21 November 2007

Vallonia costata: a new backyard snail

This snail didn't have the reflected adult lip yet. (Shell diameter=1.5 mm)

I found this Vallonia costata crawling on a rock in my backyard last weekend. The air temperature was 8 °C. Once on an even colder day when it was 3.5 °C, I observed a V. excentrica crawling on a rock. I suspect these species never fully become dormant in the winter unless it gets really cold.


I don't think I had recorded V. costata in my backyard before. It probably came with the soil or rocks that we frequently bring to our yard from elsewhere.

This species has a widespread distribution. According to Jochen Gerber*, it has been recorded from most of Europe, Russia, Armenia, Turkey, Afghanistan, Uzbekistan and the U.S. There are also records from Algeria and South Africa. It may have been introduced to some of those countries, but I believe it is thought to be native to the U.S.

Note added 10 January 2008: Earlier this week Jochen Gerber of the Field Museum in Chicago contacted me by e-mail to let me know that he didn't think that this snail was Vallonia costata. He explained that both the color of the shell ("too brown") and the snail's body ("quite heavily pigmented") did not agree with the usual colors of Vallonia shells and soft parts. I hadn't paid attention, especially, to the body pigmentation. After I did a quick literature check, I realized that Jochen was, of course, right. Pilsbry (Land Mollusca of North America, 1948) says "soft parts are transparent white" and the photo of V. costata in Fechter & Falkner (Weichtiere, 1990) shows a snail with an entirely whitish body. Jochen also suggested that this snail could instead be a Paralaoma sp. A more accurate identification will have to wait for me to find the snail again, because I didn't keep it the first time, but returned it to my yard.

*Gerber, J. 1996. Revision der Gattung Vallonia Risso 1826. Schriften zur Malakozoologie 8:1-227.


Kevin Bonham said...

Looks as much like my little globetrotting friend Paralaoma servilis as some of those living in my backyard or many of the 500+ I've stared down the microscope at in the last few weeks. The species is somewhat variable, especially the prominence of the irregular "primary" ribbing.

A cautionary note: P. servilis is most likely not the only Paralaoma capable of becoming introduced outside its range; there is another much rarer Paralaoma type thing here that certainly seems exotic given that the records are all from urban/disturbed areas but I have no idea yet (a) what it is and (b) where it came from.


Since this post was written, I have taken another similar snail from my yard. They are most likely not V. costata. But, I have not had a chance to dissect it. Also, I don't have the necessary references to Paralaoma anatomy.

I still hope to update & correct this post one day.