BY Janelle McKean
Welcome to the world of Nudibranchs. These beautiful marine molluscs look like they could be from a different universe, but you can find them living in oceans across the globe. With over 3000 extant species, there are still many to be discovered (Valdes et al. 2013). The name Nudibranchia originates from “naked gills”, named after the feathery gills on their back (Faulkner & Ghiselin 1983). Commonly known as sea slugs, Nudibranchs are broken into two types: dorid and aeolid. They range in length from a few millimetres up to a foot, have a short lifespan of around a year (National Geographic n.d.), and can be found in a variety of colors, patterns, and shapes. Some, have appendages called cerata that project from their back and head. Nudibranchs are carnivores that feed on sponges, coral, algae, and even other sea slugs. Some also posses the ability to harvest energy by eating photosynthetic organisms that contain zooxanthellae, making them “solar powered”, helping sustain them during times of food shortage (Burghardt et al. 2008; Wagele & Johnsen 2001).
Unlike most gastropods, nudibranchs lack a shell in their adult stage, which leaves them vulnerable to predators but saves them the cost of production and transportation of the shell (Faulkner & Ghiselin 1983). One wouldn’t be wrong to think their contrasting colors would attract predators, allowing for them to pick off the slugs at ease. So what would the advantage be for having such a brilliant appearance? You can rule out the idea that the hermaphroditic nudibranch uses their fancy looks to attract mates because they are blind, so how they look has no effect on how they interact with each other (Valdes et al. 2013). Instead, the main purpose of nudibranch coloration has been found to be for camouflage and defense, with an estimated 50% of species having aposematic coloring. This warns predators that they may want to think twice before making a meal of these sea slugs (Valdes et al. 2013).
It is hard to determine the development of aposematic coloration in nudibranchs, but some studies have found the involvement of adaptations other than visible warnings that have helped contribute to the evolution of color. This includes foul-tasting projections on the slug’s bodies and the capacity to acquire nematocyst-defense (Lindstrom 1999). Aeolid nudibranchs can acquire nematocysts from cnidarian pray which make their way to the projections on their backs and can be used as a form of defense should a predator try to attack (Aguado & Marin 2007).
The saying “you are what you eat” holds true in the case of some dorid nudibranchs, as one study found they acquire and begin to display pigments of the sponges that they eat. For example, the Aplysia parvula was pink when found living in pink algae but eventually turned dark green as they migrated to green-coloured algae (Faulkner & Ghiselin 1983), allowing them to go undetected in their environment. Others become toxic or unpalatable from absorbing toxins from cells of the prey they feed on (Haber et al. 2010).
Even with their highlighter bright colors, these slugs can be quite elusive, so count yourself lucky if you spot one while scuba diving. Remember, just because you have seen one does not mean you have seen them all!
Aguado, F., Marin, A. (2007). Warning coloration associated with nematocyst-based defenses in aeolidiodean nudibranchs. Journal of Molluscan Studies, 73 (1): 23-28. doi: 10.1093/mollus/eyl026
Burghardt, I., Evertsen, J., Johnsen, G. (2008). Solar powered seaslugs – mutualistic symbiosis of aeolid nudibranchia (Mollusca, Gastropoda, Opisthobranchia) with Symbiodinium. Organisms Diversity & Evolution, 8(1), 66-76. doi:10.1016/j.ode.2007.01.001
Faulkner, J., & Ghiselin, M.T. (1983). Chemical defense and evolutionary ecology of dorid nudibranchs and some other opisthobranch gastropods. Marine Ecology – Progress Series, 13(1), 295-301.
Haber, M., Cerfeda, S., Carbone, M., Calado, G., Gaspar, H., Neves, R., Maharajan, V., Cimino, G., Gavagnin, M., Ghiselin, M.T., Mollo, E. (2010). Coloration and defense in the nudibranch gastropod Hypselodoris fontandraui. Marine Biological Laboratory, 218(2), 181-188.
Lindstrom, L. (1999). Experimental approaches to studying the initial evolution of conspicuous aposematic signalling. Evolutionary Ecology, 13(7-8), 605-618. doi: 10.1023/A:1011004129607
National Geographic. (n.d.). Invertebrates, Nudibranch (Nudibranchia) [General facts about nudibranchs]. Retrieved March 16, 2015 from http://animals.nationalgeographic.com/animals/invertebrates/nudibranch/
Valdes, A., Ornelas-Gatdula, E., & Dupont, A. (2013). Color pattern variation in a shallow-water species of opisthobranch mollusc. The Biological Bulletin, 224(1), 35-46.
Wagele, H., Johnsen, G. (2001). Observations on the histology and photosynthetic performance of “solar-powered” opisthobranchs (Mollusca, Gastropoda, Opsithobranchia) containing symbiotic chloroplasts or zooxanthellae. Organisms Diversity & Evolution, 1(3), 193-210. doi:10.1078/1439-6092-00016