Invasione di velella velella e janthina sp sulla spiaggia di ostia
Janthina is a genus of small to medium-sized pelagic or planktonic sea snails, marine gastropod molluscs in the family Janthinidae, commonly known as the "purple snails" or the "violet shells".
Species in this genus occur worldwide in tropical, subtropical and warm seas.
Description
These snails are pelagic and live at the surface of the ocean. Adult snails may not be capable of swimming, and die when they are detached from their rafts; Janthina janthina larvae, however, actively swim in the water column.
The adult snails prey upon (and live near to) one of several species of pelagic animals loosely known as jellyfish. More specifically they eat the medusae of free-swimming Cnidaria, in particular the genus known as "by-the-wind sailors", Velella.
The snails are able to float securely because they create a raft of clear chitin around air bubbles formed near the ocean surface. They anchor to this raft using their foot. The snails do not have an operculum.
These snails are frequently washed ashore during storms.
The common names for this genus derive from the light purple or violet color of the shells and the violet-colored bodies. The other genus in the family, Recluzia, has olive-tan colored shells.
Species
This genus has accumulated a very large number of species names over the centuries. Most of the names that have been given are in fact synonyms of just a few species which have world-wide distributions in tropical waters. Experts disagree about some details of the synonymy.
