer (periostracum), and an increased proportion of calcite (prismatic layer, as opposed to aragonite) and organic matrix, providing potentially higher resistance against dissolution in more corrosive waters. Conversely, in temperate, higher salinity regimes, thicker, more calcified shells with a higher aragonite (nacreous layer) proportion were deposited, which suggests enhanced protection under increased predation pressure. Interacting effects of salinity and food availability on mussel shell composition predict the deposition of a thicker periostracum and organic-enriched prismatic layer under forecasted future environmental conditions, suggesting a capacity for increased protection of high-latitude populations from ocean acidification. These findings support biomineralization plasticity as a potentially advantageous compensatory mechanism conferring Mytilus species a protective capacity for quantitative and qualitative trade-offs in shell deposition as a response to regional alterations of abiotic and biotic conditions in future environments. Our work illustrates that compensatory mechanisms, driving plastic responses to the spatial structure of multiple stressors, can define geographical patterns of unanticipated species resilience to global environmental change.

Description: Current climate change as a consequence of large-scale global emissions of greenhouse gases results in an unsurpassed warming of the Arctic, leading to melting of sea ice and glaciers. This results in an increased freshening of the ocean that, in combination with warming, affects the unique features of the Arctic environment and biodiversity. While the impact of changes in ice and temperatures on physical and chemical processes is well documented, the effects on the marine biology in the Arctic remain largely understudied. Greenland is the world's largest island stretching from 59°N to 83 °N, en-compassing 12% of the world's coastline. The largely north-south orientated coastlines of Greenland constitute a unique climate gradient from the subarctic to the High Arctic, along which multiple species meet their distribution limits. However, the biology of Greenland's coastal and intertidal systems has received limited attention and remains poorly understood. Therefore, by linking ecology, physiology and genetics of a keystone species, the blue mussel, this thesis aims at increasing knowledge of the Greenland intertidal zone by specifically studying what climatic and physiological factors determine the distribution and polarward limits of intertidal species. Hitherto, it has been commonly accepted that only one blue mussel species (Mytilus edulis) inhabited the Arctic. However, by utilizing genetic tools, we revealed that three blue mussel species inhabit the Arctic, and that the blue mussel M. edulis dominates in Southwest Greenland, while the congener M. trossulus dominates in the North (Paper I). Historically, work on the distribution of blue mussels in West Greenland has only been descriptive, but I quantified the abundance and population dynamics of the genus as far north as 77°N (Paper II). In doing so, I found that sub-zero air temperatures and air exposure time are of central importance for the distribution, and that abundances are controlled at the earliest life stage, not during adulthood. In addition, I performed a series of laboratory experiments to elucidate the importance of the physiology to the distribution. One proposed hypothesis was that polar-ward distribution is controlled by failed gonad maturation and reproduction. However, I showed that blue mussels at their northernmost limit in North Greenland are capable of producing mature gonads and spawn, and that spat settles annually (Paper III). Thus, I conclude suppressed gonadal development is not a key factor in shaping polar-ward distribution limits. Another tested hypothesis is the oxygen- and capacity-limited thermal tolerance hypothesis, which suggests that low temperatures control species distribution through failed aerobic metabolism. I studied this in Paper IV but found no indications of limited aerobic performance in populations from either South or North Greenland (Paper IV). Instead, it seems that blue mussels are capable of adjusting their aerobic performance on both temporal and spatial scales (Paper V). Furthermore, blue mussels at their polar-ward limit are facing prolonged winters with limited pelagic primary production. To investigate how they survive the prolonged winter, I used fatty acids to study their food preferences, both while ice-covered and during summer (Paper VI). I found, that the population feeds extensively on diatoms while covered by intertidal sea ice, but after the ice breaks up, food consists mainly of pelagic dinoflagellates. Thus, this population likely does not face severe starvation during ice cover, as long as ice-algae growth is sustained. Finally, I exposed the blue mussel Mytilus edulis simultaneously to a natural (sub-zero air temperatures) and a chemical stressor (lead, Pb) to investigate the effects of multiple stressors on survival (Paper VII). These two stressors were chosen, because natural Greenlandic M. edulis populations are exposed to both stressors near their polar-ward limit. Chemical stress has been found to decrease thermal tolerance of some ectotherms, thus making them vulnerable to sub-zero air temperatures. I found no interacting effects in the study, but since natural populations are commonly exposed to multiple stressors in their environment, acknowledging the potential importance of such interactions are necessary to understand species ecology and distribution and should be studied further. In conclusion, the drivers of species distribution in the Greenland intertidal zone are complex, and much remains to be done. However, by combining field and laboratory work within ecology, physiology and genetics, this thesis contributes new knowledge of processes and traits determining the distribution and distribution limits of intertidal species in the Arctic. This knowledge is important for our understanding of climate change impacts - now and in the future.