This post is the second 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.
A fish that has evolved to mimic a completely different, potentially entirely unrelated species of fish is relatively impressive. It is a testament to the power of natural selection, this honing and whittling down of a creature so that it may converge on the same exact external form for the sake of protection or the easy procurement of food.
But fish all have the same overall “blueprint.” Paired fins, vertically oriented tail fin, eyes in the front, big, snappy mouth, gills behind the eyes, generally sleek and muscular…there are a number of distinctly “fishy” features that evolution doesn’t fuck around with too much. This sort of basic body plan of a taxonomic group is sometimes down as a “bauplan” and its sort of the generic physical shape and scaffolding with which a given lineage of organisms ends up modifying as different branches break off and try out different tweaks and strategies. The “bauplan” for a motor vehicle, for example, is basically four wheels with tires, a broad cabin for passengers resting atop the rolling chassis, windows, engine, headlights, perhaps a trunk in the back. Anyone can tell you that there are many differences between a Chevy Camaro, a Subaru Outback, and a Hummer, but all of them are superficial when you consider the motor vehicle bauplan they all share. Similarly, the alterations and adaptations associated with the evolution of mimicry in these fish are limited by the constraints generated by their overall body plans. A fish can only fake it so far, and imitating another animal with the same bauplan is completely within any developmental constraints.
But there are fish that manage to step outside the “vertebrate box” when it comes to pulling a fast one on their ocean community. There are some fish that are mimics of invertebrates; spineless, squishy, squirmy things that they haven’t shared a common ancestor with for well over 550 million years. These fish convincingly pass themselves off as things that aren’t even remotely built the same way, all through some clever innovation through the prism of evolution.
The stately gentleman above is a dusky batfish (Platax pinnatus), native to the tropical reefs of the West Pacific, ranging from Australia to the islands of far southern Japan. Batfish and spadefish (family Ephippidae) are beefy, disc-shaped, herbivorous fish found in the warm waters of the world. Most have very large, symmetrical, pointed dorsal and anal fins that, when extended, give them a distinctly triangular or arrow-shaped profile. Some species are of interest to sport fishermen, like the Atlantic spadefish (Chaetodipterus faber).
These imposing swimming silver dollars are bound to make an impression underwater, but if you ever came across a juvenile dusky batfish, you may not know it….because they, much like Jaleel White of Family Matters / Urkel fame, look absolutely nothing like their post-pubescent selves.
While this little guy’s high-contrast orange and black style may make him look like he was created by the Tron: Legacy design team, it is thought the odd appearance was crafted by evolution to serve a specific purpose.
You’ll note that Mr. Halloweenfish up there deviates from the adult form in more than just that vivid orange trim. Juvenile dusky batfish are shaped completely differently: they have long, elegant, flowing fins with jagged, broken edges, and a tiny head with small facial features that disappear into the sea of black scales. These fish are more thin, membranous fin material than actual meat, and appear like they’ve been carefully flattened with a rolling pin.
It turns out that baby dusky batfish share their ocean with another animal that looks remarkably similar, and it ain’t a damn fish.
At first glance, this may look like a fancy-shmancy, elegantly-riffled chocolate and orange-flavored cookie, but I can guarantee you wouldn’t want to put this thing in your mouth. In actuality, this is an orange-margined marine flatworm (Pseudobiceros periculosus), a native of tropical reefs throughout much of the Indo-Pacific. Flatworms make up the phylum Platyhelminthes, and are among the simplest bilaterally symmetrical (meaning they have two sides that mirror one another, opposed to multiple sides that mirror one another around a central point, like a sea star or a jelly) animals on the planet, lacking specialized organ systems commonly considered important by creatures like ourselves; respiratory and circulatory systems are completely absent, for example. The most familiar flatworms in the human world are parasites like tapeworms and flukes, but in the oceans, particularly on coral reefs, there are big, free-living species that swim and crawl all over the place. Many of them sport brilliant and beautiful colors, like the orange-margined flatworm above…typically to warn predators that they are toxic, taste like shit, or both.
So, mistakenly thinking P. periculosus is a delicious product of Pepperidge Farms would likely result in you aggressively spitting, scrubbing your tongue, dry heaving, and generally having an awful time.
It is thought that the young of the dusky batfish have evolved to look like poisonous and distasteful flatworms like P. periculosus (or P. periaurantias or P. affinis, other black species with bright orange margins). As full grown adults, these fish are powerful, fast swimmers, and rely on their athleticism and (in some species) habit of schooling in massive, closely-packed groups to evade becoming dinner for bigger, toothier fish. But young fish are small, solitary, and vulnerable, and trying to pass as a bitter, noxious item that predators stay clear of is certainly a workable survival strategy.
