One of the most exciting underwater encounters for a Pacific coast scuba diver is to see the brilliantly colored and bizarrely shaped Spanish shawl sea slug. The Spanish shawl is an aeolid nudibranch. These nudibranchs were named after Aeolis, the Greek god of the wind. With its purple, elongated body topped with rows of spiky, orange cerata and two bright red, antennae-like, rhinophores, the spectacular Spanish Shawl could easily have been conceived upon Mt. Olympus. Its scientific name is Flabellina iodinea.
The Spanish shawl was first identified for scientific purposes in the year 1862. Only several centimeters in length, it lives in the intertidal zone to a depth of 120 feet on the Pacific coast from Vancouver Island, British Columbia south to Baja California, including the Channel Islands and at the Galapagos Islands.
The aeolids are nudibranchs, which means, “naked gills.” They lack gills of any kind and respire directly through their skin. Their soft body is not covered with a shell. The bright orange cerata rising from Spanish shawl’s dorsal surface like licking flames are actually extensions of its digestive system. The cerata also assist in respiration because they significantly increase the surface area of the skin that is exposed to the oxygen bearing water.
Spanish shawl has a pair of spire-like rhinophores that rise directly from it head. They resemble a stack of red saucers on a stalk. Researchers believe the rhinophores serve significant sensory functions that help the animal find food and mates.
Spanish shawl’s luminous colors and unusual shape ensure their survival. Its vivid colors may seem to attract predators; however, its taste is so repellent that its coloration actually serves a warning and keeps many predators at bay.
When its colors fail to deter a predator, the attacker soon learns why Spanish shawl is a poor choice for food. The Spanish shawl eats sea anemones and fan-like hydroids. The tentacles of these animals (cnidarians) contain stinging cells called nematocysts. Somehow, Spanish shawl and the aeolids are able to ingest the nematocysts without causing them to fire off the stingers. The stinging cells pass through their digestive track and rise to the tips of the cerata where they remain armed and ready to fire into the mouth of an unsuspecting predator.
Unfortunately, for the Spanish shawl, there is a predator that is immune to the stinging cells and is able to suck the aeolid into its digestive system without harm. This “stinger–proof” predator is another sea slug: the navanax. Navanax have a striking appearance. Its tube-like body can be jet black with surrealistically patterned yellow and blue stripes and speckles. It is at least two or three times the size of the aeolids. The navanax is able to sense the slime trail left by Spanish shawl on the substrate. It follows the trail until it reaches the prey and then extends a lip-like tube through its mouth to suck it in. Although its stinging cells cannot protect Spanish shawl from navanax, it is still not helpless. It is one of several Pacific aeolids that can swim to avoid predators. By contracting muscles, it can flex laterally. These side-to-side movements together with the swaying of its cerata create a swimming motion that can carry Spanish shawl off the substrate to escape the menacing navanax.
Spanish shawl and the other aeolid nudibranchs are carnivores that eat hydroids, sea anemones, gorgonians, bryozoans and other mollusks’ eggs. Their primary feeding mechanism is the radula. It functions like a tongue and is a flexible band covered with rows of teeth. When it feeds, it holds the prey with strong jaws and scrapes the flesh into its mouth with the narrow band of radular teeth.
Now that Spanish shawl has escaped its predator and eaten its fill. Its thoughts turn to love. Being a hermaphrodite, having both male and female sexual apparatus, its opportunities to find a willing mate are greatly increased. After a successful amorous encounter, it lays its fertilized pink-orange eggs on the same hydroid it eats.
The Spanish shawl nudibranch is an undersea marvel. With is stunning colors and exotic shape, it is a favorite subject for underwater photographers. It also presents a continuing scientific mystery with its ability to eat its prey’s stinging cells and make them part of its own arsenal.