05 February 2011

Cerithidea scalariformis and its little foot

The 3-eyed, intertidal and almost-terrestrial snail Cerithidea scalariformis has been the subject of several posts (for example, this one and this one).

The picture below shows one snail photographed from below while it was crawling on a glass plate. Note the snout all the way in the front and the foot behind it, both in contact with glass.

Also note how short the foot is relative to the shell. One consequence of the discordant sizes of the foot and the shell is that the former cannot lift the latter above the substrate. The snail drags its shell behind it. Look at the picture below.

To pull its shell through wet sand and mud, its usual habitat, Cerithidea scalariformis must spend quite a bit of energy. There must, therefore, be a counterbalancing advantage to having a shell much longer than the foot that is supposed to pull it. Otherwise, evolution would have long ago done something about it.

One possible advantage of having a foot much shorter than one's shell is that when the snail is withdrawn into its shell, the foot, the last part of a snail to enter its shell, can withdraw far behind the aperture. And that is good when the aperture is under attack by a predator like a crab that can break open the aperture.

I don't actually know how far into its shell Cerithidea scalariformis can withdraw. I intend to find out in the near future.

Here is the follow-up post.


Anonymous said...

Hey Aydin. How big is the operculum in this species? And how stiff is the operculum? How well does the operculum fit the aperture? Those factors would set the limit as to how far the individual could retract into the shell.

Susan J. Hewitt


Susan, this post has a picture of the operculum:
It is soft.

Anonymous said...

Ah yes! Very nice photos and very interesting info, Aydin. I wonder what eats Cerithidea? Presumably birds, like migrating waders? But the shells are very strong, so they would not easily be crushed in a bird's stomach. I suppose racoons might eat them too.

Ah, here is an abstract of an interesting predation paper on C. californica. The author says that grapsid crabs and willets (a fairly big wading bird) eat them. I see that he willets regurgitate the shells.

Susan J. Hewitt