Amazing mimics: the mussel Lampsilis

This week I lectured on mimicry to my evolution class, a course required for all biology majors.  The lecture is a Powerpoint presentation full of amazing mimics, and is designed to show how some evolutionary principles (directional selection, frequency-dependent selection, mutualism, kin selection, and so on) play out in the making of remarkable adaptations.

Mimicry, a phenomenon discovered in the late 19th century, was also important in buttressing Darwin’s theory of natural selection, for some cases admitted of no other explanation.  Too, many ideas about how mimicry evolved are testable ones: for example, does conspicuous warning coloration in bad-tasting butterflies—coloration that is learned and avoided by bird predators—also protect the similarly colored but perfectly edible species of mimic butterflies?  This shows that evolutionary biology is not just a concatenation of plausible-sounding stories, but yields hypothesis that can be tested in the field or lab.

One of the most remarkable examples of mimicry occurs in some North American freshwater mussels in the genus Lampsilis.  Their young go through a parasitic stage, in which they must attach to the gills of fish and suck their blood before later dropping off and resuming a normal mussel-ish life on the stream bottom.  But how can a sessile adult mussel get its young into the gills of a fish?  The answer involves evolutionary modification of the mussel’s brood pouch—which contains its young—so that it attracts predatory fish.  (The brood pouch is simply an outgrowth of the mussel’s mantle.)

But I’ll let the video tell you the story:

Note that, as the video states, the mussel can’t see the fish it’s parasitizing.  In this case natural selection is literally blind.  Those mutations in the mussel that make its brood pouch look more fishlike will give it a reproductive advantage over its confrères, even if it can’t see the fish it’s deceiving.  Note as well that selection has “acted” (I’m anthropomorphizing here: selection doesn’t really “act”, for it’s not an external force but a process of gene sorting) not just on the appearance of the mussel, but on its behavior.  It has genes that make it wiggle its brood pouch in a fishlike manner.

See how precise the mimicry is: the brood pouch has fake eyespots, fake fins, and even, in some mussels, a fake mouth that opens and closes.  Mimicry is one case in which we know what the evolutionary target, or “optimum,” is, and we can see how close selection can take a species to that target. As in many other cases of mimicry, selection gets it pretty spot on.

I’ll have another example tomorrow.

27 Comments

  1. Insightful Ape
    Posted March 2, 2011 at 7:20 am | Permalink

    Fascinating. Thank you Dr Coyne.

  2. Dominic
    Posted March 2, 2011 at 8:13 am | Permalink

    The BBC had this Freshwater mussels story a couple of weeks back, funnily enough featured on Radio 4 this morning –
    http://www.bbc.co.uk/news/uk-england-tyne-12446161
    I did not realize that the young are actually parasitic on trout gills. Thanks for this!

  3. Hempenstein
    Posted March 2, 2011 at 9:08 am | Permalink

    It’s so easy to think of molluscs as nearly inert. But then I remembe Samuel Butler:

    “Even a potato in a dark cellar has a certain low cunning about him which serves him in excellent stead.

    • Dominic
      Posted March 2, 2011 at 9:37 am | Permalink

      He gave Darwin a bit of stick I seem to recall.

  4. Sven DIMilo
    Posted March 2, 2011 at 9:50 am | Permalink

    Amazing indeed.
    And it’s not just small fish that are mimicked by mussels–those small fish like darters are themselves targeted by mussels that mimic fish larvae, and others do crayfish or aquatic insects. Lots of pix from Missouri State here:
    http://unionid.missouristate.edu

    • Diane G.
      Posted March 3, 2011 at 1:25 am | Permalink

      Sweet link!

    • Posted March 3, 2011 at 9:40 am | Permalink

      Thanks for the link! Kids were all “ooohhh! mommy–what’s that!!”:-)) They love getting books on biological mimics from the library–it’s like the best Hidden Pictures ever!

  5. Posted March 2, 2011 at 10:42 am | Permalink

    I’ve covered that topic on my Blog (which is, unfortunately in French) : http://tinyurl.com/6koycyk
    I hope Google Translate will do miracles!

