Critical salinity of benthic osmoconformers in the Baltic Sea

The Baltic Sea is an osmotically challenging habitat, with a steep salinity gradient from 1 to 35. Climate change scenarios predict a salinity decrease in the brackish waters of the Baltic Sea by ca. 2. This desalination may severely impact organisms living at their physiological limit, ultimately leading to range shifts and community rearrangement. Hypoosmotic stress is known to affect survival, growth and reproduction. Still, little is known about consequences of long-term osmotic stress and physiological salinity tolerance mechanisms of benthic invertebrates inhabiting coastal Baltic ecosystems. Most marine invertebrates are osmoconformers, meaning that their body fluids are isosmotic to their environment. They acclimate to changing salinities by cellular volume regulation using intracellular osmolytes. The concept of critical salinity (S_crit) by Podbielski et al. (2016) postulates that the depletion of the intracellular organic osmolyte pool defines a critical lower salinity limit that determines loss of fitness. This study tests the concept of S_crit to estimate the lower salinity limits of six osmoconforming species from three phyla (Cnidaria, Mollusca, Echinodermata). Animals were acclimated to seven salinity treatments for four weeks to measure survival and growth (fitness proxies) in relation to tissue water content and osmolyte concentrations. Typically, species showed a decrease in survival rate with decreasing salinity, as well as a biomass decrease beyond a certain salinity threshold. Positive growth rates could only be measured at higher salinities. Tissue water content increased in five species with decreasing salinity. Our data will allow us to assess, whether the S_crit-concept is generally applicable and whether similar osmoregulatory strategies have evolved in marine osmoconformers.