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Take This Shell and Shove It: The Mollusk That Became a Worm

The views expressed are those of the author and are not necessarily those of Scientific American.


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An aplacophoran cuddled up with a bubblegum coral, according to Alistair Dove at Deep Sea News. NOAA Okeanos Explorer; public domain. Click image for source.

When you think of a mollusk, you probably have something shelled, slimy, and possibly stalk-eyed in mind. But mollusks include creatures that are none of these things.

In fact, there are mollusks that are wormy, be-spined, and eyeless. They are called aplacophorans, and scientists have long puzzled over their place in the mollusk family tree.

For molluks, shells are practically de rigeur: snails, conchs, and chitons all sport shells with varying amounts of bling (the bling kings may be these guys). But aplacophorans lack shells secreted by a mantle like other mollusks. Instead, they possess calcareous spines or spicules that give them a bristly appearance, like this. Others are more colorful, or more sparsely spined.

How do we know they are mollusks, then? They often have two major features unique to the group. Some have a distinctive food-harvesting structure called a radula that only mollusks make (if you’ve ever watched an aquarium snail scour algae from the inside of an aquarium, you’ve seen a radula in action). Some possess characteristic molluskan gills called ctenidia. But — no shell.

So are aplacophorans super-primitive molluscs that first appeared hundreds of millions of years ago and never had shells? Or are they descended from shelled ancestors who for some reason or another, decided that shells were SO 50 million years ago?

But Scientists reported in Nature in October the discovery of a 425 million-year-old fossil in the United Kingdom that seems to indicate that it’s the latter. Kulindroplax perissokomos has the worm-like body and be-spined appearance of an aplacophoran, but valves closely resembling those of the related shield-shaped mollusk called a chiton (albeit it has one less valve than the chiton standard-issue eight). Its discovery supports previous molecular evidence pointing in this evolutionary direction.

Here is a scanned reconstruction of the worm-like fossil they found and its seven shelled valves:

Sutton et al. 2012. Click image for link.

And here are the valves of a modern chiton:

Creative Commons Kirt L. Onthank. Click image for link and license.

The fossil also clearly showed signs of tiny spicules and a cuticle — a feature that aplacophorans possess today in the form of a chitinous covering. Chitin, for those of you keeping track, is also the protein that constitutes mollusk radulas, squid and octopus beaks, fungal cell walls, and the armor of crustaceans and insects.

The cuticle and spicules of Kulindroplax. Scale bar 2 mm. Sutton et al. 2012; Click image for link.

As for the spicules, aplacophorans aren’t the only modern mollusks to bear them. Chitons, too, often possess mineral spines or ornaments on the fleshy girdle ringing their valves.

Kulindroplax thus supports the hypothesis that chitons and aplacophorans share a common ancestor, given the similarity of their spicules and valves, and are together ancestral to the rest of the modern mollusks. In other words, they’re among the oldest extant members of the group.

There are two major groups of modern aplocophorans. The solenogasters eat or suck the juices from corals, comb jellies, and other cnidarians. The caudofoveates burrow in sediment. Incredibly, the evidence seems to imply the two aplacophoran groups lost their shells separately — and as recently as 40 million years ago.

Far from being primitive, aplacophorans seem to be the recently evolved, highly modified — and only recently de-shelled — descendants of ancient armored mollusks.

Jennifer Frazer About the Author: Jennifer Frazer is a AAAS Science Journalism Award-winning science writer. She has degrees in biology, plant pathology/mycology, and science writing, and has spent many happy hours studying life in situ.
Nature Blog Network
Follow on Twitter @JenniferFrazer.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. Charles Hollahan 3:59 pm 12/31/2012

    So, some seem to be parasitic while other aplacophoran are detritivores. Who needs a shell when there’s a host to protect you?

    There’s another mollusk that has a similar shape, the Serpulorbis spp. in the family Vermetidae, which dwell in a calcareous tube.

    I’ve never really thought of mollusks as slimy, not after I’ve seen hagfish, and nudibranchs aren’t really slimy either but people usually do think that. I wonder if they think that of oysters as well? Probably not after two drinks…

    Link to this
  2. 2. Bill_Crofut 10:48 am 01/3/2013

    Re: “Far from being primitive, aplacophorans seem to be the recently evolved, highly modified—and only recently de-shelled—descendants of ancient armored mollusks.”

