30 October 2006

Why is this snail hairy?


This snail, a native of eastern U.S., is known as Lobosculum pustuloides, although it may not actually belong to that genus (family Polygyridae). For today, however, I will ignore its uncertain taxonomic placement and instead discuss the hairs that cover its shell.

The hairs grow out of the periostracum, the thin skin-like layer that covers the outside of the shell. There are several species of land snails that have similar hairy shells. For example, in the U.S. the species in the genus Stenotrema, also in Polygyridae, too have hairy shells.

What are the hairs for?

To answer that question, Pfenninger et al. (2005) studied the members of the European land snail genus Trochulus. Their analyses show that the habitats of the Trochulus species with hairy shells tend to be wetter than those of the species with hairless shells. They also did some tests that showed that on a wet leaf surface a larger minimum force was necessary to move hairy shells than hairless shells. So they speculate that the hairs help the snails stick to wet surfaces such as the leaves of plants that they apparently climb when they are feeding. It is not clear to me, however, why the snails' already sticky feet and slime wouldn't be strong enough to secure them to the plants they are on.

Would their findings apply to L. pustuloides and the other hairy U.S. snails? These snails seem to spend most of their time in the litter or on rotting trunks on the ground. I don't think they are in any danger of falling down from places high above the ground.

One possibility not considered by Pfenninger et al. is that the hairs may have an antipredatory function. They may, for example, deter the carnivorous snail Haplotrema concavum that drills thru the shells of its victims if it can't fit its head thru their apertures.

Lobosculum pustuloides is a small snail. The shell of this particular adult was 4.2 mm across.

Markus Pfenninger, Magda Hrabáková, Dirk Steinke, Aline Dèpraz. 2005. Why do snails have hairs? A Bayesian inference of character evolution. BMC Evolutionary Biology 5:59 (4 November 2005). pdf


clare said...

Would they stick in the throat, do you think? Or do most predators de-shell first - like the thrush?


That's an interesting thought, Clare. Salamanders eat snails & they probably swallow them whole. The hairs could make the snail more difficult to swallow.

Snail said...

Among Camaenidae, the chloritids often have hairy shells. Not usually that hairy though.

On the subject of structures for defence, have you seen Schilthuizen et al. in the latest issue of Evolution? They looked at the elaborate ridges and vanes on Bornean Opisthostoma and concluded they were to prevent attack by carnivorous slugs ...


I read a summary by Schilthuizen himself, but haven't seen the paper itself.

BG said...

My first reading of the post left me with the impression that keeping wet leaves off the shell would make it eaiser to move around. Big wet leaves flopped on the shells must be more difficult to pull off with the cohesive effect of the water on the leaves. The hairs would prevent such large amounts of wet leaf surface area actually touching the shells.

It also looks like the hairs pick up debris which might help with camouflage.


Yes, the hairs do accumulate some debris, but I don't think camouflage would be of much help to these snails. They probably don't have any visual predators.

Anonymous said...

Are the hairs sensitive for the snail? If so, would sensitive heairs be an asset in the habitats called home by hairy snails?

Anonymous said...

Here are two other lines of inquiry:

If hairy snails tend to live in wetter places than non-hairy snails, and if hairy snails climb plants to feed which invariably exposes them to drier microclimates, might the hairs help keep the snail from dessicating?

If hairy-shelled snails tend to live in wetter habitats, are snails with shells possessing hairs less likely to drown because hairy shells are better able to wick away water than non-hairy shells?