  6. Posted March 2, 2011 at 12:06 pm | Permalink

    Very cool.

    That’s an inane comment, but I want to register that this kind of thing is not thrown away on outsiders like me.

    • Diane G.
      Posted March 3, 2011 at 1:28 am | Permalink

      I well remember one of my Ivy League profs in the early 70’s teaching us continental drift and plate tectonics, and at one point just turning from the board to face us, spreading his palms and saying “that’s just so cool!”

      Scientists are human, too. 😀

  7. rjw
    Posted March 2, 2011 at 1:15 pm | Permalink

    How dare you not mention genetic drift and developmental constraints and neutral theory! You adaptationist programme running dog.

    I’m off to leaf through Gould and Lewontin again.

  8. Grania
    Posted March 2, 2011 at 4:17 pm | Permalink

    What Ophelia said 🙂

    Posts like this are what first drew me to this site, but I have little to of value to contribute to the discussion. But I enjoy them.

    • Posted March 2, 2011 at 4:42 pm | Permalink

      Me, three.

      Jerry, please don’t ever make the mistrake of letting comment volume drive what you post. So often you post something like this that is one of the highlights of the day that nevertheless I have no business commenting on — and, I rather suspect, a great many others have similar thoughts.

      Cheers,

      b&

  9. Posted March 2, 2011 at 4:44 pm | Permalink

    Even though I feel have a pretty good grasp, for a layman (and, sadly creationist until about five years ago) of how this stuff evolves, it still briefly, very briefly, makes me think, “My goodness. Surely this could not have evolved. It MUST have been created.” It’s just a gut reaction that doesn’t last long, but still.

    • uyên
      Posted March 3, 2011 at 2:30 am | Permalink

      Me,too. But towards the end of the vid, everything became clear 🙂
      VIVA LA EVOLUTION 🙂

  10. Marlene Zuk
    Posted March 2, 2011 at 6:20 pm | Permalink

    I love the example, too, but my question is how your dept managed to get evolution approved as a required course for the majors? Has it been that way for long? And is it part of the intro series or an upper division offering? I suspect it would be virtually impossible to have it required at my institution.

    • whyevolutionistrue
      Posted March 2, 2011 at 8:28 pm | Permalink

      We got the division to require it soon after I got here (almost 25 years now). For a while it was optional and could be replaced by a Biodiversity course but now it’s mandatory, as it SHOULD be. (It’s part of an intro series.) I’m surprised that it isn’t required at Riverside!

      • Marlene Zuk
        Posted March 2, 2011 at 8:55 pm | Permalink

        To clarify, we have a 3-quarter (we’re on the quarter system) intro series, and one quarter is for ecology and evolution and population genetics etc. We just don’t have a single course on evolution that undergrads in biology have to take. When we made the quarter on eco/evo mandatory a number of years ago, there was a huge protest by the pre-meds that this was stuff they didn’t “need”. Sigh. Luckily we did prevail.

        • Bryan
          Posted March 2, 2011 at 11:03 pm | Permalink

          Seriously? What does someone majoring in Biology study if not evolution?

          • Sven DiMilo
            Posted March 3, 2011 at 7:07 am | Permalink

            Alas, more and more a Biology major can mean instead a vocational pre-Health Sciences curriculum.
            I exaggerate only slightly.

    • Diane G.
      Posted March 3, 2011 at 1:36 am | Permalink

      I’ve long felt it should be required for ALL college students. Not exactly a for-majors type course, of course, but a “anyone who wants to be considered college educated should know at least minimum about evolution” course.

      Bio being one of the biggest choices for non-majors with science electives to fill, why not formulate a course that will impart the real underlying organizing principle of the discipline, rather than make everyone memorize the Krebs cycle, etc., for one semester, something they will promptly forget?

      Esp. at a school like Chicago which, back when I thought I knew something about it, prided itself on complete education…

      /rant

  11. Posted March 2, 2011 at 8:11 pm | Permalink

    Very interesting-too interesting perhaps in that it’s taking me away from work I should be doing.

