The beautiful and mesmerizing Spanish dancer The Spanish dancer (Hexabranchus sanguineus) is the largest species of nudibranch (sea slug) in the world, capable of growing up to 90 centimetres (3 feet) in length.

It is widely celebrated in the marine biology community for its striking, vibrant crimson coloration and its unique ability to swim freely through open water rather than just crawling along the ocean floor.

Its scientific name is Hexabranchus sanguineus, which translates literally to "blood-colored six-gills". It has two specialized horn-like structures on its head used to detect chemicals dissolved in the water. The creature also has a highly visible, feathery gill plume located on its back that it uses for respiration.

While most sea slugs are bound to the substrate, this species can launch itself into the water column when threatened. It undulates its bright red or orange mantle in fluid, wave-like motions. This highly dramatic movement closely mimics the flowing, twirling skirts of a human flamenco dancer.

They inhabit the warm, tropical and subtropical waters of the Indo-Pacific region and the Red Sea. They typically live in shallow coral reefs and rocky seabeds between 1 and 20 metres deep. During the day, they strictly avoid light by squeezing into dark reef crevices. They emerge late at night to feed, making them a prized sighting for scuba divers on night dives.

The Spanish dancer is a carnivore that feeds almost exclusively on toxic marine sponges. Rather than being harmed, it assimilates the sponge's chemical defenses into its own tissues. Its brilliant red color is an evolutionary warning sign (aposematism) telling predators like fish that its flesh tastes terrible and is highly toxic.

They are simultaneous hermaphrodites, meaning an individual possesses both male and female reproductive organs. They lay their eggs in large, bright pink or red spiral ribbons that attach to rocks. These egg cases look exactly like a rose on the reef and are packed with stolen sponge toxins to prevent predators from eating them.