Doris sumptuosa [now Hexabranchus sanguineus] - “Spanish Dancer”
The “Spanish Dancer” nudibranch is named for its flamboyant “skirts” that swish around like a flamenco dancer. Hexabranchus sanguineus is found around the world, wherever the water is warm enough.
The species was first described based on a specimen from the Red Sea, where its coloration is almost blood-red (hence the “sanguineus” - blood-like). However, the color can vary from red, to orange, to bright yellow (though pure yellow is quite rare).
These are some of the largest nudibranchs out there. Adults can reach up to 40 cm in length.
United States Exploring Expedition, under the command of Charles Wilkes: Atlas, Mollusca and Shells. Augustus A. Gould, 1856.