No effect of interspecific thermal tolerance on the stability of a blue mussel, Mytilus sp., hybrid zone

Arctic coastal ecosystems are undergoing rapid transformation as accelerated warming intensifies cryosphere melt and drives poleward range expansions of boreal species. In West Greenland, the blue mussels Mytilus edulis and M. trossulus form a distinct hybrid zone that may be destabilized in response to increasing temperatures and changing salinity regimes. To assess the impacts of warming and salinity stress on the hybrid zone stability, we conducted a mortality experiment. Mussels were exposed to three salinities (5, 15, or 25 PSU) for seven days before daily exposure to one of four air temperatures (5, 30, 33, or 36°C) for 1.5 hours over the course of six days, representative of conditions in the collection area. Survival was assessed daily, and we measured the expression of six stress-related genes (hsp70, hsp90, sod, oat, p38, and p53) in M. edulis, M. trossulus, and their hybrids after six simulated tidal regimes. Mortality increased significantly with increasing temperatures and decreasing salinity across all species, with >75% mortality at 36°C, but no interspecific differences in survival or gene expression were detected. Our findings suggest that atmospheric warming alone is unlikely to destabilize or shift the M. edulis-M. trossulus hybrid zone northward. Instead, other abiotic and biotic constraints, such as prolonged exposure to sub-zero temperatures, food availability, or reproductive limitations, may limit the poleward expansion of M. edulis. Our study provides the first experimental evidence of comparable thermal and salinity tolerance among co-occurring Mytilus congeners in the Arctic, highlighting the resilience of this hybrid zone under near-future climate scenarios.