26 January 2011

Frozen Vertigo

A few nights every winter, the outside temperature where I live goes below -10° C, but not too far below; the lowest temperature I've measured in my yard within the last 3 years was -14° C. When the National Weather Service predicted that the temperature last Sunday night would be -14° C, I decided it was time again to repeat an annual experiment.

The tiny land snail Vertigo pygmaea is an inhabitant of my backyard and a frequent subject of the posts on this blog. When I collected live snails one spring that I had marked the previous fall, I determined that the species survived the winters (see this post). Otherwise, how would the snails be always out there?

The purpose of the annual experiment I carry out whenever the temperature goes below about -10° C is to determine if the little Vertigo will survive having their tails frozen off*. So last Sunday, I retrieved 8 adult and one juvenile dormant Vertigo from the yard and placed them in a small plastic container. Then I put the container on my front porch with the probe of a thermistor thermometer in it. Here is a picture of the set-up. The snail container, not quite visible in this photo, was below the wad of cotton.

The lowest temperature recorded during the night was -13.1° C. Early Monday morning I brought the snails indoors and wetted them before I left for work. In the evening, 8 of them, including the juvenile, had revived.

In North America, the range of Vertigo pygmaea extends into Canada, where the winter temperatures go much below -10° C. However, wherever there is snow cover throughout most of the winter, the ground temperature will be much less than the air temperature (see this post). Therefore, to determine the lowest temperature Vertigo pygmaea can withstand, I need to repeat my experiments either somewhere where the winters are colder than here and there is no snow or under artificially lowered temperatures.

*I don't actually know if the snails survive or resist freezing.

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