Monday, November 28, 2016

An apple-size apple snail

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

I am presently checking and correcting geographic information for the freshwater snails in the research collection of the Delaware Museum of Natural History. Though I am not constantly using the lots in the cabinets, it sometimes happens that something I find in the database doesn't make sense, so I go to the lot in question to see what the collector, and perhaps later owners, have recorded. Today I happened to look up some "Apple Snails" of the genus Pila, and they are such nice-looking shells, and so much larger than many of those I've seen of late, that I just had to share a few of them here.

Firstly, here are two views of the largest specimen of Pila in the collection, and it ought to display at about 80% of actual size if your screen resolution is 100 dpi. The shell length, with the spire eroded, is about 5 inches, or 125 mm. The smaller item next to the scale is the operculum. Many snails have these; they are armored "doors" they can use to shut themselves into a shell when a predator threatens, or to avoid drying out during a dry season.

For the second view I turned over both shell and operculum. On this side of the operculum it is a little easier to see that it grew in a spiral pattern as the shell grew.

The original designation of this species was Ampullaria leopoldvillensis Putzeys, 1898. Sylvère Putzeys was a Belgian who collected in the Belgian Congo, as it was then called, and named this species for its type locality, which is now called Kinshasa, the capital of the Democratic Republic of the Congo (which was also called Zaire from 1971-1997).

I tracked down this specimen to be sure of the spelling of Coquilhatville. Then I was able to determine that the town is now named Mbandaka. It is about 550 km upriver from Kinshasa, and while I haven't looked it up, I suspect that snails of this species are found throughout the Congo river system. I understand that just one of these makes a good meal for a child, and two or three are sufficient for a teen or adult.

While I was in that cabinet, from the same tray I picked out two other lots to photograph, to show specimens of a more "average" size for this genus. This next species seldom exceeds 2 inches, or 50 mm, but is of a very similar shape and has a lighter color:

Pila globosa Swainson, 1822 is primarily Asian, and these were collected from the Mekong River system. The abbreviation of Swainson's surname is quite common. The better-known, more prolific naturalists were all abbreviated in field notes and earlier museum labels. L. means Linnaeus (or Linné), Lam. means Lamarck, Pfr. means Pfeiffer, and so forth.

As the name implies, this species is very globular, almost spherical. It is a contender for being the original "Apple Snail", but there are several others. All members of the family Ampullariidae are called Apple Snails, though not all are quite so globular. For example, this final species has a more pronounced spire:

The scratching-out and overwriting on the museum label, printed in about 1990, shows that the species now called Pila polita (Deshayes, 1830) was originally placed (by some authorities) in the genus Ritena, of the family Neritidae. The Nerites are mostly marine snails, but many species in that family inhabit brackish estuaries and quite a number are fully fresh water-dwellers. Sometime since 1990 this species was re-examined and re-named to be among the Apple Snails, to which it bears an even better resemblance when the spire is eroded, as in the third specimen in the top row. It is a pity we don't know just where in Thailand these were found. The Apple Snail website records these from Thailand all across south Asia to the southern part of China.

It is interesting that the original collector identified these shells as Pila, while the curator or collections manager who accessioned them into the collection used a different reference work to identify them as Ritena. Later someone else changed it back. So it goes. When all we have to go on are physical characteristics of a dry shell, convergent evolution can make two species look very similar when they are actually only distantly related, and in other cases a species will look quite different from its closest cousins because of rapid adaptation to changing environments.

Now that DNA sequences are getting almost cheap, museums are beginning to go through their collections to figure out just what species certain specimens belong to. There is usually enough DNA within the shell material to use with modern techniques, though at present one must destroy one or more shells in the process. This will change. Even more exciting times are on the way!

No comments: