Wednesday, March 09, 2016

The Virginia River Snail

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

In recent months I have been performing inventory of the freshwater gastropods (that is, snails) in the collection of the Delaware Museum of Natural History. A species that caught my fancy the other day is the Virginia River Snail, formally known as Elimia virginica (Gmelin, 1791). Another "common name" used for this critter is the Piedmont Elimia, but only by those who know what "Elimia" means.

Many species now in the genus Elimia were formerly classified in Goniobasis. "Elimo" in Latin means "to perfect", that is, to make complete. "Gonio" is from French for "angle, and "basis" has much the same meaning in both languages. There is much controversy, that has been going on for a generation or so, about which genus each of several hundred species actually belongs to. Some malacologists (those who study mollusks) believe all of the species of interest here belong in one of these genera only, some the other, but the majority are still working to define just what distinguishes one from the other besides affinity to this or that early author, or early ideas about geographical distribution.

The full scientific name of any species includes the Genus, the Species epithet, and a reference to the first scientific article in which the species was described. As it happens, many workers consider that the description by Thomas Say in 1817 more properly describes this animal, rather than the earlier one by J.F. Gmelin in 1791. Controversy upon controversy! Also, the parentheses are a code that the species virginica was originally described under a different genus. Thus many museums have this species listed under the label Goniobasis virginica (Say, 1817), and some still list it as Melania virginica Gmelin, 1791.

Species in the genera Elimia and Goniobasis, whether they are one genus or two, or some less fortunate combination, comprise the largest portion of the family Pleuroceridae. "Pleuro" comes from a Greek word for "side" or "flank", and "cera" means "horn", as a ram's horn. As presently described, all species in the family Pleuroceridae are native to eastern North America, from Florida to southeastern Canada.

Here is a typical box of E. virginica shells, 210 of them. The appearance of this unwashed lot shows why we informally call these "mud snails". They seldom exceed 3 cm in length, or 1 cm in the largest diameter near the aperture.

This is a typical high-spired shell. Pleurocerids are generally high-spired and rather heavily built. They don't have the delicacy of lake and pond snails.

This species inhabits major rivers from Connecticut to Virginia, where it was first collected. It is thought that the original distribution was exclusively in rivers that empty into the Atlantic, and that occurrences elsewhere are from various introductions. The DMNH collection includes lots from Connecticut, New Jersey, New York, Pennsylvania and Virginia, which are included in that region, plus Maryland, South Carolina, Tennessee, and West Virginia. Maybe the latter are introductions, or perhaps they were just found later than the others and were there all along. A known introduction is along the Erie Canal to streams flowing into Great Lakes.

Specimens from three lots are shown in this montage. The upper two photos show items added to the collection in early days, in the 1970's. The third came along later, though the collector named on the label was active about a century ago.

If you click on the image to see it full size, some differences among populations can be seen. The shells from West Virginia have a darker stripe along the whorls, and those from Tennessee have stronger growth-line decoration compared with the others.

The angled tip of the aperture opening shows a possible reason these were originally put in the genus Goniobasis. Whether they are sufficiently "perfect" to warrant the name Elimia instead, I'll leave up to the experts.

I am sometimes asked, "Why does a collector sometimes gather hundreds of the same shell? Wouldn't some smaller number be sufficient?" I passed this question along to the curator one day, and she said, "The more the better. Range of variation is frequently studied." In the box of 210 shown above, you can see variation in color and size, and even a little variation in shape (ratio of length to width, for example).

From a single specimen, or a very small number, you primarily learn that a certain species existed in a certain location at the time of collection, assuming this is not fossil material. From a larger number, and large numbers collected at various places and times, researchers can glean certain statistics. For example, if all you had was the six shells shown in this montage, it would be hard to determine that they actually represent the same species. With a few dozen or more from each location, it is possible to learn more about how a population varies, to see if there is overlap.

The largest lot, in numbers, that I have seen, contains 16,300 shells the size of grains of sand. Perhaps this is a little extreme. But lot sizes up to a few hundred are useful, if only because the variance of a statistic such as shell length can be more precisely defined from a larger number of measurements.

In the future I will have more to say about the information that we put on lot labels, particularly because this has been revised in recent years.

For further information about this species, begin with this Wikipedia article.

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