To enhance the quality of their flatworm impression, juvenile dusky batfish do more than just look the part. These fish do “the Worm”, but instead of drunkenly flopping all over the floor at your friend’s wedding reception, they take the dance a little more seriously, gracefully undulating their fins like a woman wearing a long, flowing dress. They move slowly while doing this, hovering just above the bottom in an attempt to mimic the beautiful, billowing swimming style of a marine flatworm. They will also swim on their sides to better match the “flat” shape of the worm.
This deceptive tactic is shared across the entire batfish genus (Platax), but the young haven’t all evolved to target the same model. The longfin batfish (Platax teira) have young that appear to mimic rotting leaves, as does the young of the orbicular batfish (Platax orbicularis). The juveniles of these species in particular hang out close to shore in brackish, sheltered mangrove thickets….which drop plenty of leaves into the water for them to blend in with. Platax batavianus, the humpbacked batfish, have young that show off stunning black and white stripes with long, fan-shaped fins, but as far as know, no one has offered a potential model for the mimicry likely exhibited in this batfish. Personally, I think the young fish, with its striping and conspicuous wing-like fins, looks like it could be mimicking a small lionfish….which are armed with a marvelously, debilitatingly painful venomous sting, and are therefore a more than worthy model to emulate to keep away hungry predators.
There are a number of other fish that have a big, strong, independent adult phase, proceeded by a childhood spent cowering next to the reef and trying to convince fleet after fleet of humongous submarines passing overhead, each propelled by a stomach that is never full, that they aren’t on the menu.
For example, there are fish of the genus Plectorhinchus. These fish, members of the “grunt” family (named for the sound they make when grinding their teeth together), grow to be longer than a man’s leg and are powerful, muscle-bound, predatory fish. Plectorhinchus fish root around in the sand and rubble to uncover and crunch down on invertebrate prey like crabs or worms, and are immensely aided in this effort by their giant, voluptuous, puckered, sensitive lips….which has led to their common name, “sweetlips.”
But long before these guys are putting their DSLs to work near the top of the food chain, roving the seafloor in intimidating, ravenous gangs, they spend their awkward early years trying to be something they’re not.
One species, the harlequin sweetlips (Plectorhinchus chaetodonoides), found in the coral reefs of the Indian and west Pacific Oceans, makes a particularly dramatic transition between its elementary school form and its full-on tax-payin’, job-havin’ adult form. The leopard-spotted adults are shown in the photo above. The babies (below) look like a Holstein fucked a guppy and dressed up the resulting child like a flamenco dancer.
These little spotted buddies are solitary and stay near the shelter of the reef. It isn’t until they mature, trade in their polka dots for a classy suit of fine spots, and graduate out of their size bracket and lower rung in the reef food chain that they become the confident, shrimp-crunchin’ machines I told you about. But until then, much like the juvenile batfish, they keep alive by mimicking what is thought to be a poisonous flatworm or sea slug. I’m not aware of a specific model that’s been proposed for this species (or the other varieties of Plectorhinchus, which have similarly small, billowy young with coloration that looks very different than the adults’), and it may be that it looks vaguely “flatworm-ish” or “sea slug-ish” enough to do the trick.
What is apparent is that harlequin sweetlips juveniles have evolved a combination of visual cues, color patterns, and killer dance moves that are immediately called into action at the first sign of a potential predator. They make exaggerated swimming movements, flopping their entire body back and forth wildly, letting their broad, flaccid fins articulate with each dramatic folding of the fish’s body. This loosey-goosey undulating is something suspiciously similar to what something without a spinal column might do….something like a flatworm….or a sea slug (specifically, a brightly-colored nudibranch). They’ll also often point their noses down, and wriggle their fins and tail back and forth in a nearly stationary vertical position, further improving the ruse. Predators figure the twisting creature near the bottom of the reef is likely just a toxic blob of culinary disappointment and go about their errands, and the harlequin sweetlips lives through another day.
Poisonous flatworms are apparently one of the more successful choices for a mimic to model themselves after, because the list of wormy, pancake-like juvenile fishes is long. Another fish that appears to take the Platyhelminthes path is the white-blotched sole, Soleichthys maculosus, found in the tropical waters of the West Pacific and northern Australia. Soles, along with flounders, halibut, and plaice, make up the “flatfish” order of fishes, Pleuronectiformes. Like their common name suggests, flounders and their kin (in contrast to what depictions in certain Disney movies about mermaid minors with a hoarding problem might have you believe) are literally flattened, living their adult lives spread out across the seafloor, grotesquely compressed, like a submarine steamroller came on through while they were taking a nap. These pavement huggers typically use camouflage to blend into their sandy, pebbly surroundings, or burrow just under the most upper surface of the loose bottom substrate. But the white-blotched sole, especially in its youth, is an anti-conformist, and its black background is splotched with white, and its fins are bordered with bright orange. This coloration pattern is blatantly similar to that of a poisonous species of marine flatworm (and close relative of the proposed model for the dusky batfish juvenile) that shares its range with the sole, Pseudobiceros scintillatus.
