Snails from the Great Bath

For hundreds of years the steaming waters of the Great Bath were enjoyed by human bathers. Now the warm water is home to several species of organism. Not only the Cyanobacteria (Blue-Green Algae) that give the water its green colour, but also, more surprisingly, snails. Not the large edible snails introduced by the Romans, but small fresh-water snails.

Physella acuta around the edge of the Great Bath

These tough little snails (Physella acuta) survive the warmth (up to 39°C, warmer than our body temperature) and the cleaning: several times a year our Staff empty the Bath and sweep the algae – and the snails – down into the Roman Drain that empties into the River Avon.

Physella acuta shell

The snails are not obvious, the biggest are only 11mm long, but many congregate at the edge of the water and in the splash zone, where it is still damp but cooler. Often they can be seen crawling on the top step of the Bath in water at 37-38°C, feeding on the algae: they rasp the surface and “hoover” up anything in their path, using a tooth-covered tongue-like radula.


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

Photographs of the Great Bath in the 1890s show water lilies in the water of the Bath and if these were a warm water species introduced from North America, then the snails could have been on those plants.

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!