The Dancing Marsenia: The First Record of a Swimming Velutinid Mollusc

Among gastropods, the ability to swim has developed independently several times, mostly among Heterobranchia. Only a few species of Caenogastropoda are known to have swimming adults. Velutinidae Gray, 1840, is a family of caenogastropods with a fragile shell enclosed by the mantle. The adults of this family are benthic and ectoparasites of ascidians. Here, we present the first recorded instance of a swimming velutinid, Marsenia cf. gemma, filmed in New Caledonia. The swim propulsion method was based on the movement of the snail's foot, which followed a infinity-shaped curve. This first report changes our perspective on the biology of this family, unexpectedly placing it within the restricted group of swimming caenogastropods.