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.
Highlights
The pronounced natural salinity gradient in the Baltic Sea, ranging from 30 psu at the transition to the North Sea to less than 4 psu in the innermost sub-basins (Bothnian Bay and inner Gulf of Finland) offers a natural context in which to explore evolutionary mechanisms underlying the colonisation of extreme environmental conditions (Johannesson et al, 2011; Wennerström et al, 2013)
With our improved larval culturing system, we successfully reared Baltic Sea mussel larvae at low salinities (
Low salinity had an effect on fitness-related parameters in both Baltic M. trossulus and Baltic M. edulis larvae
Summary
The pronounced natural salinity gradient in the Baltic Sea, ranging from 30 psu at the transition to the North Sea to less than 4 psu in the innermost sub-basins (Bothnian Bay and inner Gulf of Finland) offers a natural context in which to explore evolutionary mechanisms underlying the colonisation of extreme environmental conditions (Johannesson et al, 2011; Wennerström et al, 2013). Apart from evolutionary questions, understanding how Baltic organisms cope with extreme environmental conditions holds the key to predicting and mitigating consequences of global change on marine biodiversity (Reusch et al, 2018). In this marginal sea, climate models predict a decrease. Species-specific allele frequencies change gradually resulting in allele frequency clines and a transition zone that is situated along longitude 12–13◦E (a virtual line between Öresund and Darss Sill) This is an area of maximum genetic admixture where repeated backcrossing results in the coexistence of individuals with highly different proportions of alleles specific to M. edulis (ME-alleles) and M. trossulus (MT-alleles). Despite of pervasive interspecific gene flow between Baltic mussels, substantial dissimilarities in shell morphology and physiology between Western Baltic M edulis and Eastern Baltic M. trossulus still exist (Kautsky et al, 1990; Tedengren et al, 1990; Telesca et al, 2018)
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call