Diversity and abundance of heterobranchs (Mollusca, Gastropoda) from the mesophotic and bathyal zone of the Mediterranean Sea
Margherita Toma, Federico Betti, Giorgio Bavestrello, Riccardo Cattaneo‐Vietti, Simonepietro Canese, Alessandro Cau, Franco Andaloro, Silvestro Greco, Marzia Bo
Abstract
The occurrence, relative abundance and bathymetric distribution of the deep Mediterranean heterobranch fauna were \nevaluated as a result of an extensive visual census down to 1825 m depth, carried out along the western Italian coasts, \ncovering a latitudinal range of about 600 nautical miles. Observations were conducted using Remotely Operated Vehicles \n(ROVs) in 551 sites, focusing on the deepest part of the continental shelf, the shelf edge and the upper bathyal zone. The \nanalysis of over 508 hours of video footage and more than 27,000 high-resolution photographs allowed to explore about \n594,000 m2 of rocky outcrops and nearby soft seafloors in four main coastal areas (Ligurian Sea, north-central Tyrrhenian \nSea, southern Tyrrhenian Sea, and Sicily Channel) and twelve offshore seamounts. Thirty-six species of heterobranchs, for \na total of 559 records, were identified. Sixteen were only sporadically spotted (< 1% of the total observations), while three \n(Paraflabellina ischitana, Tritoniidae nd and Peltodoris atromaculata) contributed each for more than 10% of the records. An \nextension of the known Mediterranean bathymetric distribution was reported for 80% of the observed species, with many \ntypically shallow-water taxa being also found in mesophotic environments (40–200 m). The observed marked decrease in \ndiversity and abundance suggested that Mediterranean heterobranchs are mainly a coastal benthic group. From the \ngeographical point of view, the heterobranch fauna present in the Ligurian Sea appeared significantly different from that \nobserved in the Tyrrhenian Sea and the Sicily Channel, and this latitudinal pattern was discussed. Particular attention was \ngiven to single out the bathymetric distribution of the food sources of the most frequently observed species, supporting \nstenophagy as a limiting factor for their colonisation of the deep sea.