The Sea Swallow(Glaucus atlanticus)
Also known as the Blue Sea Slug, the Blue Glaucus, and the Blue Ocean Slug, this is a species of nudibranch (sea slugs.) It will typically grow up to 3cm in length with six “feathered” appendages. This species is pelagic: it lives in the open ocean. It floats upside-down using the ocean’s surface tension and a ventral gas sac. The Swallow utilizes counter shading: it’s dorsal side is silver grey, and it’s ventral side is striped with dark and pale blues. But the most interesting thing about Glaucus is its feeding habits… it is immune to the venomous sting of nematocysts of sea jellies. So, it feeds on the most venomous ones like the Portuguese Man of War (Physalia physalis) and other surface-dwelling organisms like Janthina janthina. Glaucus, once devouring the most dangerous parts of these organisms, selects the highest-quality stinging cells and stores them in their cnidosacs in the tips of their feather-like fingers.
Learn more about the Glaucus here