Tennessee River Asian Clams

Some Corbicula clams from the Tennessee River look like they’re a new species… are they?
Answer: I don’t know. But let’s look at them.

[I haven't made a journal before. I'm not sure if this is the best way to do this, but I wanted to put my notes together in one place.]

For the past 7 months or so, I've been making my way through all of the Corbicula observations in iNat since early this year (I'm through 84% of the 15,000+). I’ve also looked at some of the old journal articles that named the species (particularly the figures if there were any), some of the more recent articles that have tried to make sense of those names, and whatever photos of museum lots I’ve been able to find.

In places where they are not native, most Corbicula have the form of the typical Corbicula fluminea sensu lato. (Perhaps I’ll write on sensu lato in another journal post.)

In some areas, additional species have been introduced: C. largillierti in South America and C. fluminalis in Europe. These species are quite similar in form, the differences are recognizable once you see enough of them.

Among the Corbicula observations from the Tennessee River drainage, I’ve noticed a form that’s unlike anything I’ve seen from anywhere else in the world (limited to photographs in iNat and the other sources I listed above). This form the coarse and widely-spaced ribs (about 1 per mm) like C. fluminea s.l. However much of the rest of the shell appears like C. fluminalis:

  • The hinge is very thick
  • The umbones are broader, higher, and often anteriorly-rotated.
  • The shell doesn’t grow as much in length as it matures; when combined with the high umbos, this means the shells are often taller than they are long.
  • Unlike in mature C. fluminea, the posterior lateral teeth aren’t much longer or straighter than the anterior teeth; this means the shell also lacks the posterior projection seen in many large C. fluminalis and results in a more symmetrical shell anterior-to-posterior (except for the umbonal rotation).
  • (I haven’t seen photos yet of fresh-enough shells to know if the coloring is any different.)


I’m just a mere hobbyist, not any type of a scientist, so I won’t be able to figure out what this unusual form signifies, nor would I even know how to go about that. I don’t live in the area (I’m in Minnesota), so I can’t go searching for live individuals for DNA sampling. I’ve been identifying these all as C. fluminea s.l., as I don’t believe there’s a better name for them, since I don’t and can’t know what their true status is.

But I can speculate…

  • This could be an “ecomorph” of C. fluminea s.l. based on local conditions, though that doesn’t explain why it doesn’t show up in other areas (conditions in the Tennessee River can’t be that unique among all the places where C. fluminea s.l. has been introduced). Nor does it explain why typical individuals still occur in the area (apparently as the majority of the individuals) or why there doesn’t appear to be any gradient between the forms.
  • This could be a population of the same species from a novel source. I believe that the invasive *Corbicula*s all reproduce asexually, so that could explain the lack of gradients. This does push against my understanding of where the species lines are drawn in asexual organisms.
  • A variant of the above could be that this was a form that was selected for during domestication. The species were introduced because they were farmed as a food source. I don’t know enough about aquaculture to figure out what’s even possible here.
  • This could be a hybrid. Just because the invasives reproduce asexually, I don’t know if they are that exclusively or if sexual reproduction is possible, just rare. This could overlap with the domestic breed thought above… perhaps hybridized before introduction due to domestication?
  • This could be a completely separate species. Origin (and appropriate name) as yet unknown. Perhaps with a small native range (which is why I haven’t seen observations from there in iNat), and not successfully introduced anywhere else. I favor this explanation because it’s the cleanest, but that doesn’t mean it’s correct. I guess it doesn’t explain why researchers haven’t noticed it yet.

Comparison of C. fluminea and C. fluminalis from Europe:

Here's my list of others' observations of this form that I've noticed so far...
Tennessee:

Alabama:

Kentucky:

Finally, here's a very different-looking Corbicula from the same area (Harden County, TN). It shows bright white nacre in a freshly-dead shell (only a little hint of something darker on the lateral teeth).

