Snails of ponds and ditches

Fresh water has its own snails, and a jar dipped at the weedy edge of a pond will often include one. The common shapes separate cleanly once you have seen them twice. Pond snails, in the family Lymnaeidae, carry a pointed spire. Bladder snails, Physidae, are smaller, glossy, and left-handed: the opening sits on the left when the apex is up and the aperture faces you. Ramshorn snails, Planorbidae, coil in a flat disc, and in some of them the body is red because they use hemoglobin rather than the usual snail pigment.

Most of these animals graze. They rasp algae, bacteria, and decaying plant tissue from surfaces. A planted pond uses that grazing. A sudden crowd of snails is usually a sudden supply of food, or of eggs brought in on a plant, not a failure of the snails’ character. They are doing the job the film of growth invited.

Breathing divides them again. Many pond snails and bladder snails are pulmonates and climb to renew the air in the lung. A snail hanging at the underside of the surface film is not trapped. It is filling that lung, and a tap on the stem will often make it drop. Eggs are laid in clear jelly on plants, glass, and stones. Unlike the buried eggs of a garden snail, these are easy to see and easy to scrape off by accident when you tidy a stem. A look at the undersides before you pull a plant is the whole precaution.

A pond snail is not a garden snail that fell in. The two are built for different water budgets. A land snail dropped into a pond drowns. A ramshorn placed on a dry path desiccates. If you fish one out while weeding, it goes back into the water it came from, not onto the bank to “recover.”