Preview Illustration of colorful nudibranchs from Ernst Haeckel.
(Ernst Haeckel, Kunstformen der Natur / Biodiversity Heritage Library)

Nudibranchs (often casually called sea slugs) are soft-bodied, marine snails that have lost all trace of an external shell. The name “nudibranch” translates from the Latin as “naked gills.” Consisting of about 3,000 species, some have developed impressive defenses, including the ability to synthesize toxic compounds (such as sulfuric acid) or the ability to "hijack" and repurpose the stinging cells of the cnidarians (jellyfish, anemones, and corals) that some of them eat. Poisonous nudbranchs are often very brightly colored to warn potential predators that they are not good to eat.