Sometimes the beaches are covered with thousands or millions of small jellyfish called Velella velella.
The blue by-the-wind sailor
The sail jellyfish is a species of cnidarians hydrozoan a pelagic exclusively feeding on zooplankton.
According to countries and regions, it can also be called raft of St. John, boat of St. Peter veil jellyfish, Jack sail by the wind or velero. Despite its name, this is not a jellyfish. Every polyp belongs to a conlony and in every colony, polyps have a specific task, eat, reproduce, defend etc.
It looks like a small oval raft topped with a veil. Tiny floats allow the Velella to stay at surface of temperate seas and moves thanks to the wind. Good news for swimmers, it does not sting !