After almost a year-long break, I resumed my survey of the land snails of Belt Woods, possibly the only tract of old growth forest left in eastern Maryland. (Previous posts on Belt woods are here and here.) The trees in an approximately 45-acre section of Belt Woods are believed to have never been cut. Yesterday, I spent about 3 hours there looking for snails.
Snail shells and the egg shells of most snail species are made out of calcium carbonate. That means that snails need calcium in their diet. In forests of eastern U.S. where there are no limestone or marble rocks and the soil is low in calcium, the snails get some, if not most, of their calcium by eating other snails' shells.
One common species that I have frequently found in the process of eating empty snail shells is Ventridens ligera. I encountered 2 such individuals yesterday. On both occasions, the snails were in damp, loose soil under pieces of wood or tree bark. The first V. ligera (picture below) had consumed most of the body whorl of the partially empty shell of another V. ligera. The red arrows point at the remaining jagged edges of the body whorl.
In the second picture you can see what is left of the shell of another V. ligera covering the aperture of a live V. ligera. The snail was apparently eating the shell from the inside.
As far as I know, V. ligera is not carnivorous. So they wouldn't normally attack other snails, including conspecifics; they probably eat only empty shells or the shells of dead snails.
These observations have 2 implications:
1. Consumption by snails is one of the significant factors that determine the life span of empty shells in areas otherwise low in calcium.
2. Missing sections of snail shells may not necessarily have been the result of a predatory attack, but may instead have resulted from post-mortem consumption by other snails.