The name "Spanish Dancer" often conjures up the image of a beautiful woman dancing the night away in an array of colorful skirts and scarves. Many people are surprised to learn that, contrary to this image, a Spanish Dancer is a type of sea slug that falls under the category of nudibrachs, also referred to as nudies. Hexabranchus sanguineus, its scientific name, is a bright red gastropod sea slug found in the tropical areas of Pacific Ocean waters, most commonly in caves or around rocky surfaces. It has also been found swimming in the Red Sea, and it is one of the largest nudibrachs, with the ability to grow up to 15.75 inches (40 cm). While Spanish Dancers are considered a type of sea slug, not all sea slugs belong in the nudibrach category, which has caused some confusion to the public.
The Hexabranchus sanguineus derives its scientific name from its readily apparent characteristics. The Latin name literally means "six gilled blood colored." The Spanish Dancer was first observed in the Red Sea by German naturalists Eduard Ruppell and Friedrich Leuckart in 1828. Since then, other variations of the Spanish Dancer have been found to be more common. Instead of the pure red color that gives the Hexabranchus sanguineus its name, a more mottled version appears to be more common, with yellow variations also existing more rarely. The sea animal's common name comes from its appearance as it moves; when it swims, its leg-like appendages, or parapodia, flap about like the skirts of a flamenco dancer.
Unlike other nudibrachs, this red sea slug has each of its six gills attached separately to its body. These gills, like those of other nudies, can be retracted when danger is near. Also similar to others of its classification, Spanish Dancers have two rhinophores on its front end. These rhinophores act as scent receptors, telling the slug when food or danger is in the area. Like other nudibrachs, the Spanish Dancer is hermaphroditic, although it can rarely fertilize itself. They lay eggs in a ribbon-like formation, which to the untrained eye may look like seaweed.
While Spanish Dancers have a specific habitat, nudibrachs in general are found in almost all waters worldwide, with the most colorful and visible specimens found in shallower tropical waters. Not much is known about them in general, and it has become a hobby for many individuals to go sea diving to find new, undiscovered varieties. It can be difficult to distinguish whether a nudibrach is an undiscovered species or a variation of an already existing one, and for this reason the most information gained about the category is from autopsied animals that have died of natural causes.