24 November 2011

Morphological scaling in snails

Here is the relation between shell volume, foot sole area and shell length for an intertidal snail.


In juveniles the foot is already shorter than the shell. During ontogeny the shell grows faster than the foot and so in adult snails the foot ends up being even shorter relative to the shell.

Note how much the rate of growth of the sole area lags behind the rate of growth of the shell volume. This is because volume is proportional to the cube of one or more linear dimensions, while area is proportional to the square of linear dimensions.

The interplay of volume and surface area and the resulting scaling effects underlie many evolutionary processes. For example, one reason why the smallest animals are all aquatic is that they lose their water content very quickly outside of the water, because their surface areas are very large relative to their volumes. At the opposite end of the range, the inner surfaces of the lungs of the largest animals always have convoluted morphologies, because the area of a flat surface would not be enough to satisfy the gas exchange requirements of the relatively much larger volume of the animal itself.

In the case of the subject snail, the consequence of this scaling effect is somewhat more mundane. When its shell gets too large relative to the foot, the snail can't lift the shell up anymore; it simply drags it behind its tail. In an earlier post, I discussed how this happens in the land snail Euxina circumdata.

This subject has had me preoccupied during most of my waking hours for the last month or so. In fact, I even lost sleep thinking over it one night. But finally, I am beginning to fit all the available pieces of the puzzle together.

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