Thank you for posting these observations, @ashley_bradford, @crisler, @engelhard, @jeffgarner, @elliotgreiner, @markmcknight, @marco_vicariotto, @reallifeecology, @rogerbirkhead, @tn_nature_nerd, @wildlander, @zealouswizard, @liviahogue, and @sbrockway !

Posted on August 04, 2023 10:57 PM by amr_mn amr_mn

Comments

One amazing thing about iNat is being able to look at so many specimens across such abroad geographic area. After you have looked a few thousand some differences really start to jump out at you. I would encourage you to talk to a malacologist and see what they think about your findings. Hopefully you can get someone interested to do some genetic work to complement your morphological observations and maybe get a coauthored publication out of it. @jeffgarner might know some folks who would be interested. In the meantime I’ll keep posting ones I find. If it does get to the point where you need some samples from down my way just let me know.

Roger

Posted by rogerbirkhead 10 months ago

Thanks for the tag, and this is why I love iNat! I observed this to get my species count up in a place that didn’t have many observations, hoping it would help, not because I thought it was particularly useful. I was on a botany trip and know nothing about clams. Happy to look for more now that I know it’s of potential use.

One of my observations was on an isolated river near the top of a mountain, far from the original source of infestation. I wonder if the clams have adapted to the location or if they crossbred with whatever species were here before. This one is just a few meters from my office: https://www.inaturalist.org/observations/43767155

I’ll make sure to keep an eye out and observe more now that I know someone’s interested, but it’s also a good reminder to us all to keep observing even when you think you have something obvious or boring in front of you. Who knows what this occurrence data could unlock in the future!

Posted by markmcknight 10 months ago

@markmcknight The one near your office is the curious one. Your other three observations of the species look typical.

Corbicula is not native to North America, so there was no original species to interbreed with.

I definitely appreciate the observations from the people that aren't really into freshwater bivalves, if only enthusiasts posted, I'd have so many fewer to look at and learn from, so thanks for adding yours!

Posted by amr_mn 10 months ago

Added three more (#11-#13) from Hamilton County, TN. Thanks @ashley_bradford!

Posted by amr_mn 10 months ago

I'm glad I can help!

Posted by ashley_bradford 10 months ago

I’ve found more observations (although most of them don’t show all of the features)

Kentucky:

Lyon County #1
I’m not sure. It’s very worn. Quite tall though

Marshall County #1, #2, #3
I don’t think the first link is one at all. It just seems unusual.

Calloway County #1, #2,

Tennessee:

Henry County #1

Stewart County #1

Alabama:

Limestone County #1, #2

More from @jeffgarner:
#1, #2

Perhaps a project would be helpful as we start to discover even more. It would make it easier to keep track.

Posted by lj_lamera 7 months ago

@lj_lamera, Thank you for those. When I first searched for these, I was scanning through the thumbnails and then only selected observations with photos of both the interior and exterior. I hadn't thought of looking back for ones I had missed.

I don't think the Marshall County #1 is this form, it's much longer, with the posterior end much more drawn out, and the posterior lateral tooth much straighter than the anterior lateral.
https://www.inaturalist.org/observations/139058673

I'd thought about a project, but I don't know how to do those, or what else I'd want to look for.
For example, would I want something I could add other forms to? (There's a different form around the Peace River in FL.)

Posted by amr_mn 7 months ago

I created a simple Observation Field that’s called “Corbicula Form.” It will be able to track the different forms. I’ve been calling these two the “Peace River” and “Tennessee River” forms. It looks like the white-nacred one you mentioned at the end is the Peace River form.

Edit: Here’s the link: https://www.inaturalist.org/observation_fields/17103

Posted by lj_lamera 7 months ago

New observations added including a few fresh shells and some from another nearby river (Sequatchie). Let me know what you think of these as well: https://www.inaturalist.org/observations?verifiable=any&taxon_id=64006&preferred_place_id=1&locale=en&user_id=markmcknight

Posted by markmcknight 7 months ago

I'll be on the watch out for these next time I'm in a creek!

Posted by bonesigh about 2 months ago

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