Nudibranchs are the marine equivalent of a garden slug. However, unlike terrestrial slugs, nudibranchs can often be quite colorful and attractive. The largest of the Hawaiian nudibranchs is the Spanish dancer (Hexabranchus sanguineus) which can sometimes achieve a length of over one foot.
While most nudibranchs crawl across the substrate, the Spanish dancer, when alarmed, can swim through the water column by flexing and extending its body in an undulating fashion. In doing so, its extended mantle waves rhythmically much like a flamenco dancer's skirt!