Introduction

Aquatic snails are found in a variety of freshwater habitats including ponds, lakes, wetlands, and streams and rivers of all sizes. They are found mostly in shallows, along shorelines and backwaters in or on aquatic vegetation, rocks, and sediments. Turn rocks around and examine crevices for snails and their eggs. Check slower or faster current habitats in streams, clearer or murkier, softer sediments in pools, on or under stream side rocks. In wetlands and backwaters of lakes or streams, check vegetation around the edges.

In all of North America, there are over 600 species (Johnson 2009, Johnson et al. 2013). In Indiana, there may be upwards of 50 or more species (Pyron et al. 2008). Along with freshwater mussels, these mollusks are among the most threatened animals on the planet.

The purpose of this aquatic snail guide is to introduce the newcomer to local aquatic snails and learn about them.

Types of Aquatic Snails

Aquatic snails basically fall into two major groups based on shell characteristics and breathing – prosobranchs (water breathers) and pulmonates (air breathers).

Prosobranch snails have a single gill for breathing and a stiff cover called an operculum. When out of the water, the cover shuts the animal off inside the shell and protects it from drying. The operculum is also used in identification. Prosobranchs vary in size, but all have stoutly built shells – not thin and flimsy. They are more commonly encountered in flowing waters – streams and rivers, either swifter or slower areas of flow. Because they have gills, they are more sensitive to water quality problems than the pulmonate snails.

Pulmonate snails have a “lung” of sorts for breathing in air, but no operculum. Their shells are generally thinner than the prosobranchs and they are more commonly encountered in standing water environments – lakes, slow areas of streams, and wetlands.

The life histories of the two groups vary. Prosobranchs tend to be longer-lived and grow more slowly than do pulmonate snails. (Dillon 2000, Johnson 2009). Most prosobranch snails have separate males and females with internal fertilization. Pulmonates are hermaphroditic, with each individual producing both sperm and eggs from the same organ. Because of their use of water for oxygen, prosobranchs are considered to be more sensitive to environmental impacts compared to the air-breathing pulmonates (Angelo et al., 2002).

Note that the terms “prosobranch” and “pulmonates” are terms of convenience, no longer of specific taxonomic rank (although taxonomy is always in flux).

Conservation Status of Aquatic Snails 

The Freshwater Mollusk Conservation Society (2014) suggests that 10% of North American species are extinct and 55% are endangered or threatened.

Pyron et al (2008) note that in Indiana: “... three species are locally extinct and many others are locally imperiled or vulnerable. Indiana DNR lists two snail species as species of special concern: the Pointed Campeloma, Campeloma decisum, and the Swamp Lymnaea, Lymnaea stagnalis.

The Indiana DNR also lists two aquatic snails as “Aquatic Invasive Invertebrates of Concern”: the Chinese mysterysnail, Cipangopaludina chinensis, and the New Zealand mudsnail, Potamoyrgus antipodarum. Mystery snails are large animals, reaching over 50 mm in diameter - they are found in Indiana. These and other exotic species must never be released into the environment. The New Zealand Mudsnail is a relatively new invader to the United States. It is a small snail (about 1/8 inch) and may be inadvertently carried by anyone recreating in or on the water. The closest location where they have been found is in eastern Colorado.

Origins of Freshwater Snails

The origin of freshwater snails is complex . Marine snails are the ancestors of the prosobranch snails - those groups with an operculum and a single gill . The group does not represent one single invasion of freshwater, but likely several over time.

Pulmonate aquatic snails, the air breathers, are descended from land ancestors, probably due to a single invasion. The pulmonates breathe inside the shell using their mantle tissue (that also makes the shell) in the same way as their terrestrial counterparts. Unlike land snails, though, the eyes of the pulmonates are at the base of the tentacles, not at the top of the tentacles.

The following links will take you to:

1. Aquatic Snails of Indiana - a tentative list of aquatic snails of Indiana .

2. Keys to Aquatic Snails - the important features of aquatic snails and a guide to major form with, perhaps, some comment on the traits of the most likely species. But this is still a chancy business. References are provided for follow-through. The Ohio Division of Wildlife has a Freshwater Snails of Ohio Field Guide which is a great picture key from the next-door state, great for iNaturalist ids - the link opens as a pdf.