This tiny guy is commonly know as the blue glaucus, sea swallow, or Glaucus Atlanticus. It is a species of sea slug that grows to around 35 mm. They float partially by means of an air bubble, which they swallow and store in their gastric cavity. They also have a rather unique defence mechanism - they store the nematocysts produced by jellyfish (their prey) in their own tissues to protect against predators.