A marvelously mimetic mollusk

I’m always glad to see an article by Natalie Angier in the New York Times science section, because I know I’m going to learn about some wonderful new trick of evolution, recounted in Angier’s characteristically lively prose.  Her piece yesterday, “Surviving by disguising: nature’s game of charades,” is about recently discovered and bizarre cases of mimicry in animals.

I’ve already posted about Angier’s first example, involving margays (a neotropical cat) making tamarin calls to lure the monkeys within reach.  (Angier notes that “The scientists published their description of the first official “field observation of margay mimicking behavior” in the journal Neotropical Primates last year, but only now is it circulating among field researchers more widely.” You heard it here first!)  She gives other examples of auditory mimicry in ants and olfactory and tactile mimicry in cockroaches.

But the strangest example is a veritable Proteus of mollusks: the Indo-Pacific octopus Thaumoctopus mimicus.  As an article in this month’s Biological Journal of the Linnaean Society describes, this species has a form of mimicry uncommon in cephalopods: deimatic (conspicuous) mimicy.  In contrast to other types of octopus mimicry, in which the beasts resemble something else to hide themselves (this includes all the wonderful ways that octopuses can change their color to mimic the background), in this instance the creature not only has a deceptive appearance, but one it flaunts before predators to convince them it’s not worth eating.

Thaumoctopus mimicus is named appropriately.  As Angier notes, it can change its appearance to look like a flounder, a lionfish, or seasnake. (It also makes other nonmimetic warning displays.) Here’s Angier’s description, a tad livelier than the one given in the paper:

Like most octopuses, T. mimicus can use its nervous system to instantly change colors into a perfect wallpaper blend. Unlike most other octopuses, the mimic will sometimes choose to make itself more conspicuous to potential predators, rather than less. If it must venture out to forage in dangerous open waters, it assumes a menacing disguise appropriate to context. Before swimming above the seafloor, the octopus gives a shudder, and presto, its flesh becomes boldly striped, its arms and body resolve into a leafy, spiny form: it’s a toxic lionfish. For skating along the sea bottom, the octopus turns its skin bumpy and beige, compresses its body, pulls its limbs to its side: it’s a toxic flatfish, undulating its fins, staring you down with its top-sided eyes.

“When it’s being pestered by a damselfish, it will turn one of its arms into a sea snake, with the contrasting banding pattern of a sea snake, and with the tip of the arm thickened to look like the snake’s head,” said Healy Hamilton, a biodiversity and informatics expert at the California Academy of Sciences and an author on the report. “Sea snakes are voracious predators of damselfish.”

And you can see all three behaviors, and more, in this video:

Here’s the toxic “peacock sole,” Pardachirus pavoninus, that the octopus is thought to mimic. (It may be a general sole mimic, too: Huffard et al. suggest that some predators may avoid eating soles simply because their flat shape makes them hard to nom.)

Two points here.  First, the octopus almost certainly is hard-wired to have these patterns and behaviors rather than making a “conscious” decision to adopt them.  The wiring, of course, would be expressed situationally: if you’re halfway in a crevasse but threatened, do the sea snake; if you’re swimming low to the bottom, do the flounder.  Huffard et al. note that other species which swim like flatfish display the behavior even if raised in isolation, never having a chance to either learn it from other octopuses or to see a flatfish swimming.

Second, the mimicry is not perfect. You can see that the “flatfish display” is, to us at least, easily distinguishable from a swimming flatfish.  But mimicry need not be perfect to give an advantage, favoring the genes promoting the behavior. Angier has this covered:

None of the octopus’s imitations are perfect, and they don’t need to be. “If the predator just takes pause,” said Dr. Healy, “the octopus can ink and jet propulse away.”

A word more about the paper.  The authors wanted to see if the flatfish-swimming display was an “adaptation,” evolved de novo in this species and its relatives, or was an “exaptation,” that is, a trait for which some of the parts were already present, evolved for other ends in an ancestor, and which were later cöopted and elaborated in this lineage to assume a new adaptive function.  They made an octopus phylogeny using mitochondrial DNA, and concluded that swimming like a flatfish was a genuine adaptation, evolved in conjunction with the long arms that can be dragged behind, resembling a fish body and fins.  They concluded this because we don’t see any of these traits in the “outgroup” species to which T. mimicus and its close relatives—all of which swim like flounders—are related.  Less closely related species don’t seem to have even the rudiments of a “flatfish display.”

Now we’re not sure that mimicry evolved because it promoted a protective resemblance to a flounder.  We don’t have evidence that you survive better if you can mimic a flounder, a sea snake, or a lionfish.  Proving that would take a lot of hard experimentation—probably in the open ocean or large tanks.  Nevertheless, the discovery of such a weird trait is the first step in trying to understand its significance.  And even if we never know for sure, we can at least marvel at evolution’s ingenuity.

Angier also mentions recent work suggesting that we humans may mimic each other for “adaptive” ends:

Yet psychologists are coming to appreciate the profound importance of nonconscious mimicry in making us feel loved and appreciated, or simply smoothing the edges of our everyday affairs. Without realizing it, when we’re conversing with friends, we match our tone of voice and speech rhythms to theirs, adopt similar body posture and even imitate their tics. Studies have shown that, when students are instructed to work cooperatively with somebody who engages in either repeated hair touching or foot shaking, the students soon start fiddling with their hair or waggling a foot. Waiters who repeat their customers’ orders word for word, or who subtly mimic a customer’s body language, earn higher tips than do waiters who paraphrase the order or forgo the gestural mirroring.

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Huffard, C. L., N. Saarman, H. Hamilton and W. B. Simison.  2010.  The evolution of conspicuous facultative mimicry in octopuses: an example of secondary adaptation? Biological Journal of the Linnean Society, 2010, 101, 68–77.