10 February 2007

Anguispira alternata angulata

Anguispira alternata angulata from MNRA. Both shells were from an area about 40 m X 10 m.

One of the abundant species in Monocacy Natural Resources Area where I did a land snail survey was what Pilsbry1 called Anguispira alternata angulata, a keeled or carinated morph of Anguispira alternata. According to Pilsbry's synonymy, this morph was first described by Férussac in 1822 as Helix alternata var. carinata. In 1896, Pilsbry & Rhoads changed the name to Pyramidula alternata carinata and then in 1948, Pilsbry came up with the new name Anguispira alternata form angulata.

Figure from Pilsbry1.

According to Pilsbry1, the shell of angulata differs from the typical A. alternata "by the distinctly to strongly angular periphery". Although he makes the point that the periphery is "hardly to be called keeled", to me "carinated", "angular" and "keeled" all mean more or less the same thing, especially when the degree of carination is variable.

Pilsbry also notes that the prominent ribs on the upper surface of the shell "are much reduced (or sometimes subobsolete)" below the periphery, that is on the bottom of the shell. Leaving aside the question of what he might have possibly meant by "subobsolete", I will point out that the prominence of the ribs on the bottom of my shells are indeed quite variable even in specimens collected from the same location. For example, the photo below shows the bottoms of the 2 shells from MNRA shown in the first picture. The one on the left in the first picture is the one on the bottom in the picture below. Its bottom is smoother than that of the other shell. And these shells were from an area about 40 m X 10 m. So speculating that there may have been differences in their habitats is not justified in this case. I don't know if shell size is a factor that contributes to shell sculpture (the shell with the smoother bottom is smaller).

Bottoms of the shells in the first photo. The one on the left in the first picture is the one on the bottom here.

Pilsbry gave records of angulata from several eastern states including, Kentucky, Pennsylvania, Tennessee, Virginia and Maryland. Earlier, F. C. Baker2 had noted that in several counties of Illinois "a form of Anguispira alternata occurs which has a carinated periphery and a low spire...It is not so abundant as the typical form. In the southern part of Illinois there is a tendency for the shell of alternata to become carinated on the periphery, even when the spire is high". The drawing on the left, from Baker, shows A. a. alternata (A) and A. a. angulata (B).

It appears that A. a. angulata is a widespread morph whose exact relationship with A. a. alternata is not yet clear. Angulata is also the dominant, if not the only morph of Anguispira alternata, in wooded lots along the Maryland side of the Potomac River at least up to Harpers Ferry in West Virginia from where Pilsbry gives a record. (There is also Anguispira fergusoni, which is indeed a separate and easy to distinguish species.) I have several alcohol specimens of angulata collected during my survey of MNRA. Their internal anatomy—if and when I get a chance to dissect them—will hopefully offer some clues as to whether A. a. angulata is distinct enough to be considered a separate taxon.

1Pilsbry, H.A. 1948. Land Mollusca of North America . Volume 2, part 2, p. 573.
2Baker, F.C. 1939. Fieldbook of Illinois Land Snails. p. 85.


pascal said...

Interesting. I've collected many specimens of A. alternata and they all seem to fit into the angulata sub-spp. I also note that the largest shell has more defined ribs on the bottom of the shell as compared to the smaller ones. However, this is from a small sample size (ca. 7 shells) and I suspect we could be dealing with genetic drift in the range of Anguispira alternata. Although the sympatry of these morphs might prove interesting (i.e. are these morphs geographical, environmental, or random?)

In my mind, "carinate" means that there is a distinct thickening of the shell carbonate along the perifery, not just a sharp change in curvature (angular). "Keeled" seems to cover both carinate and angular (see Burch, 1962 p. 14 and p. 208).

Tim Pearce said...

Besides the carina, I also see shells in SW Pennsylvania much flatter than typical A. alternata, and they seem to be much less abundant than A. alternata I have seen in Michigan. Furthermore, I have an impression (needs further research!) that A. alternata typical form is the dominant form among older museum collections, while the modern specimens I find are more carinate and even slightly keeled (to me carina means corner - so a sharper part of the curve - while keeled means an actual bump outward like the keel of a ship). Still furthermore, Gary Rosenberg at ANSP mentioned to me that his survey several years ago of Fairmont Park in Philadelphia found zero Anguispira, but specimens are common from that Park in museum collections from years ago. These ideas lead me to form a hypothesis that typical A. alternata might be going the way of the frogs (becomming less common due to some factor), while the carinate form of A. alternata might be surviving in its typically low numbers, but now that might be all we are finding.
Well, there's a hypothesis to test. I guess I would first try to verify some of the impressions. For example, it should be relatively easy to examine museum collections to see the proportions of typical versus angular forms collected over the decades from particular places.
By the way, Stephanie Clutts, student of Andy Anderson at Southern Illinois University, is examining the DNA of all the species of Anguispira and I am hopeful that she will be able to address the relationship between the typical and carinate forms of A. alternata.