    It seems to me, from an evolutionary standpoint, armor would be more advantageous for survival than its lack. Has Kulindroplax perissokomos been discovered in an environment radically different from its alleged predecessors? The abstract from the NATURE paper online doesn’t seem to be particularly helpful in answering my question:

    “The Mollusca is one of the most diverse, important and well-studied invertebrate phyla; however, relationships among major molluscan taxa have long been a subject of controversy [1, 2, 3, 4, 5, 6, 7, 8, 9]. In particular, the position of the shell-less vermiform Aplacophora and its relationship to the better-known Polyplacophora (chitons) have been problematic: Aplacophora has been treated as a paraphyletic or monophyletic group at the base of the Mollusca [3, 6, 8], proximate to other derived clades such as Cephalopoda (2, 3, 10), or as sister group to the Polyplacophora, forming the clade Aculifera [1, 5, 7, 11, 12]. Resolution of this debate is required to allow the evolutionary origins of Mollusca to be reconstructed with confidence. Recent fossil finds [13, 14, 15, 16] support the Aculifera hypothesis, demonstrating that the Palaeozoic-era palaeoloricate ‘chitons’ included taxa combining certain polyplacophoran and aplacophoran characteristics [5]. However, fossils combining an unambiguously aplacophoran-like body with chiton-like valves have remained elusive. Here we describe such a fossil, Kulindroplax perissokomos gen. et sp. nov., from the Herefordshire Lagerstätte [17, 18] (about 425 million years bp), a Silurian deposit preserving a marine biota [18] in unusual three-dimensional detail. The specimen is reconstructed three-dimensionally through physical–optical tomography [19]. Phylogenetic analysis indicates that this and many other palaeoloricate chitons are crown-group aplacophorans.”

    http://www.nature.com/nature/journal/v490/n7418/full/nature11328.html

    Link to this
  3. 3. Conches vs. conchs - Grammarist 9:33 pm 01/3/2013

    [...] For molluks, shells are practically de rigeur: snails, conchs, and chitons all sport shells with varying amounts of bling. [Scientific American (2012)] [...]

    Link to this
  4. 4. Jennifer Frazer in reply to Jennifer Frazer 9:07 am 01/4/2013

    I don’t know the answer to your question about the fossil, but often evolution de-selects for traits that were once useful but no longer. Think of flightless birds, for example, whose wings have shrunk or been repurposed for swimming. The shell may have become more costly than it was worth, or may have actually gotten in the way. Just guesses …

    Link to this
  5. 5. Bill_Crofut 9:29 am 01/4/2013

    Ms. Frazer,

    Thank you for responding.

    The problem for me with loss of traits (i.e., flightless birds) is in the form of another question: What is the evidence that the flightless birds were not flightless from the beginning of existence?

    Link to this
  6. 6. npieplow 12:46 pm 01/4/2013

    Hi Bill,

    I’ll take a crack at answering your question, though I may not have time to do as thorough a job of it as I would like.

    There are some 40 species of flightless bird in the world today. Some, like penguins, kiwis, and ostriches, are not particularly closely related to any living flying species. However, many of them, like the Flightless Cormorant of the Galapagos Islands, are clearly very similar to living flying species. The Flightless Cormorant still has wings with feathers, and it still has a keeled breastbone (which in most bird species serves as an attachment point for the enlarged breast muscles necessary for flight). If it did not evolve from a flying ancestor, why should it have wings or a keeled breastbone at all?

    Why would any bird species lose the ability to fly? It would seem to reduce a bird’s chances of survival. However, flight is an evolutionary tradeoff for birds. It is an “expensive” trait. To maintain it, a bird species must become as lightweight as possible (increasing the fragility of its skeleton) and it must invest tremendous energy in muscle power and in the constant replacement of worn feathers (both of which require the constant intake of LOTS of calories).

    For most species, flight ends up being a net evolutionary benefit, because it provides an excellent means of escape from predators and high degree of mobility that allows birds to exploit many resources unavailable to other vertebrates. But under certain circumstance, the balance may shift in favor of flightlessness.

    If a flying bird (A) does not face any land predators, like in the Galapagos; (B) has ready access to food resources that require no flight to reach, such as schools of fish immediately offshore; and (C) would be able to exploit those food resources more efficiently by trading away some of its flight adaptations, like by reducing the size of the wings so that they provide less drag underwater — then we would see a clear evolutionary path towards flightlessness, as we do in the case of the Flightless Cormorant.

    Bolstering this general idea of how flightlessness evolves is the repeating pattern we see of flightless island endemic bird species — birds restricted to islands that are very closely related to flying relatives elsewhere in the world, but have lost the ability to fly, to varying degrees. The rail family provides many excellent examples. For example, the flightless Takahe of New Zealand looks almost identical to its flying relative the Pukeko, except for being three times heavier, and the two species are only 0.3% divergent in their DNA. The Pacific Islands are full of endemic rail species that are morphologically and genetically very similar to flying species elsewhere: see http://www.jstor.org/stable/56680.

    A final line of evidence in favor of the evolutionary loss of flight is the fact that we see species around the world in various stages of losing it. Some, like ostriches, haven’t had it for millenia, and show the ancient flight adaptations in only vestigial ways (like feathers and rudimentary wings). Others, like the Flightless Cormorant, are clearly much closer to flying ancestors, with larger wings, more functional feathers, etcetera. And some, like the steamer ducks of southern South America, have lost the ability to fly more recently still. Three of the steamer duck species are only capable of “steaming” — i.e., shooting across the surface of the water using their wings and feet as paddles. The fourth retains the ability to fly, but just barely — it flies rarely and poorly, to such an extent that steamer duck experts Livezey and Humphrey (1986) described it as “mostly flighted” (http://www.jstor.org/stable/2408576).

