Thursday, December 22, 2016

The Great Pond Snail

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

This tray contains all but four of the lots of Lymnaea stagnalis (Linnaeus, 1758) that are currently in the collection of the Delaware Museum of Natural History. It is called the Great Pond Snail because it is the largest freshwater snail found in Great Britain ("Great" in this case meaning "large"). L. stagnalis is the type species of the genus Lymnaea, which is the type genus of the family Lymnaeidae. You'll notice one box contains only a pink label. That indicates the lot is a Topotype; it was collected in the location where Linnaeus's first-described specimen (the Holotype) was collected. The museum keeps types in a special cabinet. I'll get into more about types on another occasion.

The Latin prefix lymn- is equivalent to limn-, which refers to a lake. As a matter of fact, I am a bit puzzled that the family is not named Limnaeidae and the genus Limnaeus (but see below). The species was first named by Linnaeus as Helix stagnalis. He placed nearly all the spiral-shaped gastropods he knew of in the genus Helix, for obvious reasons. Later workers broke up the genus into quite a number of new genera so as to group the species more appropriately. For a time, subgenera had been created to group the species, and in fact, Lamarck renamed this species Lymnaea (Lymnaea) stagnalis in 1799, while placing several other former Helix species into that subgenus; the subgenus name is in parentheses. Later genus-splitters removed the subgenus designation. In 1875, Sandberg tried to rename the genus Limneus, considering it a justified spelling correction. Nearly everyone else disagreed because of priority rules, and so Lymnaea it remains.

Members of the family Lymnaeidae are lake snails almost exclusively, and some are also found in slow-moving streams, such as in deltas. This species in particular favors very quiet, shallow waters, even stagnant waters if the oxygen content is not too low. Thus the species name stagnalis.

I chose to photograph two of the lots. This first, catalog #60565, was collected in a shallow part of the Niagara River, on the New York side. These are the largest specimens of this species at DMNH. I neglected to put the scale in, but the museum label is 1" x 3", so the largest specimen's height is about 2.3" or 57 mm. The largest recorded specimen just exceeded 60 mm.

These shells, being nearly translucent and very light in weight, practically broadcast their environment: very quiet water and an absence of shell-crunching predators. However, they are hosts to several parasites, and intermediate hosts to at least six different flukes, including one that can severely affect humans. Thus, these are well studied because of their medical and economic impact.

They are "ubiquitously holarctic", meaning they are found throughout the Northern Hemisphere, particularly at higher latitudes, though south of the permafrost line. They apparently hunker into the mud to over-winter in England, Scotland, across northern Europe including Russia, and across the northern U.S. and Canada.

The second lot shown, #119635, contains some of the smallest adult specimens in this collection. The longest is just over 0.8", or 21 mm. Although these were collected in 1926, this indicates that the river where they were found, in or near Detroit, Michigan, must have already been heavily polluted.

Note that both of these lots have subspecies designations on the collectors' labels; one was jugularis and the other, expensa. These are now deprecated; so far as I have determined, no subspecies of L. stagnalis are recognized at present.

The labels of the second lot also show its history, or most of it. Originally collected by L. F. Merrill, it came into the hands of Grace M. Seymour, and then G. M. MacCoy, who donated it to DMNH. Sometimes, collections are accompanied by letters and other documents that tell more of the story. I haven't dug into the library records for the MacCoy collection to see if there is more to the tale.

Shell collectors who are dedicated enough to identify and index and label their specimens are dubbed Shellers. By contrast, the designation Conchologist is reserved for those who also devote time to studying shells, and perhaps living mollusks, and professional Conchologists are called Malacologists. Mollusk enthusiasts and dealers Guido and Philippe Poppe have gathered information about more than 41,000 Shellers in their site Conchology Inc., from which they sell and trade shells, and curate a kind of online museum. It is one of a handful of online resources that I have found very useful to determine the full name of someone. Interestingly, however, none of these three Shellers is included. Neither is Esley Doremus, the donor of the lot from New York.

At least we have the initials for two of them and full names for the other two. In many cases I find a label that says nothing more than a species name, a location, and a surname followed by "leg." or "legit.", the usual Latin designations for a collector. The abbreviation "col." is also sometimes used, but "coll." usually means "from the collection of"; Shellers trade and buy shells so much that for many large collections, only a small percentage of the holdings were actually self-collected by the owner.

The 78 lots of this species held at DMNH allow researchers to study areal extent, presence through time at locations or in areas that were visited more than once, and changes in animal health. I occasionally find a note among the labels that specimens were collected from the bed of a dried-up pond, indicating at least local extinction. Rather poignant, that.

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