The Bigmouth Rocksnail

kw: species summaries, natural history, natural science, museums, research, photographs

As I look through all the various species of freshwater snail, particularly in the family Pleuroceridae, I get the impression that most of them can be found in the Coosa River of Alabama. It is a biggish river, some 280 miles long, beginning near Rome, Georgia and flowing into the Alabama river north of Montgomery. It is warm enough, and sufficiently low in pollution, to host hundreds of freshwater mollusks, including dozens upon dozens of river snail species. However, saying it has low pollution may no longer be accurate, as we will see below.

In early days of collecting, a genus was a pretty broad category, and this only began to change in the 1940's as naturalists labored over the rapidly growing "tree of life" they'd been constructing since 1758 when Linnaeus proposed Binomial Nomenclature. Large collections supported more and more precise research efforts, and the ever-shifting see-saw between "lumpers" and "splitters" swayed definitely toward the splitters. After all, naming a new species—or naming a whole raft of them—was the most common way for a naturalist to make a lasting impression. At present, the extrema of the distribution of likely species in the family Pleuroceridae (see the prior post for more on this family, and this Wikipedia article) ranges from 21 species to more than 300. But when Daniel L. Graf sought to organize all the known species in this family, he called the labor "The Cleansing of the Augean Stables" (this link downloads a large PDF of his monograph, published in 2001), because there were about 1,000 "nominal species", just in this family. Clearly, some lumping has occurred since his work, more or less depending on which authority you trust most.

Among the species in the "middle of the alphabet" of those assigned to either Elimia or Goniobasis, I came across one that I was pretty sure didn't belong. So I spent a pleasant couple of hours trawling the online collections of several museums. The original label, which is in the handwriting of the collector, H. H. Smith, contains a clue, in someone else's script. The scrawl at the lower left on the brownish label reads, "Anc. occultata Smith". The same hand numbered the label "53089" at the upper left. I don't know who this person is, but someone else must have owned it for a short while, because the more recent label by C. L. Richardson has a different genus and a misspelled species. He numbered this specimen "17435" for his own collection. Smith's number had been "7083", and now the DMNH catalog number is "80433" (Yes, all of this is important).

The species Anculosa occultata Smith, 1922 has been renamed Leptoxis occultata (Smith, 1922). I checked the online data from several museums before finding one that had photos of their specimens of various species of Leptoxis, including occultata. Sure enough, these shells matched that photo the best, and agreed with the description by Smith; I looked that up also, on BHL, the Biodiversity Heritage Library. The next image is Figure 22 from that reference, but in the text, the description refers to Figure 21, which is a typo, for that figure is clearly much different, while Figure 22 matches quite well.

We see here Figure 22 and a closeup of the shells in our sample. All lines of evidence taken together convinced me that this lot was indeed properly identified by the person who wrote on Smith's label, and that someone (probably not Richardson, but more likely another collector who possessed the lot just before him) incorrectly re-identified these shells as a Goniobasis species, and misspelled the species name. Richardson would have copied that identification, as did the volunteer at DMNH who wrote "our" label in 1972. At least the volunteer was able to see that what Richardson had taken for the word "Baz" was instead "Bar", making it easier for us to determine where the shells were collected.

"Leptoxis" most likely means "small and sharp", and the genus as a whole consists of smallish shells, mostly with sharp apexes, though this species is more blunt. "Occult" means "covered", alluding to the way the aperture in some specimens is large enough to nearly hide the rest of the shell.

It is likely that going to "The Bar" on the Coosa River to have a look for these would prove fruitless. This species is listed as Extinct in the IUCN Red List. Loss of habitat, meaning in this case, loss of water of sufficient clarity, is the primary reason.

DMNH has just this one lot, collected by Smith, apparently in the 1920's. We are unlikely to obtain another, except perhaps in a trade or purchase, because the investigators who performed a survey in 1996 could find none.

I guess to most people, the extinction of a little river snail, no bigger than a marble, is no big deal. It isn't a warm, fuzzy (or feathery) critter with soulful eyes. But to me it is rather sad.