Snails wearing green heatproof suits: the benefits of algae growing on the shells of an intertidal gastropod

Abstract Positive interactions are known to play important roles in ecological divergence but have received less attention than negative interactions in studies on intertidal communities. Here, we show that growth of the mat‐forming green alga Pseudocladophora conchopheria on the shells of a intertidal gastropod ( Lunella coreensis ) provides them with benefits for living in intertidal rocky habitats. We found that P. conchopheria covered a large proportion of the shell surface in all L. coreensis inhabiting an exposed rocky shore habitat, but none or only a small proportion of the shell surface in L. coreensis inhabiting a damp boulder shore habitat. Thermal exposure experiment showed that L. coreensis originating from the boulder shore habitat had a higher mortality rate and temperatures inside the shell than those originating from the exposed rocky shore habitat. These differences disappeared when P. conchopheria was removed from the shell or was dried out, indicating that an increased coverage and dampness of the alga suppressed the increase in temperature inside the shell. These support the hypothesis that the growth of P. conchopheria on L. coreensis shells suppresses thermal stress in the hosts; the benefits of P. conchopheria may enable the hosts to exploit habitats where considerable thermal stress is present during air exposure.