Spanish Dancer

Spanish Dancer

The Spanish dancer (Hexabranchus sanguineus) is a remarkably large dorid nudibranch and marine gastropod mollusc belonging to the family Hexabranchidae. Its scientific name translates literally to "blood-coloured six-gills", reflecting its striking appearance. While commonly growing to lengths of 25 to 40 centimetres, some individuals reach up to 60 centimetres (24 inches), positioning it among the largest sea slug species in the world. Coloration varies geographically across the tropical Indo-Pacific; Red Sea specimens are almost invariably solid deep crimson, whereas populations further east present mottled patterns of red, orange, white, and yellow. Recent molecular and morphological research published in 2023 revealed that what was long considered a single species actually comprises a complex of at least five distinct species. Morphologically, the Spanish dancer possesses a soft, flattened oval body. Its anterior dorsal section features a pair of retractable rhinophores used for sensing the environment, while the posterior portion hosts six branched, contractile gills that surround the anus and are inserted independently into the body. These pale rose-coloured gills remain exposed, as is characteristic of dorid nudibranchs. Its oral tentacles are composed of a flexible membrane fitted with large digital lobes. To travel along hard surfaces, the animal glides smoothly using millions of microscopic cilia covering the underside of its muscular foot. When crawling at rest, the lateral edges of the mantle curl upwards to form a scroll-like peripheral border. However, when threatened or disturbed, the nudibranch unfurls these expansive mantle skirts and launches off the substrate into open water. By vigorously flexing and scissoring its body in a series of rhythmic undulations, it propels itself through the water column. This dramatic, billowy movement resembles the swirling dress of a flamenco dancer, giving rise to its popular common name. Hexabranchus is recognized as the only nudibranch genus capable of sustained free-swimming. The Spanish Dancer, Hexabranchus sanguineus, is a large and distinctive nudibranch known for its nocturnal activity and solitary nature, though it may be observed in pairs during mating. During the day, it typically hides under ledges or within crevices. At night, it emerges to feed, primarily on sponges, and to move across the reef. Its striking swimming behaviour, involving undulating its mantle margins, is usually a response to disturbance or a means of locomotion between feeding grounds. Reproduction in this species is hermaphroditic, with individuals engaging in reciprocal copulation. They are oviparous, laying a characteristic, ruffled egg ribbon containing numerous small, encapsulated eggs. While a specific spawning season is not well documented for all regions, reproduction generally occurs year-round in tropical waters. The larvae are planktonic, developing into veligers before settling and metamorphosing into the benthic adult form, a common life history for many nudibranchs. The exact incubation period and time to maturity are not precisely quantified in readily available literature but are typical of marine gastropods. In addition to its swimming display, the Spanish dancer incorporates potent chemical defences to ward off predators such as the moon wrasse (Thalassoma lunare) and the white-spotted hermit crab (Dardanus megistos). By consuming encrusting sponges, the nudibranch sequesters toxic macrolides and oxazole alkaloids, storing them within its body tissues and transferring them directly into its egg masses. Divers frequently observe small commensal organisms resting on the Spanish dancer, most notably the brightly patterned emperor shrimp (Periclimenes imperator), which finds shelter and transport along the sea slug's mantle folds.