17 February 2011

Truncatella's air bubble

Truncatella is a genus of coastal-terrestrial snails that have been the subjects of several posts on this blog (for example, this one and this one).

In the discussion of Truncatella subcylindrica in British Prosobranch Molluscs by Fretter & Graham (1994), there is this statement: "...on occasions, a bubble of air may be seen in the [mantle] cavity."

I have noticed an air bubble in the mantle cavities of other species of Truncatella. Here is a photo of Truncatella pulchella crawling in sea water. Notice the large air bubble in the mantle cavity under the shell.

What is the function of the air bubble? Is it an oxygen reserve for the snail?


Anonymous said...

Good question! Or... is that bubble an accidental side effect of spending a lot of time in the open air rather than underwater?

Does the bubble function as a life vest, so that if the little snail is accidentally picked up by the tide, it floats and therefore gets washed up high on the beach where it needs to be, instead of sinking to the bottom like a little piece of gravel?

Unfortunately the large tropical winkle Cenchritis muricata (Linnaeus, 1758) has an extremely opaque shell, so you can't see how much air that one has inside its shell. However it spends virtually all the time in the air. Many of them seem to be never submerged for any length of time, and are pretty darn well almost land snails. That creature always amazes me, how some of them can tolerate climbing around on the hot, hot rocks that are at the absolute uppermost reaches of the splash zone. I suppose with those bigger guys you could drop them in water and see how big a bubble of air came out of the shell when it sank.

Susan J. Hewitt

Anonymous said...

It seems to hold its shell in a very horizontal orientation, perhaps this bubble prevents the shell from dragging when crawling in the water. Do any other gastropods do that? Although, it is a terrestrial creature...