snail of a different stripe, or my snail Y12345

June 13, 2005

in Natural history

Snail1I came across this attractive land snail while doing some gardening. I was curious, as usual.  For me, it doesn’t seem right not to know and understand the creatures sharing my property.  And while I don’t want to become too mollusk-oriented here, considering the slug post,  this one turned out to be too fascinating to not write about.

The snail is Cepaea nemoralis. It goes by many common names, Banded Wood Snail and Grovesnail seem to be the most frequently used.  I soon learned (and you’ll anticipate my next statement) that they are not native to North America. C. nemoralis and its sister species C. hortensis are from western Europe and have been widely introduced, often via plant materials but occasionally intentionally. They have been on this side of The Pond for about 150 years.

It turns out C. nemoralis is a remarkably well-studied organism because it is, I was intrigued to discover, one of the most polymorphic species of any known animal, possessing extreme intraspecific variation of mitochondrial DNA.

The polymorphism is readily apparent if you run across many of these snails. The base color of the shells may be various shades of yellow, pink, or brown, and the shells may be unbanded, or have up to five bands.  Bands may be complete, fused, or not fully pigmented (some handsome photos here).  To keep things straight, a shorthand has been developed to categorize shell color and pattern, beginning with a letter signifying shell color, and five digits to record the number of bands.  My snail would have the notation Y12345.  Had any bands been fused, they would be noted parenthetically, and split bands would be shown with subscripts. I don’t know how bands that change color along their length are recorded, nor the variations in the color of the shell’s lip, or the snails body — all of which are further examples of polymorphism in this gastropod.

What makes a snail — this snail — so variable? It turns out, a whole lot of things. In any given population of C. nemoralis, several factors may act together, and their relative importance will vary from site to site.  I found the classic paper on the subject, Polymorphism in Cepaea: A problem with too many solutions? both aptly named and a fine overview of the forces that influence polymorphism in Banded Tree Snails. Just a few examples:

Darksnail_1 Habitat: More structurally diverse habitats tend to have banded snail populations, while snails in more uniform habitats (grasslands, dunes) have fewer or no bands, thought to help in camouflage against predators.  Color of the base of the shell is also influenced by habitat, again facilitating camouflage. All the ones I’ve found in my garden are yellow with bands. I found this one in a shady wet woods.  Not only is the shell brown, so is the body.

Climate: Climate may influence shell color in two ways.  First, shell color helps the snails thermoregulate. Darker shells absorb more heat than light shells; therefore, yellow shells tend to predominate in hotter climates.  Rainfall also plays a role. Although most C. nemoralis have dark-lipped shells, populations with high percentages of white lips tend to be found in wetter areas.  In the same places, dark-lipped C. hortensis (the uncommon morph) are found in higher proportions.  Rainfall affects both these species in the same way (increasing the number of the less-common morph), but the mechanism is not known.

Conspicuousness — C. nemoralis is the most active of a suite of similiar species, making it more conspicuous to predators.  This may further influence the high degree of polymorphism in this species.

All this selective pressure is complicated by elaborate genetics, which I don’t pretend to fully understand.  I do know that since snails don’t move very far, they tend to mate with close, often related, neighbors, and that will cause the frequency of gene alleles to change over time. Looks like the sequencing of their DNA was completed fairly recently, so I’m sure more there will be even more interesting revelations on the evolutionary ecology of what Thomaz et al. called “a classic organism in ecological genetics.”

And I just thought I found a pretty snail, and wondered if maybe I could find something interesting about it.

Jones, J.S., B.H. Leith, and P. Rawlings.  1977. Polymorphism in Cepaea: A problem with too many solutions? Annual Review of Ecology and Systematics 8:109-143, with 201 references.

Terrett, J. A., S. Miles, and R. H. Thomas.  1996.  Complete DNA sequence of the mitochrondrial genome of Cepaea nemoralis (Gastropoda: Pulmonata). Jrl. Molecular Evol. 42:160-168.

Thomaz, D., A. Guiller, and B. Clarke. 1996. Extreme divergence of mitochondrial DNA within species of pulmonate land snails. Proceedings of the Royal Society Ser. B: 263: 363-368.


{ 10 comments }

Rurality June 13, 2005 at 2:20 pm

That kind of thing is how I can look up and realize that I've been on the internet for hours longer than I'd intended. 🙂

"I do know that since snails don't move very far, they tend to mate with close, often related, neighbors…"

Could have been said of most people too, not that many generations ago!

Aydin June 13, 2005 at 6:18 pm

Actually Cepaea hortensis is usually considered to be native to the U.S. For example, see Hubricht. 1985. Distributions of native land mollusks of eastern U.S. Fieldiana, No. 24.

Nuthatch June 14, 2005 at 7:53 am

Ah…a journal I don't have access to. I'll see if I can find a copy of that paper.

Aydin June 15, 2005 at 9:55 am

"Hubricht 1985" is more like a paperback book of distribution map after distribution map. If you want, I can e-mail you the map for C. hortensis.

You have C. nemoralis in your garden & in the woods? The banded snail in the woods may be something else; a native snail.

I thought I could write something up about C. nemoralis, but I am going to back out of that "deal". I just don't have time for that now. Sorry.

Patricia A Hodge September 20, 2006 at 11:34 am

We just found one on a gravel road, yellow with stripes. very interesting.. uppper New York State Pat

kt December 12, 2006 at 8:56 am

I have been looking for distribution maps for cepaea hortensis and nemoralis in the US for my university project, so if the kind owner of Hubricht 1985 could email them that would be great (ninjakai@hotmail.com)! Also both cepaea hortensis and nemoralis are european species later introduced into the US, which is supported by fossil evidence.

Anneke May 13, 2008 at 7:16 pm

I'm glad I discovered this article! I have lived in my area for 53 years (almost) and NEVER seen these sorts of snails. We had ones very like this when I spend about 2 years in New York City (City Island, to be precise) but these had blue stripes mixed in with the brown.

Last fall, standing waiting for my ride at the end of a day, I looked down and discovered an enormous one meandering through the grass outside my office building in an industrial park. A few weeks ago, I found some empty shells and brought them home to photograph. Last Thursday, I found another in the middle of the parking lot and brought it home to add it to the collection.

After photographing it, it sat on my desk until Monday evening when I decided to use it in a piece of "found object" art.

Luckily, before getting out the hot glue gun, I fiddled with the shell to decide just what to do with it. It was then that I noticed that… "Hello!", there was someone home!

That was 5 days or sitting on my desk without food or watter. I had no idea they were so resilient!

Interestingly, the lip of its shell was all shattered. In the last 24 hours, this has been repaired and the whitish band has is pristine and solid.

I understand that those with a whitish band are a different species, though it is difficult to tell as the one without goes through a phase where it also has the same band. I shall have to see if

He (she?) now has a nice terrarium and yummy food.

Flower Girl July 3, 2008 at 2:47 am

im very happy to read this article…and im very thankfull to google to help me out in its search…really interesting article…I was in search of these type of snails pics to paste them im my journal… Thanks for this once again….

Kamagra Discount March 18, 2009 at 9:43 pm

really interesting info.. thanks!

Pat in Red Hook July 3, 2009 at 8:57 am

Thanks for this info! I have an abundance of stripey snails in my Brooklyn backyard this year. They made their first, limited, appearance last summer.

Here's a picture of one on my Flickr site:
http://www.flickr.com/photos/11700355@N06/3055091000/

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