Lethal and sub-lethal responses of rocky shore gastropods to extreme temperatures

Intertidal communities are more frequently experiencing extreme air and sea temperatures as a result of anthropogenic climate change, with heatwaves increasing in intensity and duration. Most studies exploring the thermal tolerances of intertidal species to extreme temperatures haven't directly studied early life stages. We explored whether locally extreme tide-out temperatures were lethally impacting premature Boreal (Littorina littorea and Steromphala cineraria) and Lustanian (Phorcus lineatus and Steromphala umbilicalis) intertidal gastropod species from southwest England. Two separate experiments tested lethal and sub-lethal responses to simulated local heatwave and cold spell temperatures. Two sensitive early size classes (new recruits and year 1+ juveniles) for each species were exposed to simulated extreme and ambient tide-out temperatures in replicated boulderfield (air) and rockpool (seawater) microhabitats per experiment. Realistic experimental temperatures were determined by EnvLogger temperature data recorded locally in sun-exposed, shaded and rockpool intertidal habitats. For each nine-day experiment, specimens were exposed for six hours daily to four laboratory thermal treatments simulating both ambient conditions and extreme temperature events. These experiments were designed to test the following questions: are premature trochids and littorinids more sensitive to extreme temperatures than adults, and is there a difference in response between species of Boreal and Lusitanian origin sampled from the same geographical region? We identified that the premature intertidal trochid species may not be more thermally sensitive than adults, although premature L. littorina may have a slightly lower lethal heat limit. Survivorship in extreme heatwave air temperatures (40.0–42.9 °C) was lowest for lowshore and highest for highshore species, relating directly to species' vertical shore zonation. There were minimal to zero mortalities in all other thermal treatments. Following stress-induced inactivity in the extreme heatwave air temperature treatment, S. cineraria and P. lineatus specimens all died. In contrast, L. littorea and S. umbilicalis specimens recovered in some instances. Intensifying and more frequent aerial heatwave events occurring unseasonably are likely to cause population declines, local extinctions or subtidal retreat for premature lowshore Boreal intertidal species, influencing recruitment to adult populations, especially where rockpools are not present to provide thermal refugia. Conversely, mid-highshore Lusitanian species are likely to be more resilient to high air temperatures during low tides. Cold spell temperatures did not lethally or sub-lethally impact any juvenile gastropod species, suggesting Lusitanean species will thrive as a result of increasingly milder winters.