Janthina janthina Linnaeus 1758

violet snail, purple bubble snail

The empty purple shells are often found washed up on beaches. Less commonly there are strandings of live animals. They live on the ocean surface supported by a transparent float of hardened mucus bubbles. They eat bluebottles.
They are widely distributed in warm seas. Each animal is released live from the parent, starting life as a male, later transforming into female.

Naming notes:-
from the Latin ianthinus meaning violet-blue.