    One question-do the mussels take enough blood to harm the bass? The narrator spoke of it as an arms race, a term I’ve normally heard associated with predator-prey relationships. Or in this case are the mussels simply a nuisance the bass tries to avoid, like blackflies and humans?

  12. Diane G.
    Posted March 3, 2011 at 1:38 am | Permalink

    What a cool sequence showing the glochidia attaching to the gill plates!

  13. Sven DiMilo
    Posted March 3, 2011 at 7:12 am | Permalink

    It suddenly occured to me that nobody has yet mentioned the other most salient fact (besides the remarkable mimicry thing) about these animals:

    As a group, freshwater mussels are probably the most severely endangered animals on the planet. They are worse off than frogs and turtles, even.

  14. freddyc
    Posted April 24, 2011 at 9:01 pm | Permalink

    How would it know the specific mimic it has to develop and then what to do with it? If it does’nt get it right wouldn’t the species have become extinct? I mean a lot of things have to happen all at once for the species survival..it just seems random or accidental “correct ” components coming together in unison just do not make sense to me..

  15. Diane G.
    Posted April 26, 2011 at 12:23 am | Permalink

    Well, that’s the whole beauty of the phenomenon! The mussel, as you suspect, doesn’t (can’t!) “know” anything. Jerry described this above:

    the mussel can’t see the fish it’s parasitizing.* In this case natural selection is literally blind. Those mutations in the mussel that make its brood pouch look more fishlike will give it a reproductive advantage over its confrères, even if it can’t see the fish it’s deceiving. Note as well that selection has “acted” (I’m anthropomorphizing here: selection doesn’t really “act”, for it’s not an external force but a process of gene sorting) not just on the appearance of the mussel, but on its behavior. It has genes that make it wiggle its brood pouch in a fishlike manner.

    Remember that this wonderful mimicry has evolved over a long period of time, and in concert with the evolving reactions of the bass. Imagine that an early mussel happened to develop a (heritable) brownish spot on its brood patch. An early bass, catching a glimpse of it, “thought” “Ooooh, darter!” and gulped it up. The larvae of this mussel had then been successfully connected with their host and went on to produce more mussels with brownish spots. But meanwhile, the bass that ended up with the higher parasite load did more poorly than the more discriminating bass, which then began to survive in greater numbers than the impulsive bass. Then one of the brown-spotted brood-patch mussels developed more spots, looking more darter like, and the fish were again deceived. And AGAIN, the fish “learned” (by virtue of degree of parasite load) to be even more discriminatory. So the evolutionary tennis match goes on and on until you end up with a very incredible darter-lookalike brood pouch on the mussel, and a fish that will only snap at very very good mimics. It can certainly be hard to get one’s head around this process when we just see the finished product**, but if you break it down into gradual steps and remember the co-evolution of species, it will become clearer, I hope.

    (As to your question, “if it doesn’t get it right, would the species have become extinct?”–not necessarily. “In the beginning” the mussel had to be successful in some degree to even be able to start this whole mimicry development. But broadcasting your larvae (if that was its previous method) into the water column is going to be far less successful than having your target host devour them directly. One might expect mussels “pre-mimicry” to have persisted at far lower population levels, perhaps, than are now possible…Remember that selection acts on variation, but variation arises largely by chance. Another parasitic mussel could have chanced into an entirely different strategy; say, developing some sort of olfactory lure for its host…)

    *It would probably have been a bit more apt to say, “the mussel cannot ‘see’ the fish it’s mimicking in order to attract the fish it’s parasitizing.”

    **”Endpoint” is really not the case either; all we see now is the current situation. Who knows what direction this relationship could take in the future? If something else should come along and wipe out the darters, say, pretty soon the fish would have no search image for them and the mussels would be back at square one. 😀

  16. giorgio
    Posted December 13, 2011 at 7:39 am | Permalink

    I believe in “some form” of evolution, but something else, something yet to be scientifically discovered, is happening here. The chance of the mussel to grow such a specialized “appendage” just by random mutation would probably take longer than the age of the universe. No, something else is happening here, something that will be explained scientifically.


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