While some flatfish are somewhat well-known for producing their own distasteful secretions (like the Red Sea moses sole (Pardachirus marmoratus), which excretes a milky fluid containing pardaxin, an effective, toxic predator repellent), the flamboyant white-blotched sole is very edible. Because of this, its mimicry must be convincing. Air tight. It’s not enough to look like a flatworm from a distance (being flat already gives you a leg up in that regard), you have to fall into the role head over heels and really sell the whole damn thing, soaring your method acting skills to Daniel Day Lewis-ian heights.
Young white-blotched soles completely alter the way they swim (much like the dusky batfish and the harlequin sweetlips), channeling their inner invertebrate, as can be observed in the video below from The Blenny Watcher:
When flatfish move across their two-dimensional environment, they kick off the bottom with a thrust of their tails and glide like a living frisbee to another spot to settle down, keeping close to the bottom like a hovercraft, moving swiftly with rapid strokes of the tail and back edges of their fins. But the white-blotched sole in the video doesn’t do that. No. This little guy cautiously and methodically scoots along the bottom like someone trying to covertly edge themselves closer and closer to the door so they can escape an awkward, under-attended house party without being noticed. Like I’ve said before, it’s hard to show definitively that mimicry is occurring, but when behavior matches up with appearance like this, it’s pretty convincing.
With all this talk of baby fish pretending to be flatworms, you’d be forgiven if you thought that was the extent of cross-phylum mimicry in the ocean. Adult fish are known to try their hand at this kind of deception, and with very different organisms. Flatworms are easy, if you think about it; decked out in radiant colors, and with a fairly basic body shape and no complex structures to imitate, they are also found commonly in the more productive reefs. Poisonous flatworms are an obvious “choice” for a model. But there are more creative options out there.
Consider the frogfish. This family of fishes (Antennariidae) is kin to the famous deep-sea anglerfishes and monkfishes (large, ugly critters regularly fished in the North Atlantic). Like the rest of the taxonomic order to which they belong, frogfishes are ambush predators, lying in wait either buried in sand or perched on a rock or coral head for a small fish to cluelessly stumble into the range of its lightning fast jaws. Frogfishes also typically use a wormy lure (called an “esca”) on their face that they jiggle seductively to entice a meal to come just a liiiiiittle bit closer. Most frogfish engage in a specific kind of mimicry known as “mimesis”, a type of camouflage that makes you look like your (typically living) surroundings. Different species of frogfish have evolved projections from their skin and patterning that makes them blend into their surroundings; from backgrounds like sponges, to seaweed, to coral, frogfishes are really fucking good at disappearing in plain sight. When you actually look at them, they are basically chubby wads of used chewing gum with fin “arms” and a face. Frogfish always look like someone just slapped an ice cream cone out of their hands and onto the ground.
The disappointed three-time winner of the “World’s Fuzziest Peach Contest” pictured above is the striated frogfish (Antennarius striatus). It is an incredibly common species, and is widespread, ranging into every warm temperate to tropical marine body of water on Earth. Most of the time, they look like the individual above; like Carrot Top’s cousin that he keeps insisting lives “by” the ocean. But every so often, they look like this:
Occasionally, a completely black morph without any patterning shows up. These guys look like someone took a tangled hair plug from a shower drain and used it to decorate one of those black turds you get from drinking Pepto Bismol. This fish looks like it should be sultrily voiced by Tim Curry and plotting to destroy the Last Rainforest.
In the Philippines in 2005, these black morphs were observed congregating alongside a species of urchin. This urchin, Astropyge radiata, is nearly entirely covered in long, needle-sharp venomous spines that pack a burning sting…hence its common name, the “fire urchin.” As I’ve mentioned once before, getting stung by a venomous urchin is a less than pleasurable experience, and anything capable of inducing agonizing chemical rage like that is bound to be a good potential model for a mimicking species.
