They might look like rock or semi-precious stones but they are actually creatures known as by-the-wind-sailors, or velella velella in Portuguese.
They are usually only a few centimetres wide and look a bit like jellyfish, but are in fact groups of tiny animals living together.
Each has a little structure on top, like a sail, which helps it travel across the surface of the sea. The bright blue is a pigment that helps protect them from sunlight.
They are hydrozoans – tiny predators who live in groups in salt water, and are related to sea anemones and coral.
Marine specialist Frankie Hobro, of Anglesey Sea Zoo, said the creatures were “beautiful”, but warned people not to touch them because of their sting.