Invertebrates
Spanish Dancer
Hexabranchus sanguineus · also called Spanish dancer nudibranch, blood-red sea slug
The Spanish dancer is a sea slug the size of a forearm, coloured like arterial blood, that spends most of its time doing nothing more dramatic than grazing sponges. But lift one off the reef and it transforms: the body flattens, the edges start to ripple, and the animal swims away through open water in a swirl of scarlet.
Overview
Hexabranchus sanguineus is the largest nudibranch in the world, reaching 40–60 cm, and lives on reefs across the tropical Indo-Pacific and the Red Sea. It grazes sponges and takes up their defensive chemicals for its own protection.
The dance
Normally it crawls with its mantle edges rolled up. To travel or escape, it flattens out and swims by flexing the whole mantle in waves — a rippling, skirt-like motion that names the animal. Its scientific name, Hexabranchus, “six gills,” refers to the branched breathing tuft on its back.
Red for danger
The colour is a warning. The sponge-derived toxins in its skin — some studied as anticancer leads — leave it with few predators. Its egg ribbon, a frilly scarlet rose, is more toxic still and left unguarded. Small emperor shrimp often ride on its back, protected by its poison.
Status
Least Concern — widespread and common, not fished, and of no conservation concern.
Spanish Dancer: common questions
Why is it called the Spanish dancer?
For the way it swims. Most sea slugs cannot swim at all and simply crawl, but the Spanish dancer, when it lifts off the reef, unrolls its large, flexible mantle and drives itself through the water by sending waves along it, folding and unfolding the edges. The motion, in a big animal coloured deep red with paler pink flashes, looks strikingly like the swirling, flaring skirt of a flamenco dancer, and the resemblance gave the species its common name in several languages. It does not swim often or for long, usually just to escape a threat or to relocate, but when it does the display is one of the more memorable sights on a night dive.
Is the Spanish dancer poisonous or dangerous to touch?
It is chemically defended but not dangerous to handle briefly. Its skin contains toxins that it acquires from the sponges it eats, and these deter fish and other predators; the same class of compounds is of interest to pharmacologists for possible anticancer and antifungal use. For a person, touching a Spanish dancer is not harmful, though as with all reef life it is better admired than handled, both to avoid stressing the animal and because its protective mucus coat is easily damaged. The one part to leave strictly alone is the egg ribbon, which is significantly more toxic than the adult.
What is a nudibranch, and how big can they get?
A nudibranch is a shell-less marine snail, a type of sea slug, and the group is famous for spectacular colours and shapes; the name means "naked gills", after the exposed breathing structures many of them carry on the back. Most nudibranchs are small, from a few millimetres to a few centimetres, because they are slow-moving animals with soft bodies and no armour, relying on toxins and camouflage rather than size. The Spanish dancer is the giant exception at up to half a metre or more, and a large one gliding over the reef at night can be mistaken at first glance for a piece of drifting red cloth.
Why does the Spanish dancer carry shrimp on its back?
The relationship is a partnership that mainly benefits the shrimp. Emperor shrimp, small and often coloured to match, live on the mantle of the Spanish dancer and on other large slugs and sea cucumbers, where they pick up mucus, food particles, and waste, and where the host's toxicity shields them from predators that would otherwise eat such an exposed, slow shrimp. The slug appears largely unaffected, possibly gaining a small cleaning benefit. Several other reef invertebrates host similar commensal shrimp, and finding the shrimp is often how a diver first spots a well-camouflaged host.