Predation governs the eulittoral distribution of a ubiquitous Mediterranean gastropod

Abstract

Topshell gastropods are among the most important intertidal biofilm grazers. Present throughout the Mediterranean, this sea’s only exclusively eulittoral trochid, Phorcus turbinatus, is spatially separated from its infralittoral congeners, P. articulatus, P. richardi and P. mutabilis. Despite its ubiquity, data on its fundamental ecology are lacking. By a series of laboratory and field experiments, I show that predation fixes the lower boundary of the vertical range inhabited by P. turbinatus precisely at lowest astronomical tide (LAT). Predicted in situ predation risk rapidly increases from 30% d–1 at the mean position of P. turbinatus (+0.31 m) to 84% d–1 at 1 m below LAT. At least nine predators across all major phyla prey on P. turbinatus. Important in terms of abundance, zonation and frequency of predation events are the two muricids Stramonita haemastoma (+0.08 m) and Hexaplex trunculus (−1.73 m) on exposed and sheltered coasts, respectively, along with Thalassoma pavo (−0.56 m) and Hermodice carunculata (−1.24 m). Physiological constraints seem to be of secondary importance since P. turbinatus survives up to 1 week fully immersed in situ. Therefore, predation risk likely is the ultimate driver of vertical distribution and thus physiological adaptation of littoral Mediterranean trochids.