Seasonal impact on the thermal tolerance window of a keystone coastal grazer, the common periwinkle Littorina littorea.

Temperature influences the performance of marine ectotherms, and understanding species' thermal responses is key to predicting climate change impacts. This study investigated seasonal acclimatization effects on the thermal tolerance of the common periwinkle Littorina littorea. Snails were collected in winter and summer on the North Sea Island of Helgoland and exposed to gradually increasing temperatures (1.5 °C/h from the sampling temperature, 10-16 °C to 34 °C). Behavioral responses (antenna extension and movement onset) were assessed at target temperatures (10/16, 22, 25, 28, 31, and 34 °C), alongside cellular responses measured via activity of catalase (CAT), Hsp70 content and 1H NMR-based metabolite profiling. Both winter and summer groups showed delayed behavioral responses above 22 °C, indicating a similar behavioral breakpoint temperature across seasons. Metabolite data revealed temperature-dependent changes only in the winter group, with elevated succinate, glutathione, glutamine and propionate levels at 28 °C, indicating anaerobic metabolism and onset of critical thermal limits. While the upper suboptimal temperature appeared similar between groups (22 °C), the upper critical temperature (Tc) was detected only in the winter group (28 °C). Summer snails had significantly higher baseline Hsp70 levels and slightly higher CAT activities, indicating enhanced cellular protection in conjunction with a seasonal shift in Tc. Overall, the results show that seasonal acclimatization alters acute thermal responses in L. littorea and behavioral metrics are confirmed as valuable sublethal indicators for assessing heat tolerance in marine invertebrates.