24 October 2006

Helicodiscus parallelus revisited


This is the land snail species Thomas Say described twice under different names, the first time as Helix lineata and the second time as a freshwater snail with the name Planorbis parallelus. I already discussed this in a previous post.

Hparallelus2Here is a live Helicodiscus parallelus I found in Belt Woods recently. The diameter of this snail's shell was 3 mm. As Pilsbry, citing E.S. Morse, noted in Land Mollusca of North America (1948), H. parallelus is "blind" in that it doesn't have visible eye spots at the tips of its upper tentacles.

Say didn't give figures of his specimens. I don't know if anyone published figures of this species before Amos Binney did, as Helix lineata, in 1857 in volume 3 of his Terrestrial Air-Breathing Mollusks of the United States etc. However, Binney's drawing showing the shell from the side doesn't look like a typical H. parallelus. The spire is too high and the aperture is too descending. Compare Binney's drawing, reproduced below, with my photographs of shells here and in my previous post.



aaigen said...

These snails can be found within rotten wood. I suppose the flat shape (rare in land shells) and tiny size allow it to fit into thin cracks.


Strobilops spp. also live in rotten wood, but they don't have flat shells.


Comment from Tim Pearce sent by e-mail:

You could address the elusive difference between Helicodiscus parallelus (Say, 1817) and H. notius Hubricht, 1962. Right, Say's types, if they ever existed, seem to have disappeared. Binney's figures look like an atypical specimen. Pilsbry (1948: 627) suggested using Council Bluff, Iowa as type locality, but I am not sure (would have to scrutinize the code) what weight such a statement carries. Pilsbry's (1948: 626) illustration of H. parallelus is indistinguishable (in my opinion) from Hubricht's (1962) illustration of H. notius. The late Wayne Grimm told me that Pilsbry had mistakenly illustrated H. notius and called it H. parallelus.

Is Hubricht's H. notius a junior synonym of H. parallelus? Did Pilsbry illustrate the wrong species as H. parallelus? Or is the situation more complex than this?

I have seen specimens answering to H. parallelus in pristine woods and in residential yards. Does one species tolerate that much habitat heterogeneity, or would this be anecdotal evidence supporting the existence of more than one species?

To address this question I would measure a bunch of characters on a bunch of shells from different places, apply morphometric analysis to see if I can find bimodality (or some hint that there is more than one morphological entity). If there are more than one morphological form, then examine the DNA of the different forms to look for more evidence whether they look more like one species or like more than one. Another approach would be to examine DNA of specimens from different geographic areas (and habitats) with the same question about whether they look like one species or multiple species.


Comments from Harry Lee submitted in an e-mail:

Pilsbry (1948:625) gave an abbreviated synonymy of this species. A. Binney (1851: 260) reported a dozen citations back to the original description(s) by Say. Among these are four
illustrations. Two are in rare European works (F鲵ssac and "Chemnitz second edition"), and two in scarce American works, the earlier of which was Binney (1840: pl. 22, fig. 6). This figure (actually three as you demonstrate) is identical to the one you cite (1857: pl. 48, fig. 1). The remaining illustration appeared in the first edition of Gould's Invertebrates of Massachusetts (1840), and, if it is the same as that in the second edition (1870, fig. 667, p. 405), it is an excellent one, closely matching the shells your photographs, and bearing little resemblance to the Binney illustrations.