15 November 2010

Shell shape changes in snails

The shapes of the shells of most snail species change during growth. Probably all snails when they hatch out of their eggs have a near-spherical shape, because their eggs are near-spherical (but here is an exception). As the snail grows, either the shell diameter or the height grows faster than the other dimension, giving the shell its final, adult shape. On top of the overall change in shape, the body whorl may descend as the snail nears adulthood further changing the shell shape.

In some species, however, the overall shell shape stays more or less the same during growth. Here is an example.

This is a Helix species. The shell on the left is, obviously, a juvenile, while the one on the right is an adult.

In the next photo I equalized the shell heights of the juvenile and adult shells and then overlapped their images. I also made the color of the juvenile shell reddish to make it easier to distinguish it from the adult.

The most outstanding difference between the juvenile and the adult is the area of the aperture, which is wider in the former. That is probably a universal characteristic of juvenile snail shells; shell aperture almost always gets narrower during growth. I don't know why.

1 comment:

Fred Schueler said...

The skulls of Crocodyles are also very isomorphic from hatchling to large adult.