|Physella acuta around the edge of the Great Bath|
|Physella acuta shell|
This species is thought to have originated in North America, in the Carolinas, and in the last 200 years they have spread to 6 continents. How the snails arrived in the Great Bath we do not know. Darwin wrote of fresh-water snails being spread to new ponds on the feet of birds. Although mallard ducks (Anas platyrhynchos), visit the Great Bath yearly, I think it is more likely they have been introduced by human agency.
|1890s photograph of water lillies in the Great Bath|
Native British fresh-water snails are no longer found in the Great Bath, although after the Romans left, Lymnaea truncatula and Planorbis albus thrived in the marshy ruins of the Roman buildings. Physella acuta survive because they can live at temperatures up to 40°C, and because they breed rapidly. Only 6 weeks after hatching, an individual can start to lay eggs of its own. Like other snails, they are hermaphrodite, i.e. each individual produces both sperm and eggs. Usually 2 individuals copulate and exchange sperm, but a single individual can fertilise its own eggs. So just one snail, surviving the cleaning process, could, in theory, repopulate the whole Great Bath.
The snails were featured on television in The One Show in October 2009.
Next time you visit the Roman Baths, see if you can spot the snails – but do not fall in!