The Ruddy Anculosa

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In the early and middle 1800's most new species of the "mud snail" family Pleuroceridae were put into two genera, either Melania or Pleurocera. These and a few other genera were all in the family Strepomatidae, which was later changed to a subfamily, Pleurocerinae, in the family Pleuroceridae.

Today's species was named Anculosa rubiginosa in 1841 by Isaac Lea, a prolific student of freshwater mollusks. At present its exact classification is contended, and I have provisionally listed it as Leptoxis rubiginosa (Lea, 1841) until the matter is settled. The note by John D Parker summarizes what he knew in the 1950's. Other populations in the Coosa River have been found, and also in some nearby rivers, possibly as introductions from boating or water bird migrations.

I looked at these specimens as compared to other species from the same river, and figured that Dr. Lea was kind of reaching for a name. At the time, the genus Anculosa had dozens of named species, with more to come. These are "ruddy" only by comparison with others that are darker brown or gray-brown. I wonder, does the term "ruddy" include a color I'd call Roan?

I have recently spoken to an expert on the American freshwater snails, who tends toward the interpretation of the Pleuroceridae as containing somewhere between five and 20 true species, with numerous minor local varieties that are nonetheless parts of a few large, interbreeding geoclines.

The term "cline" refers to a graded variation over some spectrum. In paleontology, a chronocline is a gradual change over time. For example, a common living species of scallop has on average 18 ridges on the main part of the shell (ridges on the ears are not counted). Perusing a collection containing many specimens of this species, one might find examples with different numbers of ridges, ranging from 16 to 21. In a sequence of fossil beds covering a modest span of time, perhaps less than a million years, we might find that a gradually changing environment has reduced the vigor of the surf, and that the oldest scallops of the same species have fewer and heavier ridges. If you collect enough specimens from such a section, you might be able to chart the change in average number of ridges, and perhaps the changing minimum and maximum number. Glory-seeking students working in an area with spottier coverage of the same sedimentary sequence might decide that the earliest specimens, perhaps with 12 ridges, and later shells, with 15 on average, are different species and should get new names in the genus containing the modern species. But the reality is a gradual change over time with no distinct change. The whole sequence is a chronocline.

We need to be aware that the term "geocline" is being used by geologists in place of the older term "geosyncline", referring to a basin filled with a sequence of sediments. I am using the term in its phylogenetic sense, of a variation in the appearance of a species across some region. The most famous example of a geocline is the Herring Gull, a familiar species of North America and northern Europe. Careful examination of specimens from different places shows that they vary across that region in size and background color, just a little. However, further east across Siberia, the variations continue, particularly a continued darkening of the gray color; and further West across northern Alaska, the color gets paler. Across the Bering Strait, you see what appear to be two species of whitish-grayish gulls, and I have been told they don't interbreed when an occasional stray crosses the strait. Yet these two "species" are actually local varieties, the two ends of a continuous ring, a geocline of gulls.

With that in mind, we might ask, how could multiple populations of small mud snails, living in the rivers of the eastern United States from Georgia to Massachusetts, be in genetic contact with sufficient frequency to maintain their status as a single breeding population? Water birds. In particular, long-legged wading birds such as egrets and herons. Herons tend to stand still for long times, wholly immobile, watching for prey. As slow as they are, snails would have plenty of time to crawl onto a heron's foot or leg. During the season of breeding and just after, snail spat land everywhere, and some will land on the feet of wading birds. Later on the bird flies off with its cargo, and some snails will exit once it is again standing in water, perhaps in a stream dozens of miles away. In human terms, gene flow by this means may seem very slow, but in terms of genetic speciation, it is more than fast enough to keep the gene pool mixed.

Species classification is continually under tension between "lumpers" and "splitters". For the first century or so of mud snail studies in the U.S., the splitters prevailed, leading to a situation in the late 1990's that about 1,000 species had been named in the family Pleuroceridae. It is much more likely that there are fewer than 100 true species in this family, and if my friend is right, perhaps fewer than ten. I foresee a possible flurry of defining various "forms", such as happened some years ago with the species Liguus fasciatus (Müller, 1774) of Florida and Cuba, a walnut-size tree snail that has about 50 color forms in Florida and 70 or so in Cuba. Their differing colors are often quite striking. But they are all now considered a single species.