Spanish Dancer – Hexabranchus sanguineus

Very few divers have seen a Spanish dancer dancing; those who have had this opportunity immediately understand the origin of its popular name. The source of its scientific name is Hexabranchus sanguineus from the Latin, Hexabranchus = six gills, and sanguineus = blood colored. This marine dance is reminiscent of the famous flamenco dance, including the body twists, exactly like the gypsies who started dancing in this folkloric style hundreds of years ago.
The Spanish dancer belongs to a group called “sea slugs” (basically snails without shells). The dancer spends most of its time crawling on the sand at the foot of the reef in its search for its favorite food – sponges: these are primitive creatures, living in a fixed place, nourished by filtering small food particles in the water – the filtering is performed by using both an entry, and an exit filter. The dancer can be found in depths of up to 50 or more meters, a depth that most divers do not reach.
The dancing slug can be found anywhere from the tropics to the Indo-Pacific. The Spanish dancer was first identified in the Gulf of Eilat in 1828 by Ruppel and Leuckart. It is one of the largest in its group, and it reaches a length of 40 cm; a few even reach a length of half a meter, or more. This naked snail has six external gill clusters in the rear section of its body. It retracts, and extends its gills from a type of centipede formation that protects the gills. Here too, one notices the fascinating similarity between dancers above, and below water: the gills extend in the same manner as “La Gitana” (the flamenco dancer) waves her fan marking the start of the dance. The dancer has hornlike formations on the front of its body that act as feelers which the Spanish Dancer uses to “smell” the water, and find its favorite food. The Dancer’s eyes, which are more than likely used to find a mate, are situated at the base of these horns.
It feeds mainly on poisonous sponges which it eats while it crawls slowly along the sea floor, leaving a trail of gnawed crumbs.
The Spanish Dancer can be identified by the color of its bright red cloak, and its edges, which are white in some species. In most cases, it is difficult to identify the dancer when its cloak is closed. In certain areas of the world, especially Hawaii and Tanzania, they can also be found in shades of yellow.
The sophisticated dance performance can only be seen when it is escaping from predators. When this slug feels threatened, it swims with a twisting body motion that helps it to maneuver towards a safe haven in the water. The Spanish Dancer received its name because of its fascinating swimming movements. Very little is known about the reasons for its bright colors, most researchers agree that the colors are a warning sign to other fish that they are toxic. In this case the warning is applicable, since while eating the sponges it produces venom made from the different toxins in the sponges with which it protects itself, and its offspring. The Dancer’s eggs are protected from predators by the self-defensive venom inside the egg.
The slug’s similarity to dancers does not end here: the eggs it lays resemble the orange / red lace that gypsy dancers used to add to the hems of their dresses.
In every batch, there are thousands of eggs from which the young slugs hatch. In the initial stage they swim until they change their shape, after which they settle on the seabed, and become young slugs. In principle, all slugs are all hermaphrodites, meaning, each individual has both female and male organs. Despite their ability to self fertilize, in most cases, they prefer donating, and receiving sperm to maximize the diversification of their genes. Sometimes, by looking closely, one can see small crustaceans that live on their bodies, feeding on the food particles that stick to the mucus covering its body. These crustaceans have a red color that makes it difficult to identify them on the slug’s body, and this also serves to protect the crustaceans from predators for most of their lives.

Author: Aviv Levy – Scientific Manager, Underwater Observatory Marine Park

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