Physiological and cellular responses of Mytilus galloprovincialis to hyposalinity stress in contrasting urbanized coastal environments

Global climate change and anthropogenic developments have intensified the coastal hydrological cycle, increasingly subjecting marine ecosystems to acute hyposaline conditions primarily from urban stormwater runoff. This study investigated the physiological responses of the invasive Mediterranean mussel, Mytilus galloprovincialis, to osmotic stress by studying two populations in Los Angeles County, CA with contrasting hydrology: Ballona Creek (BC), a concrete channel characterized by highly variable salinity, and Marina Del Rey Harbor (MDR), a hydrographically stable marine environment. We collected mussels during the dry (fall) and wet (winter) seasons and exposed them to salinities ranging from 40 to 5 psu over 7 and 14 d periods. We quantified hemolymph osmolarity, metabolic rate, clearance rate, cellular stress responses via Hsp/Hsc 70 relative protein abundance, mortality, and body condition index (BCI). Hemolymph analysis confirmed that M. galloprovincialis is a euryhaline osmoconformer from 40 to 15 psu but transitions to hyperosmotic regulation under severe hyposalinity (< 15 psu). Whole-organism performance metrics (metabolic and clearance rates) were significantly elevated during the wet compared to the dry season across both populations. Metabolic and clearance rates peaked at intermediate salinities (25 psu) before undergoing a severe drop (up to 95%) at the lower threshold (5 psu) signifying metabolic depression tied to defensive shell valve closure in response to hyposalinity. Site-specific differences were evident where the variable saline environment experienced by the BC population displayed a primed cellular defense to hyposalinity. BC mussels were characterized by a 4.1-fold increase in Hsp/Hsc 70 relative protein abundance at 20 psu and superior survival under chronic stress, with a median lethal salinity (LS50) of 13 d at 10 psu compared to just 7 d for MDR mussels. Additionally, chronic 14 d exposure to 15 or 25 psu significantly reduced BCI for mussels from both sites. Together, the reduced feeding and metabolic depression under sever hyposalinity suggest limited energy acquisition and may have resulted in greater mortality. Ultimately, this research highlights that local environmental histories shape physiological resilience, which will dictate how marine bivalves cope with future climate-driven precipitation anomalies.