Mesmerizing Marine Mimics: Sea Slug Style

This post is the third in a six post series outlining the evolution of mimicry within the ocean realm. These posts detail various ways in which organisms may copy other organisms in appearance and behavior, and the evolutionary context for how these mimic-model pairings have come to be. The first entry in this series goes over some fundamental introductory concepts and definitions regarding mimicry in general.

The previous entry in this series tackled fish that masqueraded as their flippity-floppity boneless distant cousins, generally to keep themselves from getting devoured by one of the endless multitudes of hungry mouths that tirelessly dart back and forth underneath the waves. The entry before that looked at fish that mimic other fish as part of the same evolutionary strategy. It’s worth mentioning that while I really like fish (I study the things, for Christ’s sake), not every bit of mimicry in the ocean involves gills and fins. Plenty of invertebrates (the perhaps less-than-charismatic things like worms and mollusks) in the oceans also engage in mimicry of other species. Sometimes, just like in the last entry, these imitations can jump across to completely different phyla (a major organizational grouping of life right below “kingdom” (like the animal kingdom, Animalia) and above “class” (like Mammalia, which includes mammals within the phylum Chordata)). It’s also important to consider that the phylum to which all vertebrates belong, Chordata, is just one grouping compared to a shitload of so-called “invertebrate” phyla…like thirty of them (the exact number depends on who you talk to…there’s plenty of taxonomic/phylogenetic disagreement to go around). Familiar, cute vertebrate critters like pandas and cats and goldfish and parakeets get a lot of attention, but in reality they make up a tiny sliver of animal diversity. The vast majority of the animal species on planet Earth are “invertebrates”…a commonly thrown out figure is something like 97% of all animals. In light of this, focusing specifically on some gooshy guys for one entry in this series seems only fair. I guess you say I’m…throwing them a bone….or something.

One group of invertebrates that is deserving of focus in the context of marine mimicry are the nudibranchs (pronounced “noo-dee braynks”). Their name means “naked gill”, which refers to their unique respiratory system. While they are regularly referred to as “sea slugs”, nudibranchs represent just one large taxonomic group of marine-living “slug” (the other groups include things like the solar-powered sacoglossans, a member of which is the “sea sheep” that everyone was losing their shit about a month or two ago, or the huge, ink-squirting sea hare, which is part of lineage distinct from nudibranchs). To reduce confusion on what variety of sea slug one is talking about, many folks refer to them by the shorthand “nudis” (pronounced like one would for “nudies”…although taking care to distinguish between things like “nudi photographs” and “nudie photographs” in casual conversation is STRONGLY RECOMMENDED).

Nudibranchs are nested within the Phylum Mollusca, which contains notoriously slimy, muscular, and often tasty animals like squid, octopus, clams, and mussels, along with snails and slugs. They are also “gastropods”, which are a class of mollusks commonly equipped with a muscle-bound sliding “foot”, and include everything from abalone to your typical garden snail. Nudibranchs make up a modestly-sized branch of the gastropod family tree (which is MASSIVE, with at least 60,000 species), but their 2,000 or so species are found in essentially every marine habitat, from shallow reefs to the deep-sea. They are called “slugs” due to their lack of a shell, but the land-lubber “slugs” that keep terrorizing your lettuce plants under the cover of night are far more closely related to terrestrial snails than they are to the maritime mollusks at hand; nudibranchs are not particularly closely related to either domestic variety of gastropod. The majority of species are remarkably delicate and tiny, growing only to about the size of a fingernail, but some are burly even by gastropod standards, like the Spanish dancer nudibranch (Hexabranchus), named for its gorgeous red colors and elegant swimming style reminiscent of a flamenco dancer, which can occasionally reach the size of a watermelon. The one pictured below, found here on O`ahu a couple of years ago, is significantly smaller.


Photo: Jake Buehler

Nudibranchs are renowned for their bizarre, alien appearances and explode-in-your-face colors. Honestly, these things are like extraterrestrials ala Avatar meets three hours into a balls-to-the-wall mushroom trip. There are blue dragon nudibranchs, which look like pipe cleaners designed by Dr. Seuss. There are variable neon nudibranchs, which appear to be fuzzy velvet posters that have come to life. Many are so intensely colorful they look man-made. The Hopkin’s rose nudibranch is pretty much identical to those unnerving, pink, moving, Koosh ball things in Jim Henson’s 1982 film The Dark Crystal:


But let’s be honest, everything in that movie is unnerving. Remember the landstriders? *shiver*

