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This unusual sea slug, the pelagic aeolid nudibranch Glaucus atlanticus, also known as the "sea swallow" has a flattened body with rayed cerata and dark blue stripes along the edge of its foot. This sea slug lives floating upside down at the surface of the water, as shown here from above. Its body is dark and pale blue ventrally (the upper surface as it floats), and silvery grey on its true dorsal side.

This species typically grows to 6 cm in length, and it feeds on medusas, including venomous ones, and also on other pelagic gastropods. Because this sea slug concentrates the stinging power of the medusas, it can deliver a very painful and somewhat dangerous sting to humans.