27 April 2005

Spacious shells 2

In a previous post I explained that in most pulmonate land snails the shell seems to be larger than the snail’s body. I listed three reasons why it would be good for a snail to have a shell larger than its body. One advantage is that a larger shell allows a snail to carry water in its mantle cavity. Another advantage I mentioned is that when a snail loses as much as all of the extra portion of its shell to an injury, it will still have enough shell left to withdraw its entire body into. I will now illustrate the latter argument with real-life examples.


The picture above shows two shells of Albinaria lerosiensis, a clausiliid from southwestern Turkey. The shell on the bottom is an ordinary looking specimen, while the one on top looks peculiar; not only does it have a malformed body whorl, but the shell also appears to be too short for its diameter.

Turning the shells sideways, as in the picture below, reveals what happened to the top shell: it lost at least a whorl. But the snail managed to survive and rebuild its shell. The rebuilt whorl is, however, distorted, perhaps because the snail’s mantle―the organ responsible for secreting the shell―had also suffered an injury.


The second example is a shell of Triodopsis juxtidens, a common species from Maryland forests. The photograph below clearly shows that the snail lost almost a half of its original body whorl. It nevertheless survived and repaired its shell (the repaired section is without the characteristics ribs). However, the repaired shell is slightly smaller than the original one as indicated by the arrows pointing at the remnants of the original lip, which are in front of the rebuilt lip.


We can draw two conclusions from these two examples: (1) these snails were able to survive the loss of substantial portions of the body whorls of their shells, because what was left intact was still large enough for them to withdraw their bodies into for protection; (2) in both cases the rebuilt shells ended up being slightly smaller than the original shells. That the snails could fit into smaller shells shows that the original shells were too large for the snails’ bodies. It is not entirely clear why rebuilt shells are often smaller than the original ones. One disadvantage of this is that the snail now has less space for carrying water.

Do prosobranch land snails also have extra space in their shells? That will be the subject of a future post.

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