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:

• fluminalis vs. fluminea s.l., from the same location on the Rhine, from the same observer.
• Italian observation with both species. fluminalis is the lower shell, the upper two are fluminea s.l. (the next photo)

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

• Hamilton County #1, #2 (second photo above), #3, #4, #5, #6 (first photo above), #7 (young), #8 (eroded), #9 (possible, but longer posteriorly), #10
• More Hamilton County: #11 (mixed with typical), #12, #13 (two valves of this form, one typical)
• Meigs County, external only
• Knox County, multiple shells

Alabama:

• Madison County (third photo above)
• Morgan County
• Lauderdale County #1, #2 (Juvenile, only potentially this form).
• Colbert County #1. #2

Kentucky:

• Lyon County, Lake Barkely
  Not in the Tennessee River, but an impoundment of the Cumberland River!

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 !

Publicado el agosto 4, 2023 10:57 TARDE por amr_mn