Janthina is a genus of sea snails which float, impassively, in the oceans. They are thin-shelled, exquisitely shaded in two hues of purple and mauve and stay afloat by constructing a raft of air-filled bubbles. They feed on small jellyfish and medusae, also coloured purple. When there are prevailing south-westerlies for two weeks, long enough to bring these essentially tropical snails close enough to our coastline, the western seaboard of the British Isles may experience mass beachings of these snails. When this happens the strandline turns purple.