Being young in a changing world: how temperature and salinity changes interactively modify the performance of larval stages of the barnacle Amphibalanus improvisus

The fate of key species, such as the barnacle Amphibalanus improvisus, in the course of global change is of particular interest since any change in their abundance and/or performance may entail community-wide effects. In the fluctuating Western Baltic, species typically experience a broad range of environmental conditions, which may preselect them to better cope with climate change. In this study, we examined the sensitivity of two crucial ontogenetic phases (naupliar, cypris) of the barnacle toward a range of temperature (12, 20, and 28°C) and salinity (5, 15, and 30 psu) combinations. Under all salinity treatments, nauplii developed faster at intermediate and high temperatures. Cyprid metamorphosis success, in contrast, was interactively impacted by temperature and salinity. Survival of nauplii decreased with increasing salinity under all temperature treatments. Highest settlement rates occurred at the intermediate temperature and salinity combination, i.e., 20°C and 15 psu. Settlement success of “naive” cyprids, i.e., when nauplii were raised in the absence of stress (20°C/15 psu), was less impacted by stressful temperature/salinity combinations than that of cyprids with a stress history. Here, settlement success was highest at 30 psu particularly at low and high temperatures. Surprisingly, larval survival was not highest under the conditions typical for the Kiel Fjord at the season of peak settlement (20°C/15 psu). The proportion of nauplii that ultimately transformed to attached juveniles was, however, highest under these “home” conditions. Overall, only particularly stressful combinations of temperature and salinity substantially reduced larval performance and development. Given more time for adaptation, the relatively smooth climate shifts predicted will probably not dramatically affect this species

From geography to genes: evolutionary perspectives on salinity tolerance in the brackish water barnacle Balanus improvisus

How species respond to changes in their environment is a fundamental question in biology. This has become an increasingly important issue as anthropogenic effects of climate change and biological invasions have major impacts on marine ecosystems worldwide. In this thesis I investigated the role of salinity tolerance from an evolutionary perspective, using a wide range of techniques, spanning from population genetics and common-garden experiments to characterizing potential genes involved in osmoregulation in barnacles. I used the acorn barnacle species Balanus (Amphibalanus) improvisus, which displays a remarkably broad salinity tolerance, to investigate how this trait has influenced the species' potential to establish in new environments, and respond to projected near-future salinity reductions in coastal seas. I also examined physiological and molecular mechanisms that may be involved in osmoregulation in B. improvisus. I further analysed population genetic structure using microsatellites and mitochondrial DNA, and related the results to anthropogenic and natural dispersal dynamics on both global and regional (Baltic Sea) scales. I found high genetic diversity in most populations, with many shared haplotypes between distant populations. This supports the hypothesis that maritime shipping is an important vector for the dispersal of the cosmopolitan species B. improvisus. Nonetheless, natural larval dispersal is also important on smaller geographical scales, such as within the Baltic Sea. Marked genetic differentiation between northern and southern Baltic Sea populations raises the question whether there is restricted gene flow within the Baltic Sea, creating potential for local adaptations to evolve. To investigate the extent to which the broad distribution of B. improvisus along the Baltic Sea salinity gradient is explained by local adaptation versus physiological plasticity, I performed a common-garden experiment in which multiple populations were exposed to different salinities and multiple fitness-related phenotypic traits were recorded. The experiment confirmed that phenotypic plasticity, rather than local adaptation, explained the broad distribution of the species along the salinity gradient. Interestingly, all populations of B. improvisus performed best at low and intermediate salinities in many fitness-related traits (survival, growth and reproduction), although other traits (e.g. shell strength an juvenile growth) indicated higher costs associated with low salinity. A candidate gene approach was used to investigate the molecular basis of broad salinity tolerance in B. improvisus by characterizing the Na+/K+ ATPase (NAK) of B. improvisus – an ion transporter commonly involved in active osmoregulation in many species. We identified two main gene variants in B. improvisus (NAK1 and NAK2), and found that NAK1 mRNA existed in two isoforms that were differentially expressed in different life stages and adult tissues, suggesting an active role in osmoregulation. Lastly, I summarise current knowledge about salinity tolerance in barnacles and outline new research directions to further our understanding of the physiological and molecular mechanisms involved in salinity tolerance in barnacles