19 November 2007

143 shells of Mesodon zaletus


Recently I got my hands on this lot of 143 shells of Mesodon zaletus from Tennessee. According to the collector, the shells were representative of a single population. This was a good opportunity to look at the variabilities of various shell characteristics in this species.

Here are some of the things I noticed.

Shell height was quite variable. There were relatively flat shells and then there were shells with higher spires. The next photo shows 2 examples from the extreme ends of the spectrum.


The characteristic parietal tooth of the species was also variable in dimensions.


Two shells in the entire lot did not have parietal teeth. Here is one of them.


I don't know if the parietal tooth continues to grow throughout the life span of a snail. The toothless shell pictured above had a fully developed lip and so it was not a young adult. I suppose a rare mutation or some developmental abnormality may block the formation of the parietal tooth. Whatever the underlying cause is, it does not otherwise have lethal consequences, at least not always, for this particular snail became an adult. In one other species that I am familiar with, Neohelix albolabris, which is normally toothless, the opposite occurs: there are occasional specimens with a parietal tooth.


Tim Pearce said...


If you look closely at that toothless shell, you can see that some of the parietal callus is missing, and it is missing exactly where the tooth would be if there were a tooth. My guess is that the living snail had a tooth, but that the tooth has broken off or fallen off. So I would probably not use this specimen as an example of a toothless M. zaletus.

Did you mean to include some graphs showing the shape of the diameter distribution curve, or other curves?

Great work.

Stephanie Clark said...

Hi Aydin

I agree with Tim, your toothless shell has lost its tooth at some stage. I know I found a specimen of this species in northern Alabama a year or two ago and as I was cleaning some of the dirt out of the aperture the parietal tooth completely separated from the shell with only the slightest bit of pressure.