Snails - reaching the crunch-point

I like snails; I may garden, but I like them anyway, and I really hate standing on them as they roam across footpaths at night. However, there are many dangers awaiting snails - hungry corvids for example - and damage can occur that doesn't leave them squashed. However, apart from minor chips and so on, it often feels like they must die if their shell is broken, after all dehydration seems likely aside from any internal damage. This specimen of the common garden snail (Cornu aspersum or Helix aspersa depending where you look - I won't go into the taxonomic disagreements here) found in our garden yesterday shows that this is not necessarily the case...

A garden snail showing a large shell-injury

The shell injury showing where membranes have rejoined to it from the body. There are also cracks on the spire, indicating the considerable extent of the damage it survived.

Although it may not be obvious from the photos, this is not a new injury and the snail was highly active. The exposed surface is dry and has re-attached to the broken edges of the shell; although snails can, and do, mend minor shell-damage as they grow (by secreting new calcareous material), that is unlikely to happen with such a large hole. So, how did the snail survive?

Well, assuming no majo damage to the body itself, the main issue is water loss. The shell retains moisture but of course water can be lost through the main opening (aperture). Therefore, during dry and/or cold conditions, the snail withdraws and seals the aperture using a thin 'skin' of dried mucus, called an 'epiphragm'. Less obviously, it can also avoid damage from freezing, not only by seeking sheletered locations, but also through an antifreeze mechanism in its haemolymph (equivalent of blood). In warmer dry conditions, the edge of the internal 'skin' or mantle (i.e. where it meets the edge of the aperture) can change its permeability to water and thus further help prevent drying-out (Machin, 1966).

A broken shell however presents a whole new aperture through which water can be lost. A search for publications on snail-shell mending, show that Andrews (1935) looked at snails of the genus Neritina and noted a variety of mechanisms and examples, but Durning (1957) described the process from a medical perspective. He noted that the snail secretes a glycoprotein matrix in order to provide a substrate onto/into which calcium carbonate can be secreted in turn i.e. shell material can't be produced directly onto the edge of breaks, and there are few cells there to secrete directly. So, a matrix is needed. This is something that will be happening here to some extent, but the membrane around the mantle also seems to have toughened and, unless the damage is much more recent than it appears to be, this snail survived the recent long winter with this hole - a common species that I'm now looking at from a totally new perspective.

References

Andrews, E.A. (1935). Shell repair by the snail, Neritina. Journal of Experimental Zoology 70(1): 75–107.
Durning, W.C. (1957). Repair of a defect in the shell of the snail Helix aspersa. J Bone Joint Surg Am 39-A(2): 377-93.

Machin, J. (1966). The evaporation of water from Helix aspersa IV. Loss from the mantle of the inactive snail. Journal of Experimental Biology 45: 269-278.