This case, where black frogfish are slowly shuffling alongside a herd of sea urchins, is, possibly, fairly unique in regards to sea-bound mimicry. If indeed the frogfish were associating with the urchins to better resemble one for its own protection (with the idea that larger predatory are unlikely to bite down on something that looks covered in toxic pokers), a “wolf in urchin’s clothing,” then this is sort of a new one for frogfish biology. This would be Batesian mimicry; when a species has evolved to look or behave similarly to another species that is harmful/dangerous/distasteful, garnering the mimic a level of protection. This is different than looking like a tuft of sea grass or a pile of rubble to avoid detection, because those are neutral objects…background elements. Batesian mimicry has the purpose of making the mimic look like it can cause more trouble than it actually can. It’s hard to tell if the frogfish are blending in with the urchins for their own protection, or for making their disguise towards prey more effective; as in, is this just good ol’ mimesis, since no tasty little fish would expect an urchin to be a threat? Urchins aren’t exactly known for lunging out of nowhere and snapping up fish faster than you can blink. It’s possible there’s three distinct forms of mimicry happening at the same time. 1) Batesian mimicry (frogfish looks like a venomous urchin, so nothing tries to fucking bite it), 2) mimesis (frogfish looks like slow, benign, herbivorous urchins, causing small prey fish to never suspect a thing), and 3) “aggressive mimicry” (frogfish uses lure to persuade dinner to mozie on over to its awaiting mouth hole). I’m not aware of any other creature on the planet that manages to be a lying, tricky, untrustworthy bastard in so many different categories.
If this is the case, there are some interesting evolutionary angles to consider, especially when you remember the black frogfish are just a single morph within a species. How often do these black frogfish and the urchins overlap with each other, and where? Does this opportunity for Batesian mimicry make the black morphs more “fit” than the other, more common, tan and yellow morphs?
Science noted this possible occurrence of mimicry for the first time only a decade ago, so there are still plenty of unanswered questions.
The striated frogfish may have dabbled in a diversity of mimicry methodologies, but there’s another animal in the ocean that has a repertoire of characters it can pull out whenever it needs to, and is a prodigal genius of targeted bullshittery; a masterful Kevin Spacey of underwater imitation.
I’m of course talking about the undisputed king of marine mimicry, the mimic octopus (Thaumoctopus mimicus). This small, wiry, normally white and deep brown striped cephalopod is found in open sand flats in the tropical West and Central Pacific, as well as around the Great Barrier Reef. Thaumoctopus is a bit of a dickish, overachieving curve-ruiner when it comes to mimicry, far and away surpassing the capabilities of any other animal in the ocean with at least a dozen different models it can mimic.
This Swiss Army knife of deceit can casually contort its body and long, trailing tentacles into whatever animal seems like the best option at the time. This is a video showing just a few tricks up its eight sleeves. Lionfish. Sea snakes. Mantis shrimp. Jellies. Crabs. Tube worms. Sea squirts. Sponges. Flounders. The list goes on. Thaumoctopus‘s CV is extensive enough to land it a spot on the cast of SNL…and to be honest, it might be a bit of an improvement.
The mimic octopus is impressive as shit, but, as I’m sure you, astute reader, have noticed….the mimic octopus is not a fish. I’m listing this mollusk here not to go over the minutia of its mind-blowing powers of duplicity, or to really talk about it much at all, as mimic octopus have been talked about plenty since their discovery in 1998. No, I’m still actually aiming to talk about a fish that mimics an invertebrate….an invertebrate that just happens to be the most adept and versatile mimic in the ocean.
To understand what I mean, you need to check out this video below from Luiz Rocha from the Cal Academy of Sciences, and featured here:
Behold, the antics of the black-marble jawfish (Stalix histrio), which appears to mimic the tentacles of an on-the-move Thaumoctopus, behaving like a ninth arm as it sticks close by. Jawfish, members of the somewhat recently-evolved family Opistognathidae, are vaguely blenny-like fish with big, blunt heads, large mouths, and narrow, tapered bodies. They are homebodies, and spend much of their time submerged in burrows defending their tiny territories. Because of this, jawfish aren’t all that great at the “swimming in the open” thing that most fish do. They are also quite itty-bitty, with no toxins, spines, or mace to protect themselves with.
It’s been offered by the scientists that first described this relationship that Stalix histrio has found a means to move out in the open safely and securely, by hitchhiking along yet another completely unrelated mimicking species in possibly the only known case of “mimic-ception” on Earth. Think about that. The mimic octopus is so powerfully talented mimic of sea life to grant its own protection, that a teeny species of fish has evolved to mimic it for its own safety. That’s how good at its job the mimic octopus is. I’m not entirely sure the jawfish isn’t hanging out with one of the mimicry greats on some kind of mimicry apprenticeship or something, and is seeking to learn the tricks of the trade from a mentor. The entire thing sort of feels like when there’s a fictional TV show existing entirely within the world of a fictional TV show. Nature shouldn’t be that meta. I’m not sure I can handle it.
This post series will continue: Part 3 will tackle mimicry among the spineless and slimy…
Image credits: Intro sweetlips photo, adult dusky batfish, juvenile dusky batfish, orange-margin flatworm, sweetlips adults, sweetlips juvenile, P. scintillatus, brown frogfish, black frogfish, mimic octopus, mimic octopus as flounder
© 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.
I just knew that octopus and jawfish had to be there.
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