27 August 2009

How a snail moves - Part 1

Somewhat confusingly, the vertical axis is the horizontal displacement against time on the horizontal axis.

This graph is from Lissmann (1945). It shows the horizontal displacement in time of a mucus gland on the sole of a Helix pomatia, a land snail, that was crawling up on a vertical glass plate.

The most significant piece of information to be gained from this graph is that the movement of a point, any point, on the sole of a snail is not continuous, but intermittent. That is the case even though the overall movement of the snail is continuous.

Part 2

H. W. LISSMANN. 1945. The Mechanism of Locomotion in Gastropod Molluscs: I. Kinematics
J. Exp. Biol. 21:58-69. pdf


Dr Dan Holdsworth said...

The really interesting thing about slugs and snails, though, is the mucus they secrete. According to McNeill Alexander (who has tested the viscosity of the stuff) the foot mucus of a snail is a really quite sophisticated substance.

Up to a set amount of shear stress, it behaves like an elastic solid. Over that shear stress it behaves like a viscous liquid. The net effect is that the slug sticks to a surface until the muscular contraction wave on its foot exceeds the shear transition level (which it does only locally, not over the entire foot) at which point that part of the foot and that part only slides forwards.

As a means of moving, this is very, very efficient indeed; slow but very energy efficient since the slug doesn't need to expend energy in hanging onto a surface, nor does it have to be too fussy about what it crawls over. The mucus is interesting stuff too; it is a glycoprotein which is mostly glyco and not very proteinaceous, and which is secreted at about 99% water.

The mucus a slug produces is therefore very cheap to produce, since it can actually find carbohydrates quite easily in its food; lipids and proteins are much less easy for a slug to acquire.

Finally a slug is unusual amongst invertebrates in having muscles which work even at very low temperatures; the slug slows down a lot in such conditions but as potato farmers will woefully tell you, the slugs can and do remain active quite a lot of the time in winter.


The problem with secreting large quantities of something that is 99% water is that it is, well, 99% water. That is why slugs stranded on hot & dry sidewalks on sunny summer mornings succumb to dehydration. Of course, one can argue that hot & dry sidewalks were not around when slugs were evolving.