The scale is in millimeters.
This species has never been on this blog before. Theodoxus fluviatilis is an aquatic snail and I don't often write about aquatic snails. These specimens are from Köyceğiz Lake in southwestern Turkey where we collected for about half an hour one morning last summer.
As its name implies, T. fluviatilis lives in rivers and also in lakes and brackish waters with salinities up to 13 parts per thousand, according to Fretter and Graham1.
Many of the live snails I pulled out from the lake had tiny eggs in rows attached to their shells near their apertures. Fretter & Graham1 describe the eggs as "a flattened sphere made up of approximately equal halves sutured together around the equator." Apparently, the 2 halves are separated when the young snails escape.
Theodoxus fluviatilis with eggs. Snail's operculum is closing the shell aperture.
Another characteristic of this species and its relatives is that as they grow they resorb the inner walls of their shells (that is, the previous outer walls that became the inner walls after they were covered by subsequent whorls). As a result, the inside of the shell becomes a little bit more spacious than it would otherwise be. In the picture below you can see the inside of a T. fluviatilis shell thru the hole I opened in its wall. The diagram shows the location of the missing (resorbed) inner walls that would be seen from the direction of the red arrow. The edge of the remaining portion of the inner wall (blue arrow) gives the false impression of being the columella. The actual columella, which was resorbed, would be located directly under the apex of the shell.
Inside of a Theodoxus fluviatilis shell.
Vermeij2 suggests that the function of resorption of inner shell walls is to increase the available space within the shell. He speculates that in carnivorous species, resorption makes room for the swallowed prey. Increased space inside its shell may also help an intertidal snail to store more water in its shell for times when it is exposed to air. In the case of T. fluviatilis, which is neither carnivorous nor intertidal (although they do live quite close to the shore), I suspect resorption creates more room for eggs.
1. Fretter & Graham. 1994. British Prosobranch Molluscs.
2. Vermeij. 1993. A Natural History of Shells.