02 January 2006

Sleeping the winter away

In areas where the climate is the Mediterranean type, land snails aestivate during the dry and hot summers and are active during the fall, the mild winters and the spring. In contrast, in the eastern U.S., where there is no dry season, land snails are active throughout the summer and spend the cold winters inside their shells in a dormant state. For the past 4 years or so, I have been studying the colonies of Oxyloma retusa (family Succineidae) at a small local lake. Although O. retusa is a land snail, it always lives near the water, usually within a few centimeters of it. In the spring and summer I find them abundantly crawling on the mud or on cattails growing out of the water. However, in the winter the snails are very difficult to locate. The snails just disappear, somehow survive the winter and reappear late in March in the same spots.

I have followed their generation cycle for several seasons. In early April of every year I find only juveniles, which suggests that only the juveniles survive the winter. All the adults die early in the fall. There is a slight possibility that all the snails die during the winter and juveniles are brought every spring by the water birds from the south. However, one piece of evidence eliminates this mechanism as a major contributor to the snail population. Early in the spring most snails' shells have a "winter stop"—a line across their shells showing where the edge of the shell was the previous fall before they stopped growing. In other words, the snails with a winter stop are most likely to have wintered at the spot.

That is, however, circumstantial evidence. I wanted to prove directly that the snails spend the winter at the same location where they spend their springs and summers. The only way to do that was to find live snails there in the winter. So about a week ago on a cold day, took my camera and went out to my usual study site, a long (about 6 meters) and narrow (only a meter or so wide) piece of muddy land extending into the lake. I spent about a half an hour in the cold wind searching thru the piles of rotting leaves and cattail stalks. People were probably watching me from their warm homes across the lake and wondering what on earth I was doing.

I succeeded. I found one snail, only one, but that was enough. As I had suspected that the snails in the winter would be juveniles, this one was tiny, just 4 mm long (adults grow to be more than 10 mm long). Stuck to a piece of rotting leaf and completely oblivious to its surroundings, it was waiting for the warm weather. I positioned it in the sun and quickly took several pictures of it. The arrow in the left-hand picture below points at the edge of the snail's body within its shell. Unlike most other pulmonate land snails, succineids cannot withdraw too far into their shells. (See my previous posts here, here and here for examples of how far into their shells other pulmonate snails can withdraw.)

oxylomdcmbr

I still needed to prove that the snail was alive and that it wasn't just a dead, frozen snail. So I brought the snail home. Once it warmed up it came out of its shell and started exploring its new surroundings.

oxylomdcmbr3

There must be more snails out there, but they are hard to find. Perhaps I am not looking at the right places. Their habitat is frequently covered with ice during the winter. It is possible that to avoid freezing the snails migrate away from the water towards higher ground, although the one I found was not far from the water. I don't think they can bury into the soil, because their shells are very fragile. In any case, now I have enough evidence to confidently conclude that the juvenile Oxyloma retusa survive the winter at their summer habitat. The snails that were there early in the fall will be the ones that will start growing in late March of this year. I will probably go back there early in March and do another search.

2 comments:

Tim Pearce said...

Good job finding that live juvenile! Do you think some individuals might survive the winter as eggs? Another way to demonstrate that the snails are staying all winter would be to mark some (is it possible to find juveniles in the fall and mark them, for example with a spot of fingernail polish?).

Anonymous said...

My son and I found a snail in our Oklahoma yard in November. Since frost nights had just started, we took it into the house and kept it in an open jar. It ate some, and then "ran away" several times until I placed a plant next to the jar. Immediately, the snail found the underside of a leaf, and placed himself there in what looks to be hibernating mode. My son pulled the snail off, and it woke up, ate some, and then hurried back to the leaf and shut itself up again. In contrast to Mr. Örstan's snail, our snail does not seem to care that the house is warm; it is set on hibernating.