The white-lined dirona looks like the ghost of a feather boa, and the highly-venomous blue glaucus is the size of a nickel and glides through the open ocean like a six-winged dragon made out of periwinkle glitter. Some nudibranchs look remarkably like the tiniest, fluffiest bunny you’ve ever seen. The diversity of otherworldly forms and color schemes among the nudibranchs is impressive, and much of this has its roots in the evolutionary biology of these animals. Being soft, mushy, and slow is no way to go through life in the ocean…in particular because your life will invariably be cut short. An easy meal doesn’t stay unexploited for long. Many nudibranchs have managed to maintain their svelte, silky, shell-less figures and avoid extinction by finding other ways of defending themselves. Some take toxins they acquire in their diet (often from sponges) and concentrate it in their bodies, becoming distasteful or outright dangerously poisonous to would-be predators. It is then thought that the insanely, psychedelic colors are an advertisement of their toxicity, telling the ocean at large that they ain’t nothin’ to fuck with. Others that feed on cnidarians (things like jellies and anenomes) can store their food’s venomous stinging cells, putting them in fleshy extensions that cover their backs like a forest of blistering despair. Nudibranchs are like Rogue…if she had to cannibalize other mutants to absorb their superpowers through the lining of her stomach.

Because of this habit of sneakily taking “you are what you eat” to the next, noxious level, nudibranchs are known to be mimicked by other species that do not have the capacity to become nasty-tasting or embedded with the biochemical Angel of Death.

Consider, for example, the pimpled phyllidiella (Phyllidiella pustulosa), a small nudibranch native to the tropical waters of the Indo-Pacific. It is a commonly encountered nudibranch, and is notable for regularly venturing out into the open in the middle of the day, a brazen act for a defenseless little slug nugget. However, the pimpled phyllidiella protects itself with toxins that it steals from the sponges it slurps up as it slowly slides across the coral reef. Fish in particular steer clear of this nudibranch, possibly because of its inherent toxicity, but also potentially because it resembles a blood sausage infected with small pox.


Fish will literally eat each other’s shit, but Mr. Clearasil “Before” Photo over here is just too icky.

It’s a winning evolutionary move for the nudibranch, and one that has been exploited by a completely different kind of soft-bodied co-inhabitant of the reef. This go-getter? Pseudoceros imitatus, often referred to as just the “mimic flatworm.” As a flatworm, it is a member of the phylum Platyhelminthes, and as I’ve described in the previous entry in this post series, this means it is nowhere near nudibranchs in the animal phylogenetic tree, and is a member of a group characterized by having internal organ systems about as substantive and fleshed out as Presidential candidate Donald Trump’s foreign policy musings. Actually, come to think of it, Trump’s socio-political ideas have a lot in common with parasitic flatworm infections; both propagate via mouths that have stayed open when they really should have been shut….and through liberal expulsion of feces.


Make America Eat Again…and Again…..and Again….Stop Asking Me Why and Just Put Food in Your Face, Goddamnit, You Ain’t the Only One Who’s Gotta Eat Around Here

Anyways, the mimic flatworm, unlike many other varieties of closely related flatworm that DO produce defensive toxins (and are themselves models for various mimics), is benign. Fortunately, it has evolved to look strikingly similar to the co-distributed pimpled phyllidiella, and has the same pale purple patches of zit-like bumps interspersed with black striping.

The mimic flatworm has even modified its sensory antennae (on the left side of the image) so that they better resemble the rhinophores (the blade-shaped “ear”-like structures on the heads of nudibranchs that act like noses/tongues, sensing chemicals in the water) of the model nudibranch, rolling them into upright, black cones. One brief, passing glance at this flatworm slithering out in the open by a passing fish (who had previously had a sorrowful run-in with a toxic pimpled phyllidiela) and it’s unlikely the fish would be able to tell the difference, and would continue on its way….and the worm lives to see another day.

