Who is the sea swallow? (in the picture)

The sea swallow, blue angel, is a species of small, blue sea slug. These sea slugs are pelagic: they float upside down by using the surface tension of the water to stay up, where they are carried along by the winds and ocean currents. Glaucus atlanticus feeds on other pelagic creatures, including the venomous siphonophore and the Portuguese man o' war.