Spiny river snail (Io fluvialis)

Spiny river snail on rock
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Spiny river snail fact file

Spiny river snail description

GenusIo (1)

The spiny river snail (Io fluvialis) is one of the largest aquatic snails in all of North America. It ranges in colour from brown to olive-green on the outer surface of the shell, while the inside of the shell is often marked by striking purple bands. This snail gets its common name from the large, armoured spines that grow on its shell, although interestingly there are some populations that lack such spines (2). As with most snail species, the male spiny river snail tends to be smaller than the female (3).

Length: up to 5 cm (2)

Spiny river snail biology

The spiny river snail is a freshwater species and extracts oxygen from its aquatic environment using a single gill (4). It is a relatively long-lived snail, sometimes living for up to 15 years (2). The spiny river snail feeds on the algal coating of rocks, as well as other organic debris beneath the water's surface of the rivers it inhabits (1) (2) (4).

The female spiny river snail generally lays 20 to 100 eggs during the spring (1), when water temperatures reach 15 degrees Celsius (2). The eggs are laid in lines or swirls on a smooth surface such as rocks or even empty mussel shells, and are purplish grey in colour. The eggs are very small, measuring on average just 0.35 millimetres, and hatch after around 15 to 20 days (1) (2).


Spiny river snail range

Occurring only in the United States, the spiny river snail was once found in Virginia, Tennessee and Alabama (1). It was formerly widespread across the entire catchment of the Tennessee River above Muscle Shoals, Alabama; however, it is now only found in the Clinch, Powell and Holston river systems of southwest Virginia and northeast Tennessee (2).


Spiny river snail habitat

The spiny river snail is found in rapidly flowing, well-oxygenated waters of rocky shoals or sandbars lying just below the surface of the water, but not in still water. It prefers a water depth of up to 1.5 metres below the surface (1) (4).


Spiny river snail status

Classified as Endangered (EN) on the IUCN Red List (1)

IUCN Red List species status – Endangered


Spiny river snail threats

The main reason for the decline in the spiny river snail’s population is the reduction in water quality from industrial and agricultural pollution. Dam construction has also caused further degradation of the habitat in which the spiny river snail lives. The dams effectively turn the river into a reservoir, eliminating current flow and increasing water depth, thus altering the habitat for the spiny river snail to such an extent that the snail is unable to tolerate the changed environment (4).


Spiny river snail conservation

Conservation and recovery efforts for this freshwater snail include artificial culture, water pollution control, and most importantly, habitat protection and restoration (4).

In the 1970s, the spiny river snail was reintroduced into the North Fork Holston River (5) and the tail waters of Nickajack Dam, Tennessee, in the hopes of recolonising the upper reaches of Guntersville Reservoir (6). This resulted in several populations being restored by the early 1990s (7). This intensive conservation effort is still being continued by Virginia’s Aquatic Wildlife Conservation Center (8).


Find out more

Learn more about the conservation of the spiny river snail:



This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:

This species information was authored as part of the Arkive and Universities Scheme.


Simple plants that lack roots, stems and leaves but contain the green pigment chlorophyll. Most occur in marine and freshwater habitats.


  1. IUCN Red List (August, 2011)
  2. Virginia Department of Game and Inland Fisheries: Aquatic Wildlife Conservation Center (August, 2011)
  3. Ohbayashi-Hodoki, K., Ishihama, F. and Shimada, M. (2004) Body size - dependent gender role in a simultaneous hermaphrodite freshwater snail, Physa acuta. Behavioral Ecology, 6: 976-981.
  4. Johnson, P.D. (2003) Sustaining America’s aquatic biodiversity - freshwater snail biodiversity and conservation. Fisheries and Wildlife, Virginia Cooperative Extension, Virginia Tech, and Virginia State University. Available at:
  5. Ahlstedt, S.A. (1979) Recent mollusk transplants into the North Fork Holston River in southwestern Virginia. Bulletin of the American Malacological Union, 21-23.
  6. Mirarchi, R.E. (2004) Alabama Wildlife. Volume One: A Checklist of Vertebrates and Selected Invertebrates: Aquatic Mollusks, Fishes, Amphibians, Reptiles, Birds, and Mammals. University of Alabama Press, Tuscaloosa, Alabama.
  7. Ahlstedt, S.A. (1991) Reintroduction of the spiny riversnail Io fluvialis (Say, 1825) (Gastropoda: Pleuroceridae) into the North Fork Holston River, southwest Virginia and northeast Tennessee. American Malacological Bulletin, 8:139-142.
  8. Freshwater Gastropods of North America: Freshwater Gastropods of Virginia Project (August, 2011)

Image credit

Spiny river snail on rock  
Spiny river snail on rock

© Dick Biggins / U.S. Fish and Wildlife Service

U.S. Fish and Wildlife Service, Asheville Ecological Services Field Office


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