A new glowing mollusc, Bathydevius caudactylus, has been discovered in ocean's midnight zone. About the size of an apple, this bioluminescent sea slug has a paddle-like tail and a large gelatinous hood. Unlike typical sea slugs, which live on seafloor or near surface, it thrives at depths between 3,300 and 13,100 feet, making it the first known deep-ocean sea slug.