Clam, Snail, and Caddisfly Case

2012 October 6

While we were netting aquatic life in the pond west of the house on May 6, 2012, we turned up a number of things that were not arthropods. Like this little fellow.

I’m not quite sure whether this is a young specimen of one of the 45 species of freshwater mussels that are known to live in Michigan (and ultimately get to be several inches across), or an adult “pea clam” which never get much bigger than this. It is certainly a bivalve mollusc, though, which includes all manner of clams, mussels, scallops, oysters, and similar creatures. All of these have two-part, hinged shells that they can close up to defend themselves, and that are made up mostly of calcium carbonate (calcite and/or aragonite). The vast majority of bivalves live in the ocean, where there is more dissolved calcium available in the water (which makes it easier for them to grow thick shells in the ocean). The freshwater clams I’ve seen generally have thinnish shells, which makes sense given that they would have a harder time acquiring enough calcium to make them.

The shells grow at the edges, so the little bulges on the hinged side of the shell are the original portions. There is some temptation to count the “growth rings” to see how old it is, the same way one would count the rings in a cut tree stump to determine its age. Unfortunately, there are a lot of rings, some fairly pronounced (which might be annual rings) and others hard to distinguish (which might correspond to episodes of storm runoff, or weekly temperature variations, or sunspots for all I know)

At a rough guess, though, I can make out a series of fairly distinct bands of fairly uniform width, and if I assume those are the annual bands it is probably around 8 years old. Not bad for something about the size of the nail on my little finger.

This, incidentally, is why the State of Michigan has prohibited harvesting of the native freshwater mussels. They have historically been very plentiful, but that has been because there was nothing much that could eat them, so they could live for a long time (tens or even hundreds of years) and accumulate. Back when they were heavily harvested both for food and for making mother-of-pearl buttons, the older mussels were being removed faster than they could be replaced by the younger ones, and the numbers dropped precipitously in areas where they were being fished out.

Of course, it’s not like they don’t try to reproduce. Freshwater mussels produce a lot of eggs every year. It’s just that it is hard for them to get started. They have a somewhat involved lifestyle, where the newly-hatched clams latch onto the gills of fish to be carried around. They parasitize the fish until they are big enough that their shells make a decent defense against getting eaten, and then drop off. For this to work, they need to (a) get hold of a suitable fish, (b) avoid getting removed by the fish at some point, and (c) drop off of the fish when it is over a suitable substrate (a soft, sandy bottom that it can burrow into). The odds of this working right are pretty remote, so only a tiny fraction of mussel eggs get a chance to mature.

There were also snails. Snails are molluscs, too, but instead of two-part shells like bivalves, they only have one-part shells. These also grow from the edge, and most snails grow the shells as a coil so that they don’t have this long projection of their outgrown shell trailing behind them.

Having a shell is a pretty effective deterrent against predators, and so a number of species of arthropods have developed shells, too. Unfortunately, they don’t have the ability to make calcium carbonate shells, so they need to make their shells out of something else. Caddisflies, for example, like to make theirs out of bits of debris held together by silk. Like this little house of straw that we found at the same time as the clam and the snail.

This was an older shell, although still pretty strong.

The caddisfly was long gone, too.

This must have been a big one, this caddisfly house is much bigger than the ones I’ve seen in the past. We probably have a lot more caddisfly species living around here than one would normally think.

2 Responses
  1. October 8, 2012

    This was a very interesting post on housing projects by living organisms.
    I have previously encountered the caddisfly’s admirable propensity to make frugal use of detritus in the vicinity in the creative fashion that is depicted in this post—- somewhere else (I think it was on a pond blog or in one of Thoreau’s essays) but I have never seen such a large sculpture of the nature you have here. This stunning tunnel-building woven together by the caddisfly seems rather too ornate for such a minor creature but perhaps he too aspired for greater status than just a simple straw hut and this is the consequence of his thirst.

    I’d never encountered the rather hairy story you have described here of the reproductive trials and tribulations of the freshwater mussel.

    Actually , I’d not encountered many of these charming fairy tales of living organisms before reading about the vivid and charismatic characters on your blog.

    I live in Alberta (you know that place where we have toxic tailings ponds so it isn’t surprising that I’ve not heard of such tales. In our part of the world we are more used to reading about ducks dying in gloop, oil spills in recreational lakes and stories of mutated fish and rare cancers appearing in human beings living downstream from the Athabasca River).

    Who knew that freshwater mussels were thin shelled (and maybe even thin skinned)? Who knew that they would do such work for love? Or rather for their own survival and the persistence of their genes.

    I had no idea that they had to lug around on a passing cruise vessel of a fish in order to reach a suitable locale for development where presumably they burrow into the bottom of the lake and do what? How do freshwater mussels make more of themselves? You never told us.

    I go look on the Internet as I am avoiding social studies 9. Younger boy has a test and I have to understand the Canadian federal government in order to test him about chapter 1 (How effectively does Canada’s political system govern Canada for all Canadians?—uh, badly).

    The story of mussel love (as usual for all shelled creatures) turns out to be hideously complicated and perhaps this is why you refrained from exploiting the matter. I go to this site to reveal what you left out and make a mess of the explanation:

    As far as I can understand this yapping here;
    1) Eggs are laid by females
    2) They go from ovaries to some other place which they call “suprabranchial chambers”.
    3) Sperm magically appears to fertilize the eggs ( I suppose a male was passing by doing his job of fly-by-stream mating)
    4) Eggs are fertilized and attach to gills of the female—they yap about water tubes but I have no idea what these are and it is too late to go look up more information.
    5) Mucus acts as the glue for all of this binding of babies to mum. Yuck.
    6) Mum has brood chambers in her gills.
    7) Sometimes all 4 gills are employed for incubators (who knew that they had four gills? I had no idea).
    8) The baby (fertilized egg) develops into something called a glochidium which sounds vaguely threatening.
    9) This larval form is huge in a gill (“0.1 to 0.4 mm wide”). I imagine the mum has a hard time breathing. They yap about the baby’s cuteness (“with two valves closed by an adductor muscle and a long larval thread, in some (Anodonta, etc.) the valves bear hooks ventrally, but others (Unio, Quadrula, Lampsilis) lack hooks.”)
    So not only does mummy mussel have to contend with fat babies in her water tubes of her gills but they might have parasitic hooks.
    10) Eventually mum gets fed up and excretes babies (larvae) from what is called here an “exhalent siphon”(is there no way to just say this simply without making her sound like a whale all the time?)
    11) The babies hit bottom or are like spiderlings and taken by the currents (in this case the water currents).
    12) Those with hooks apparently hitchhike as you have described to new locales where the fishing is good. These hooked forms attach to the fish exteriors.
    13) The hookless larvae attach to the interior of the fish host into the gill areas.
    14) In either case, they are encapsulated (much like worms in pigs it seems).
    15) The cysts protect the larval form (I think) and they chow down on the host fish.
    16) Later the cysts weaken and the mussel escapes to become a free living mussel. Nice.
    17) I never knew fish had parasites that form cysts like the roundworms that infect pigs and humans.
    Interesting. You left all this information out. Next time don’t forget to put in more parasitology as it always interesting to see what good use other species makes of higher life forms.
    18) I’d better go back now to younger boy’s social studies book. He has a test on Friday.

  2. October 10, 2012

    But Julie, if I didn’t leave all this information out, you wouldn’t have the fun of adding it in!

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