The Spanish Dancer is one of the largest and most captivating species of sea slug. This marine animal is a type of nudibranch, meaning “naked gills,” a shell-less gastropod mollusk known for its striking appearance. Its common name is descriptive, given because its movement through the water resembles the flowing ruffles of a flamenco dancer’s dress.
Physical Characteristics and Scientific Identity
The Spanish Dancer is scientifically identified as Hexabranchus sanguineus, a name that translates from Latin as “blood-colored six-gills.” It belongs to the phylum Mollusca and the class Gastropoda. This species is an exceptionally large dorid nudibranch, commonly reaching lengths of 25 to 30 centimeters. Some individuals are reported to grow up to 40 centimeters, making it one of the largest sea slugs known.
Its soft, flattened body is typically a vivid palette of mottled red, orange, and pink, sometimes speckled with small white dots. Like all nudibranchs, it lacks a protective shell. Located at the anterior end of its body are a pair of retractable sensory organs called rhinophores. These are used to detect chemical cues in the water to find food or mates. Near its posterior end, it features six distinct, exposed, contractile gills that are used for respiration, giving the species its scientific designation of “six-gills”.
The Signature Swimming Behavior
The most distinguishing trait of the Spanish Dancer is its ability to swim, a behavior uncommon among sea slugs, which generally only crawl along the substrate. While it spends most of its time creeping across the seafloor on its muscular foot, it transitions to swimming when threatened. The swimming motion is a defensive escape maneuver, allowing the creature to flee from potential predators.
To initiate the swim, the Spanish Dancer unfurls the wide, ruffled edges of its mantle, known as the parapodia. It then propels itself through the water column using rhythmic, undulating contractions of its entire body. This fluid, wave-like movement, combined with its red and orange coloration, creates the visual effect of a fabric skirt swirling dramatically in the water, which is the source of its common name.
Ecology, Diet, and Chemical Defense
The Spanish Dancer inhabits the warm, tropical, and subtropical waters across the Indo-Pacific, ranging from the Red Sea and East Africa to French Polynesia. It is most frequently found in shallow waters, typically on coral and rocky reefs where its primary food source is abundant. As a specialized predator, its diet consists predominantly of specific types of sponges.
This selective feeding allows the Spanish Dancer to sequester potent chemical compounds from the sponges it consumes, particularly those from the family Halichondriidae. These sequestered compounds, which are macrolide toxins, make the sea slug unpalatable or toxic to most predators. Its bright coloration acts as a form of aposematism, a visual warning signal that advertises its toxicity. Furthermore, the nudibranch incorporates these defensive chemicals into its large, pink or red egg ribbons, providing chemical protection for its offspring against predation.