Nudibranchs or sea slugs are widely recognised to include some of the most spectacularly ornate animals in the sea. Various families such as dorids are renowned for their brilliant colours, so vibrant that it seems almost impossible to believe that such an animal really exists and is not just the creation of an imaginative six-year-old who has just been given a large box of felt-tip pens for their birthday. But as flashy as such taxa are, I'm looking today at a family whose members are not quite so colourful, but are no less spectacular.
The Aeolidiidae are found worldwide, mostly in tropical waters but with some species extending to more temperate climes. Instead of the neon colours of the dorids, Aeolidiidae and other aeolid nudibranchs tend to more ornate morphology. Their body is ornamented (often densely) by outgrowths called cerata. The cerata are outgrowths of the digestive system that function like gills, increasing surface area to allow the diffusion of gases in and out of the blood. They also play a significant role in defense from predators. At the most basic level, cerata can defend against predators in a similar manner to the detachable tail of a lizard - breaking off to distract attackers from the main body of the nudibranch (Miller & Byrne, 2000). However, members of the Aeolidioidea, which includes the Aeolidiidae and the closely related families Glaucidae and Facelinidae, have taken this even further.
Most aeolids are predators of cnidarians, and Aeolidiidae in particular feed almost exclusively on sea anemones. Sea anemone tentacles bear dangerous stinging cells, nematocysts, that are lethal for most other animals, but aeolidiids can graze on them with impunity. Not only are they not harmed by the anemone's nematocysts, but they are actually able to transfer the nematocysts, undigested, into the ends of the cerata, where they are converted into an added defense for the aeolidiid itself. The nematocysts sit in a vacuole at the end of the ceras called a cnidosac, just below the epidermis. When the ceras is forcefully stimulated, the nematocysts burst through the skin, stinging any would-be attacker (Kälker & Schmekel, 1976). Such recycled nematocysts lose none of their original toxicity, and aeolids that feed on particularly toxic cnidarians, such as the planktonic Glaucus which feeds on siphonophores like Portuguese men-of-war, can be just as dangerous to handle as their prey.
Prey-derived toxicity is not uncommon among animals - members of such disparate taxa as arrow-poison frogs, pitohui birds and monarch butterflies all derive their toxicity from their diet, as do non-aeolidioid nudibranchs. The Aeolidioidea are still unique in that it is not merely the chemical products of their prey that they are appropriating, but actual cellular structures. How such a remarkable characteristic evolved would certainly be a fascinating question to research. Miller & Byrne (2000) found when studying cerata function in Phidiana crassicornis, a member of the family Facelinidae, that nematocyst discharge in defense seemed to be only secondary to the detachment of the cerata themselves, and not all predators caused nematocysts to discharge. They therefore suggested the possibility that the cnidosac may not have originally evolved for defense, but rather as a means of sequestering nematocysts and avoiding harm to the aeolidioid itself. The defensive possibilities would then be something of an added bonus.
It is also noteworthy in this regard that while other nudibranchs do not harvest stinging cells from their prey, they may take up other kinds of cells. Many marine animals such as corals contain symbiotic unicellular algae, zooxanthellae, and a number of different types of nudibranch, including some species of Aeolidiidae, have been recorded as containing zooxanthellae from their prey animals (Wägele & Johnsen, 2001). These zooxanthellae are usually contained within the epithelium of the digestive glands. It is unclear in many cases (and probably varies between species) just how functional the purloined zooxanthellae remain, but studies on some species have demonstrated that they are a significant factor in survival through periods of low nutrition. In members of the aeolidiid genus Spurilla, the digestive gland is branched and spreads into places such as just under the epidermis or the oral tentacles, allowing the zooxanthellae to be exposed to the light. Could there be a connection between the more widespread sequestration of zooxanthellae and the uniquely aeolidioid sequestration of nematocysts? Could the processes that evolved to allow the one have been co-opted to serve the other?
Kälker, H., & L. Schmekel. 1976. Bau und Funktion des Cnidosacks der Aeolidoidea (Gastropoda Nudibranchia). Zoomorphology 86 (1): 41-60.
Miller, J. A., & M. Byrne. 2000. Ceratal autotomy and regeneration in the aeolid nudibranch Phidiana crassicornis and the role of predators. Invertebrate Biology 119 (2): 167-176.
Wägele, M., & G. Johnsen. 2001. Observations on the histology and photosynthetic performance of “solar-powered” opisthobranchs (Mollusca, Gastropoda, Opisthobranchia) containing symbiotic chloroplasts or zooxanthellae. Organisms Diversity & Evolution 1 (3): 193-210.