    There’s quite a bit of published research on the evolution of flightlessness — you might want to start with that last link (payment required) or a Google Scholar search to learn more about the various theories and the evidence to support them.

    Link to this
  7. 7. Bill_Crofut 12:36 pm 01/5/2013

    Hi npieplow,

    Re: “If…the Flightless Cormorant…did not evolve from a flying ancestor, why should it have wings or a keeled breastbone at all?

    The response that comes immediately to mind is: economy of design.

    Re: “…evidence in favor of the evolutionary loss of flight is the ostriches…”

    Is there any evidence that the ostrich was able to fly prior to its alleged loss of the ability to fly?

    You’ve indicated a restriction based on time constraint. Have you one particular online paper (without cost) you would recommend as a starting point for my research?

    Link to this
  8. 8. Charles Hollahan 12:57 pm 01/6/2013

    Well, the fact that flightless birds have vestigial components of wings, keel-bones, and so forth suggest that they evolved from ancestors that flew. “Economy of design” suggests design, not evolution.

    There’s a lot of material posted on the web and if you search “evolution of flightless birds” then you’ll find plenty of free material, some good and some poor, that you can read. There’s a recent study of genetic material that states that the large flightless birds developed multiple times and not from a single ancestor – the way it was once thought, http://www.livescience.com/2846-theory-flightless-birds-shot.html .

    Sea hares are shell-less slugs that evolved from mollusks which once had shells, the larval stage of the animal has an identical form as other planktonic forms of the same class – a veliger which is basically a tiny snail in a shell, but if you look inside the adult then you’ll find a vestigial shell that amounts to a shield. Other sea slugs lack even a vestigial shell. Embryology and development are the best places to look for connections, that is until genetics became so useful. Now a look at the genome will do.

    Parasitic organisms will often look so unlike their free-living relatives that identification becomes very difficult without genetics or natural history. The article above illustrates that. What I find amazing is how people found links so well without genetics and genome research is reinforcing a lot of insights while altering and modifying others.

    Link to this
  9. 9. Bill_Crofut 8:44 am 01/7/2013

    Charles Hollahan,

    What is the evidence any characteristic in flightless birds is vestigal? Is that not, rather, a conclusion based on approaching the evidence from the standpoint of evolutionary bias?

    Thank you for providing the url which will initiate my research.

    Link to this
  10. 10. Bill_Crofut 9:30 am 01/7/2013

    Charles Hollahan,

    Re: “We think the most likely hypothesis is that the tinamous, rheas, cassowaries, emus, and kiwis had an ancestor that could fly, and that flight was lost at least three times (once in ostriches, once in rheas, and once in the emu-cassowary-kiwi group),” Braun said….The finding is also exciting from a developmental standpoint, Braun said, given how similar most ratites look. The ratites apparently all arrived at similar body shapes through different evolutionary routes, a concept called convergence (for example, birds, bats, insects and pterosaurs all have or had wings, but they are not all closely related).”

    http://www.livescience.com/2846-theory-flightless-birds-shot.html

    The hypothesis of loss of flight at least three times, brings to mind another anatomical quandry expressed by Prof. Frank B. Salisbury:

    “Even something as complex as the eye has appeared several times; for example, in the squid, the vertebrates, and the arthropods. It’s bad enough accounting for the origin of such things once, but the thought of producing them several times according to the modern synthetic theory makes my head swim.”

    [1971. Doubts about the modern synthetic theory of evolution. THE AMERICAN BIOLOGY TEACHER, September, p. 338]

    Regarding the hypothesis of convergence, Prof. W. R. Thompson wrote:

    “What such cases like those of anatomical ‘convergence’ and general homology actually demonstrate is that there are large numbers of organisms differing considerably in the details of structure but constructed on the same fundamental plan. However, this is no proof of descent from one original ancestor of this anatomical type. This itself requires proof.”

    [1956. Introduction. In: Charles Darwin. Origin of Species. Everyman Library No. 811. London: J. M. Dent and Sons. Reprinted with permission. Evolution Protest Movement. 1967. NEW CHALLENGING ‘INTRODUCTION' TO THE ORIGIN OF SPECIES. Selsey, Sussex: Selsey Press Ltd., p. 11-12]

    Link to this
  11. 11. Charles Hollahan 12:43 pm 01/7/2013

    Bill, you’re not an open-minded person, what evidence do you have that the world is more than 6000 yrs old? Why do you believe everything in the bible?

    I don’t really care what evidence you have. I won’t waste any time answering you again. In fact, I like getting a review now and then but the science of genetics is moving so much faster than any one person can keep up with that any time you spend writing in these blogs is a waste, but I’m not interested in stopping you. Go ahead. Knock yourself out. You won’t make any converts here. I won’t go to creationist’s sites and write about evolution there – you can believe whatever you want to. Most people here are amused by fundamentalists, except for four or five of the writers who really get worked up by it, you’ll get much more bang for your time with them.

    Link to this

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