This little guy struggling to regain its footing on my fingertips is a Truncatella pulchella. The snails in the genus Truncatella spend their entire lives on land, but never too far from the sea. In fact, they live so close to the sea that during storms thousands of them must wash away, never to return.
The picture on the left shows a typical Truncatella habitat in Florida: piles of rotting seaweed stranded on a sandy or rocky beach only a few meters from the sea. In the depression in the foreground, next to my notebook, I found quite a number of them.
These small snails are among the handful of gastropods that truncate their shells; a seemingly wasteful practice that must nevertheless have some adaptive value. As a Truncatella nears maturity, the apex of the shell breaks off and the snail seals the resulting hole, ending with a more or less flat topped shell. In the picture below you can see 3 adults of Truncatella caribaeensis with truncated shells and one subadult with an intact apex.
These snails appear to have evolved terrestriality relatively recently on the geologic time scale. According to Gary Rosenberg1, one species endemic to Barbados may have become terrestrial within the last million years and may in fact be "the most recent animal to become fully terrestrial."
Next in this series: How Truncatella caribaeensis moves.
Previous entry in this series was Batillaria minima.
1. Rosenberg, G. 1996. Independent evolution of terrestriality in Atlantic truncatellid gastropods. Evolution 50:682-693.