Massive strandings of Velella velella (Hydrozoa: Anthoathecata: Porpitidae) in the Ligurian Sea (North-western Mediterranean Sea)

Velella velella, the so-called by-the-wind sailor, is a common member of the open-ocean pleustonic fauna, worldwide distributed in tropical and temperate regions. Thanks to their sail protruding above the sea surface, floating polymorphic colonies of this hydrozoan are carried by winds, and tend to aggregate in large swarms, that often get stranded along the shores. Although these events are commonly observed in springtime along the Ligurian coasts (North-western Mediterranean Sea), no quantitative characterization was ever made. The aim of this study was to characterize the stranding events that occurred in spring 2016 along the Ligurian coast, by evaluating the influence of the local sea conditions and by quantifying the abundance of the stranded colonies in each event. Their size-frequency distribution and biomass were examined, and the associated mollusc fauna identified and counted. The magnitude of these episodes was so relevant that, along the Ligurian coasts, the V. velella strandings constitute one of the most important biological deposition of organic matter; nevertheless, the ecological role of these remarkable and stochastic accumulations of chitin along the coast is still unexplored.