Last snail standing? superior thermal resilience of an alien tropical intertidal gastropod over natives in an ocean-warming hotspot

Ocean warming threatens the stability and survival of marine ecosystems, exposing species to lethal and sub-lethal effects. This is true particularly at their warm distributional edge, and more so, in extreme and highly variable environments like the intertidal zone where they can be often exposed to temperatures close or beyond their thermal limits. When thermophilic non-indigenous species invade such extreme environments, they should have a physiological advantage over native species. However, this assumption was rarely investigated. To test this, we investigated the thermal environment and vulnerability of four intertidal gastropods, two native topshells and two limpets, one native , and one Indo-Pacific, in the rapidly-warming southeastern Levantine basin, the warmest part of the Mediterranean Sea. Species metabolic performance was tested under a wide water temperature range, 18–37 °C, by measuring their respiration rates (a proxy for metabolic rates) and survival. The optimum of the three native snails ranged between 29 and 33 °C, and they all died at 35 °C, though at different rates. The measured optimum of the native limpet, was higher than that assessed in the western Mediterranean, suggesting either phenotypic plasticity or genotypic adaptation for warmer conditions. With the recent warming of 2–3 °C, all native snails have become much closer to their thermal edge. The tropical limpet survived, however, at 35 °C for weeks and its respiration rates did not decrease under this extreme temperature. After few more decades of Levantine warming, the tropical limpet might thus be the last large mid-intertidal snail remaining, and the main one to sustain the important function of a macro-grazer on endo- and epi-lithic algae at this zone.