Yesterday's post was about the tiny semi-terrestrial snail Assiminea succinea and a simple immersion test I had subjected them to in Florida last week. Tim Pearce, in his comment, questioned the identification of the snail. Because the snail in the pictures had eyes at the tips of its tentacles and because no operculum was visible in the pictures, Tim wondered if the pictured snail was instead a common land snail.
A little bit of background information may be necessary to clarify Tim's comments. The land snails that are familiar to most people have 2 pairs of tentacles with the longest pair carrying eyes at their tips. They also don't have opercula. Those are the pulmonate snails in the group Stylommatophora. Among their evolutionarily closest relatives are the snails in the Basommatophora, the adults of which also lack opercula, but have their eyes at the bases of their 1 pair of tentacles. Here is an example: Melampus bullaoides.
Now the plot starts to thicken. The species in the 3rd pulmonate group the Systellommatophora also lack opercula, but have eyes on the tips of their tentacles. In contrast, all other non-pulmonate land snails have opercula and their eyes are at the bases of their 1 pair of tentacles. Here is an example: Pomatias elegans.
To further thicken the plot, I should add that most marine snails, the polyphyletic prosobranchs, have their eyes at the bases of their 1 pair of tentacles and many have opercula.
So, if the snail that I identified as Assiminea succinea didn't have an operculum, it would likely to be a stylommatophoran, although, as Tim noted, it did not have 2 pairs of tentacles. The latter trait is, however, not a universal one among the stylommatophorans as Tim also noted. For example, the snails in the genus Vertigo have only 1 pair of tentacles. Here is an example: Vertigo gouldi.
Confusing? Tell me about it.
Fortunately, to settle the matter quickly I remembered that I had additional pictures of the subject snails and their opercula. Here is a set showing how the little Assiminea succinea comes out of its shell. In the 1st photo the aperture is mostly covered by the thin operculum, while in the 2nd photo, the snail's foot is pushing the operculum (arrow) out of the way. Those little elongated things visible thru its shell, and also outside the shell, are fecal particles, which is another characteristic that separates non-pulmonate snails from at least the stylommatophoran pulmonates; in the latter group, as far as I know, the feces are always in the form of strings and never expelled as individual particles.
I will leave the discussions of the evolutionary implications of all of this, which Tim's touches upon in its comment, to future posts.
Assiminea succinea is in the family Assimineidae. Here is a brief discussion of the characteristics of the family. A new Assiminea species, A. mesopotamica, was described last year by Glöer et. al (Mollusca 25:3, 2007). Their description notes both the presence of an operculum and the location of the eyes.