For many years believed to contain only a single species in the Indo-Pacific region, Hexabranchus sanguineus, and one other species from the Caribbean, Hexabranchus morsomus. A research paper, published in 2023, has revealed that Hexabranchus sanguineus is actually a complex of five species spread across the broad Indo-Pacific region.
They are commonly referred to as Spanish Dancer, a name received from its ability to unroll its normally furled up edges of the broad mantle skirt, revealing brightly coloured patches producing a startle display when disturbed (dilematic behaviour). Dorso-ventral flexions of the body and rippling of the mantle edges provide a flamboyant display and swimming action.
They are large dorid nudibranchs, wide, flattened and soft-bodied without spicules, the lateral edges of the mantle rolled inwards over the edge of the dorsal surface of the body. The lamellate rhinophores are clubbed at the distal end, angled posteriorly and able to contract into separate pockets. The gill branches are mulitipinnate and contractile. The gills are separately inserted but unable to retract fully. The number of gill tufts is variable often more than the six indicated by their name. The gill tufts form a circle around an elevated anal papilla. The oral tentacles are large, plate-like and fleshy with papillae along the outer edge.
They feed upon sponges.