Salinity Driven Selection and Local Adaptation in Baltic Sea Mytilid Mussels

Abstract

Baltic blue mussels can colonise and dominate habitats with far lower salinity (<10 psu) than other Mytilus congeners. Pervasive gene flow was observed between Western Baltic Mytilus edulis living at high salinity conditions and Eastern Baltic M. trossulus living at lower salinites, with highest admixture proportions within a genetic transition zone located at intermediate salinities (Darss Sill area). Yet, we do not understand the impacts of low salinity on larval performance, and how salinity may act as an early selective pressure during passive larval drift across salinity gradients. This study tested whether larvae originating from two different populations along the natural salinity cline in the Baltic Sea have highest fitness at their native salinities. Our results suggest that Eastern Baltic M. trossulus (Usedom, 7 psu) and Western Baltic M. edulis (Kiel, 16 psu) larvae display better performance (fitness components: growth, mortality, settlement success) when reared at their respective native salinities. This suggests that these populations are adapted to their local environment. Additionally, species diagnostic markers were used for genetic analyses of transition zone (Ahrenshoop, 11 psu) mussel larvae exposed to low salinity. This revealed that low salinity selection resulted in a shift towards allele frequencies more typical for Eastern Baltic M. trossulus. Thus, salinity acts as a selective pressure during the pre-settlement phase and can shape the genetic composition of Baltic mussel populations driving local adaptation to low salinity. Future climate change driven desalination, therefore, has the potential to shift the Baltic Sea hybrid gradient westward with consequences for benthic ecosystem structure.

SNP discovery and genetic structure in blue mussel species using low coverage sequencing and a medium density 60 K SNP‐array

“…This can be explained by a better efficiency to detect introgression across a semi‐permeable barrier to gene flow with an increased number of markers distributed along a larger portion of the genome, as discussed by Fraïsse et al (2016). In conclusion, the species distribution patterns revealed by lcWGS data in this study support previous data for the Baltic Sea (Knöbel et al, 2021; Stuckas et al, 2017), Southwest England (Hilbish et al, 2002; Vendrami et al, 2020), Mediterranean (Boukadida et al, 2021; Simon et al, 2021), USA (Saarman & Pogson, 2015) and Chilean (Araneda et al, 2016) populations. Such observations provide a valuable insight into species distributions and admixture in the blue mussel species‐complex.…”

“…Broad‐scale population structure and population dynamics in this species complex is therefore predominantly shaped by interactions between oceanography and the biology of each species. Both pre‐ and post‐settlement selection drive geographical and ecological segmentation, and contribute to determining species distribution and in shaping hybrid zones (Bierne et al, 2002; Bierne, Bonhomme, & David, 2003; Knöbel et al, 2021; Koehn et al, 1980). Moreover, the success of mussel aquaculture is tightly coupled to the environment, across all stages of production, with environmental change also influencing key performance traits including growth, survival, and susceptibility to disease (Nascimento‐Schulze et al, 2021).…”

“…In blue mussels, nuclear and mitochondrial DNA markers have been widely used to investigate evolutionary processes (Quesada et al, 1998; Rawson & Hilbish, 1995; Zouros et al, 1994), species genetics (Koehn, 1991; McDonald et al, 1991), hybridization patterns (Bierne, Borsa, et al, 2003; Riginos & Cunningham, 2005; Stuckas et al, 2017) and selection (Bierne, Bonhomme, & David, 2003; Fraïsse et al, 2016; Knöbel et al, 2021; Koehn et al, 1980). Linkage maps have been developed for Mytilys edulis (Lallias et al, 2007), whilst low‐density SNP‐panels have been used to delineate species and genetic lineages within the Mytilus genus (Simon et al, 2021; Wilson et al, 2018), to investigate the shell trait‐species correlation (Carboni et al, 2021) and also for pedigree reconstruction (Nguyen et al, 2014a).…”

Capacity for Cellular Osmoregulation Defines Critical Salinity of Marine Invertebrates at Low Salinity

“…Altogether, a critical salinity range of 7-10 can be suggested, in which a majority of blue mussels can still survive, yet with energetic trade-offs due to costs for cellular osmoregulation that affect growth, which was demonstrated by a 64% reduction in growth. While they can, hence, tolerate lower salinities under laboratory conditions, M. edulis-like genotypes (as used in our study) are replaced in the field by M. trossolus-like genotypes already at salinities <10 (Stuckas et al, 2017;Knöbel et al, 2021). The distribution limit of M. edulis-like mussels in the Baltic Sea is therefore defined by M. edulis' physiological limits.…”

Towards environmentally friendly finfish farming: A potential for mussel farms to compensate fish farm effluents

“…The brackish nontidal Baltic Sea is colonized by M. edulis and M. trossulus ; the species inhabit various hard and mixed bottom subtidal habitats. Due to its wider salinity tolerance, M. trossulus is distributed almost throughout the Baltic Sea, while M. edulis occupies the westernmost higher salinity sub‐basins (Kijewski et al, 2019; Knöbel et al, 2021; Stuckas et al, 2017). Nevertheless, there exists no pure M. trossulus in the Baltic Sea with all mytilids having various fractions of M. edulis alleles in their genomes (Kijewski et al, 2019).…”