Glaucus atlanticus: The Blue Sea Slug, Blue Ocean Slug, or the Sea Swallow
This awesome mollusc is a drifter, one of the few species that floats the open ocean in both temperate and tropical waters, across most of the Atlantic, Pacific, and Indian Oceans. Though it’s never close to the beaches (aside from periods of intense wind), the tidal currents pull the Blue Sea Slug along the shorelines, a few miles out to sea.
They subsist on a diet of Portuguese Man-o’-wars, other extremely dangerous hydrozoans, and sometimes each other - they’re beautiful, but deadly if eaten. The nematocysts that are consumed by these guys are stored in their cerata (outgrowths). Though the nematocysts don’t fire from inside the cerata, the venom that they’d normally inject while being part of the Man-o’-War is still present, and active as a compound.
Toxic though they are, they still face seabird strikes while floating out at sea, and as such, half of them is a dull grey color, and the other half is bright shining blue.
Fun fact! Gastrodon from Pokemon is supposedly based off of the Sea Swallow, and just like this mollusc, it has different coloration based upon the sea that it’s caught in.
Det Kongelige Danske videnskabernes selskabs skrifter, Vol. 7: Anatomiske Bidrag til Kundskab om Aeolidierne. Rudolph Bergh, 1864-1868.