The Pokémon “Shellos” and its evolution “Gastrodon” (whose pink West Sea variant is one of my favorite Pokémon) resemble a sea slug. Don’t just take my word for it, though—in their Pokédex entries, Shellos and Gastrodon are referred to as the “Sea Slug Pokémon”. The Pokémon Goomy is referred to as a Mollusk Pokémon in many languages, and although sea slugs are a type of mollusk, the resemblance is less apparent. The team behind the games really didn’t have to change much from their real-life inspiration, though, as sea slugs are some of the most unreal-looking creatures in our oceans. As Helen Sullivan writes about sea slugs, which are also known as nudibranchs, in the Smithsonian Magazine, “It is really, really difficult not to compare them to Pokémon characters” (2024).
Nudibranchs belong to the class Gastropoda, which falls under the phylum Mollusca (mollusks). The term sea slug is used to broadly refer to 11 different subgroups under Gastropoda that do not have hard outer shells. True nudibranchs account for only one of these subgroups, but the others are sometimes also referred to as nudibranchs for convenience (Davey & Willan, 2020).
When reading about species of nudibranch, one caught my eye for its name and bright orange coloration: the clown nudibranch. Even the name makes them sound like some sort of Pokémon! They are quite small—around 3 cm on average for adults—but their orange spots would probably make them hard to miss. The name, appearance, and distinctive characteristics of this creature gave me the idea to make a faux Pokémon card for the species, whose scientific name is Ceratosoma amoenum.
Dorid
Nudibranchs can have a few different body plans that differ in shape, gills, and more. C. amoenum has a dorid nudibranch body plan, which means it has two protrusions on its front end and a bundle of protrusions on its back end. The two protrusions that resemble antennae are called rhinophores while the bunch of protrusions on its backside are actually its gills! This distinctive characteristic is where nudibranchs get their name from. Nudibranch means “naked gills” (Sullivan, 2024).
Not to be confused with…
When researching for this article, I found that there are other nudibranch species with a similar appearance and common name as C. amoenum. Trophia catalinae also has a white body with orange spots, although its spots are raised (known as “tubercles”), and its rhinophores and gills are orange rather than magenta. It’s referred to by a similar name: “sea clown trophia” or simply “sea clown”, so you can see where the confusion might arise. Trophia modesta is a species that was recently distinguished from Trophia catalinae in a 2020 study, and it is commonly known as the “modest clown dorid” (Jung et al.). Unsurprisingly, it looks more similar to T. catalinae than it does to C. amoenum, although its body is more translucent than T. catalinae’s. Still, an amateur nudibranch enjoyer may get these species confused at first glance.
Defensive glands
Nudibranchs often contain chemical defenses within their bodies. A protective tissue called the mantle contains glands that secrete the toxins (Davey & Willan, 2020). Like other nudibranchs, our clown friend doesn’t produce these toxins on their own—they obtain them from their diet, which consists primarily of sponges (Winters et al., 2018). This chemical defense is critical for the soft, shell-less nudibranchs.
Shell loss
Speaking of not having a shell, both the English and Japanese name for Shellos mean “lost shell”: Karanakushi in Japanese, with kara meaning “shell” and nakushi being derived from nakushite (without) or nakusu (to lose); and “shell loss” (Shellos) in English (“Shellos”, 2026). The name origin references one of the nudibranch’s most defining characteristics. Gastropods have an external shell in their larval stage, although members of order Nudibranchia lose it before they reach adulthood, with C. amoenum being no exception (Davey & Willan, 2020).
Hermaphroditism
As members of the Gastropoda class, sea slugs are hermaphrodites. Each individual has both male and female reproductive organs. I know what you may be wondering, but no, they can’t impregnate themselves, although they often donate and receive sperm simultaneously with another individual. Some nudibranchs even shed their penises after intercourse and gain a new one soon after (Sekizama et al., 2013). Talk about freaks of nature….
I considered designing a creature based on the clown nudibranch for my fake Pokémon card, but why would I when the real thing looks like that? I’ll leave the creature designs to the Pokémon team for now. I hope you’ve come to see that sometimes reality can be stranger than the fiction it inspires, and that our world is full of amazing yet often overlooked creatures like the nudibranch.
References
Davey, N., Willan, R.C. (2020). Super sea slugs: a guide to the sea slugs of New Zealand. (Version 1). National Institute of Water and Atmospheric Research (NIWA).
Gastrodon. (2026). Bulbapedia. Retrieved April 17, 2026, from https://pokemondb.net/pokedex/gastrodon
Jung, D., Gosliner, T.M., Choi, T., Kil, H., Chichvarkhin, A., Goddard, J.H.R., Valdés, A. (2020). The return of the clown: pseudocryptic speciation in the North Pacific clown nudibranch, Triopha catalinae (Cooper, 1863) sensu lato identified by integrative taxonomic approaches. Marine Biodiversity, 50, 84. doi.org/10.1007/s12526-020-01107-2
Sekizama, A., Seki, S., Tokuzato, M., Shiga, S., Nakashima, Y. (2013). Disposable penis and its replenishment in a simultaneous hermaphrodite. Biology Letters, 9(2). doi.org/10.1098/rsbl.2012. 1150
Shellos. (2026). Bulbapedia. Retrieved April 17, 2026, from https://pokemondb.net/pokedex/shellos
Sullivan, H. (2024, May). Dive Into the Exotic World of Nudibranchs, the Spectacular Slugs of the Sea. Smithsonian Magazine. www.smithsonianmag.com/science-nature/dive-exotic- world-nudibranchs-spectacular-slugs-sea-180984013/
Winters, A.E., White, A.M., Dewi, A.S., Mudianta, I.W., Wilson, N.G., Forster, L.C., Garson, M.J., Cheney, K.L. (2018). Distribution of Defensive Metabolites in Nudibranch Molluscs. Journal of Chemical Ecology, 44, 384-396. doi.org/10.1007/s10886-018-0941-5
