The intertidal periwinkle <i>Littorina littorea</i> actively chooses endolith-infested mussel beds as thermally benign habitats under extreme heatwave conditions

ABSTRACT Extreme events such as heatwaves have become more frequent, severe and long-lasting. Microbial endolithic communities, which have long been considered as parasitic and detrimental to intertidal mussels, have increasingly been considered as thermally advantageous for mussel beds through the increased albedo related to the white shell discolouration, which lowers body temperatures and increases survival rates. Despite the putative beneficial thermal effects endolith-infested mussel beds may confer to infauna using these beds as habitats, we are still critically lacking information about the ability of highly motile species to actively choose infested beds over noninfested ones. The present study was opportunistically conducted during the unprecedented mid-July 2022 atmospheric heatwave that coincided with midday low tides to investigate whether the common periwinkle Littorina littorea actively selects infested beds over noninfested ones, and whether L. littorea inhabiting infested beds had lower body temperature than those found in noninfested beds and on bare rocks. Littorina littorea is shown to consistently actively select endolith-infested Mytilus edulis patches as thermally benign microhabitats under conditions of extreme heat stress. This behaviour drastically decreased L. littorea body temperatures, which were on average 8.7 °C and 10.2 °C cooler than individuals observed in noninfested mussel patches and on bare rock, respectively. This decrease notably surpasses by far the heat-induced microhabitat choice reported in the literature and the putative beneficial effects of aggregation behaviour and evaporative cooling as thermal adaptations to heat stress. They were, however, comparable to the thermal benefits of postural behaviours reported for high-shore littorinids that experience some of the most stressful intertidal thermal conditions on a global scale. The ability of L. littorea, and eventually other motile infauna species to choose thermally benign endolith-infested mussel beds may then alleviate thermal stress through an increase in their thermal safety margin.