03 October 2006

Withdraw or die!

The intertidal snail Batillaria minima was the subject of a post back in June. Long-time readers of this blog may recall that deep retractibility of land snails into their shells and the resulting conclusion that they build oversized shells have been the subjects of several posts (for example, here).

While photographing B. minima one day last June, I also tested their retractibility into their shells. During that process, I noticed that many snails had repair scars on their body whorls. This observation led to a quick study, which has just been published in No. 14 of Triton, the Journal of the Israel Malacological Society. You can download the pdf version from here.

In summary, the study demonstrated is that some B. minima survive the attacks of a predator, because the snails can withdraw to a position about a quarter of a whorl behind their apertures, which is possible because they have oversized shells. This led to a more general conclusion:

"From an evolutionary point of view, a snail would build an oversized shell only if the protection and other possible advantages offered by such a shell compensate for the extra time, energy and material required to build and maintain it. In B. minima, the vital antipredatory function of the oversized shell provides the necessary
compensation."


Note: Some time after I submitted this manuscript, I found a paper (Dudley, Nautilus, 94:162-164, 1980) about the comparison of the frequencies of repair scars on B. minima and Cerithium lutosum. The paper did not discuss retractibility.

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