Other species from other invertebrate phyla hop on the Toxic Pretender Train as well. Take, for example, the blackspotted sea cucumber (Pearsonothuria graeffei). It, like all sea cucumbers, is an echinoderm, a member of the phylum Echinodermata. The echinoderm umbrella includes familiar ocean creatures sea stars, sea urchins, sand dollars, bristle stars, and of course, sea cukes. Echinoderms are actually very closely related to our own phylum, Chordata, and have a number of bizarre distinguishing features, like locomotion driven by a complex hydraulic system and five-part radial symmetry (meaning that they have a body plan equally divided around a central axis, and thus have no right and left “sides” like we do). Sea cucumbers make up the echinoderm class Holothuroidea, and are closely related to their urchin cousins, but tend to be more leathery and sausage-shaped than little, hard, spiny balls. Sea cucumbers like the blackspotted sea cucumber are scavengers, methodically shuffling through the sandy bottom of the ocean, pushing whatever rotting particles they can find into their mouth with their scoop-shaped oral tentacles and extruding the waste (and lots of sediment) out the other end. When disturbed, many sea cukes (including this species) will shoot sticky strands of their own viscera at their assailant from their own asshole, which, you know, is kind of off-putting. These specialized organs are very roughly analogous to gills or lungs.

Yes, you read that right, sea cucumbers, when scared, have their breath taken away…but only because they shit themselves hard enough to blast part of their respiratory system out of their ass. I’ve heard of some fearful sharts in my day, but I’m pretty sure that takes the cake.

The blackspotted sea cucumber is a fairly standard “holothurian”, butt stuff included. Lethargic. Dull in coloration. Looks a bit like a plastic bag wrapped around a blackberry bramble. Or a mosquito-bitten colon. Or kind of like the alien from John Carpenter’s The Thing was trying to morph into a gherkin, but got interrupted halfway through by Kurt Russell.


“Well, looks like we’re in a bit of a pickle, aren’t we?” *lights flamethrower*

The blackspotted sea cucumber is mainly a tropical Indian Ocean species, but can also be found into the Indo-Australian archipelago (Indonesia, Papua New Guinea, etc.) and the Philippines. They can grow to about a foot in length, and precious few animals would dare attempting to eat them at their full adult size…I mean…that’s potentially a lot of weaponized butt-guts to deal with. So what does this all have to do with mimicry? Well, the young of this species of sea cucumber look very different from the adults.

Take a gander at the baby blackspotted sea cucumber:


It’s the thing that looks like a piece of bird shit.

You see, when the blackspotted sea cucumber is just a wee little sea cukelet, it has white (sometimes bluish) blotches interspersed with black lines, and large, exaggerated thorny projections capped with orange or yellow….very different than the brownish, spotted adult form. Attracting attention to oneself through in-your-face colors isn’t a good idea if you a dinky foodstuff like the young sea cucumber above….unless you wish to get something else across to your co-denizens of the deep. It is perhaps no coincidence that this unique, childhood appearance is remarkably close to that of a variety of nudibranch (one that’s actually in the same taxonomic family as the pimpled phyllidiella above).

This nudibranch is Phyllidia varicosa. It is also known as the “scrambled egg nudibranch”, which is possibly the most viscerally descriptive name for an organism I’ve heard since I read about dog vomit slime mold. And it’s an accurate description…er…sort of.


Behold, the Great Easter Turd

The scrambled egg nudibranch is, unsurprisingly, poisonous and gross-tasting as hell, and fish keep this guy miles away from their mouths once they realize what those pretty pastel colors are dressing up. While the blackspotted sea cucumber is a youngster, it is an effective mimic of these toxic nudibranchs, possibly providing itself protection from fish predators. However, as it eats, and craps, and grows larger, it vastly exceeds the maximum size of its nudibranch model…and the effectiveness of the mimicry breaks down….which isn’t helped by the gradual color change to more muted tones. Luckily for the blackspotted sea cucumber, while the young are helpless in the face of predators, adults apparently become toxic themselves as they age, so all is well in mature, drab post-pubescent sea cucumber land.

This post series will continue: Part 4 will focus on fish that mimic body parts of other animals, use parts of their own bodies to mimic the whole bodies of other animals, or use parts of their own bodies to mimic other parts of their bodies….trust me it’s not as confusing as it all sounds….

Image credits: Introductory nudibranch image, pimpled phyllidiella, tapeworm and intestinal roundworm background image for Trump/Tapeworm 2016 composite, adult blackspotted sea cucumber, juvenile blackspotted sea cucumber, scrambled egg nudibranch

© Jacob Buehler and “Shit You Didn’t Know About Biology”, 2012-2015. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Jacob Buehler and “Shit You Didn’t Know About Biology” with appropriate and specific direction to the original content.

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2 thoughts on “Mesmerizing Marine Mimics: Sea Slug Style

  1. Man oh man,in many ways you remind me of David Attenborough , just no censor ha ha . You are a credit to Biologists and Ethologists everywhere . Keep up the good work . You have a keen eye for discussing and explaining animal and species behavior in ways the average person can understand and have respect for .

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