Littorina littorea show small-scale persistent tidal height and habitat partitioning that is resilient to dislodgement through specific movement rates

Dislodgment, in particular by breaking waves, is a widely acknowledged source of stress, if not mortality, in rocky shores intertidal gastropods. This raises the question of understanding if snails dislodged from a specific habitat have the ability to navigate back to their original substrate, and how this is achieved. In this context, we investigated the consequences of dislodgement on a population of Littorina littorea inhabiting two proximate habitats (a seawall ranging from the low to the high intertidal zone and the related low intertidal rocky platform) located on an isolated offshore reef over a three-month period. Using tagged undisturbed, disturbed, translocated and transplanted L. littorea, we assessed a potential tidal height preference and found that L. littorea showed a clear tendency to remain on their original tidal height, with recapture rates ranging between 100% and 88%, respectively 1 and 78 days after dislodgment. We subsequently measured the recapture rates of individuals dislodged from three distinct tidal levels on the seawall and from a horizontal platform. Dislodged L. littorea showed a clear tendency to return to their original tidal level, with return rates ranging between 32 and 48% 78 days after dislodgement; snails from the horizontal platform were never found on the proximate seawall. In addition, high frequency observations conducted on individually tagged dislodged L. littorea during the first 8 successive low tides of our survey indicate that individuals originating from the seawall consistently showed higher movement rates and directionality towards the seawall, while individuals originating from the platform showed no directionality and reduced migration rate. These results show that dislodged L. littorea navigate back to their original substrate and tidal height. Putative processes include a combination of geotaxis, chemotaxis and rheotaxis. At relatively short time scales (i.e. days to months), L. littorea show both tidal and habitat partitioning that is distinct from the movement patterns previously reported in littorinids over a range of temporal scales.