Spatial and temporal genetic structure of Velella velella (Hydrozoa, Porpitidae) and its predator Janthina pallida (Gastropoda, Epitoniidae)

The by-the-wind sailor Velella velella is a cosmopolitan, pleustonic colonial hydrozoan, common in the Mediterranean Sea. It forms offshore aggregations that occasionally strand on the coasts, following strong wind conditions. Colonies are frequently discovered around Janthina pallida Janthina pallida, a pleustonic gastropod that feeds on them and floats on the secretion of a bubble raft. Because of the massive occurrence of these species along the Italian coasts, and their often neglected ecological roles, it is pivotal to increase the knowledge of their genetic structure. Analyses of the genetic variability of V. velella and J. pallida, stranded on the beach in springs 2016 and 2017 at ten locations along the Ligurian and Tyrrhenian seas, show the existence of panmictic Mediterranean populations. Nevertheless, V. velella showed high genetic variability with a high number of singleton haplotypes, while J. pallida showed a recent population expansion. Considering the ecological and socio-economic importance of gelatinous zooplankton in today's changing marine ecosystems, these results lay the basis for a better understanding of the ecological role and population dynamics of Mediterranean pleustonic species.