Shawnee rocksnail (Lithasia obovata)

The Shawnee rocksnail (Lithasia obovata) is a member of the Pleuroceridae family of molluscs, which vary in colour from pale yellow to dark brown or black. Members of this family have solid, right-facing shells with the mantle opening facing towards the rear. The spiral operculum, which is the horny or calcareous plate that is attached to the foot and is used to close off the shell when the soft parts of the body are retracted, is typically not circular in shape. The Shawnee rocksnail’s eyes are located on the outer sides of the bases of the tentacles ⁽²⁾.

As in other Lithasia species, female Shawnee rocksnails are typically larger than males. Males of this species do not have external sex organs, and the female lays its eggs through a special egg-laying sinus found on the right-hand-side of the foot ⁽²⁾.

There is little information available on the biology and ecology of the Shawnee rocksnail. However, it is known to be a freshwater ⁽¹⁾ benthic species ⁽³⁾ which breathes through gills ⁽²⁾. As with other members of the Pleuroceridae family, the Shawnee rocksnail is thought to be a feeding generalist ⁽²⁾, consuming detritus and other organic material scraped from a variety of substrates ^{(2) (6)}. This species is believed to have a lifespan of about ten years ⁽²⁾.

The Shawnee rocksnail is endemic to the United States ^{(1) (3)}, where it is widely distributed across a large range including the Ohio River and its tributaries in Pennsylvania, Ohio, Indiana, Illinois, Kentucky and Tennessee ⁽³⁾.

A freshwater mollusc ⁽¹⁾, the Shawnee rocksnail is a benthic species found in big rivers ⁽³⁾ and streams ⁽²⁾ where it is restricted to isolated, shallow, rocky habitats ^{(2) (3) (4)}. This species has a relatively large ‘foot’ which is an adaptation to life in shallow areas and enables it to survive wave action ⁽⁵⁾.

The Shawnee rocksnail has yet to be assessed on the IUCN Red List.

The Shawnee rocksnail is thought to be globally secure and is not considered to be threatened with extinction, as it has a wide distribution and a presumed large population. However, this species is classified as Critically Imperiled in Illinois and as Vulnerable in Pennsylvania ⁽³⁾. It is believed that threatened populations are at risk as a result of the Shawnee rocksnail’s affinity for isolated, shallow habitats and its inability to travel to deeper dispersal pathways ^{(2) (4)}. The Shawnee rocksnail is also sensitive to cold temperatures, meaning that it is less capable of colonising new habitats ⁽⁴⁾. In addition, the colonisation of the invasive zebra mussel (Dreissena polymorpha) poses a threat to the Shawnee rocksnail ⁽²⁾.

There are currently no known conservation measures in place for the Shawnee rocksnail.

Find out more about the Shawnee rocksnail:

• NatureServe Explorer - Shawnee rocksnail: 
  http://explorer.natureserve.org/servlet/NatureServe?searchName=Lithasia+obovata+

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Benthic

Relating to the lowermost region of a body of water such as an ocean or lake, or to the organisms that live there.

Calcareous

Containing calcium carbonate, chalky.

Detritus

Litter formed from fragments of dead material.

Endemic

A species or taxonomic group that is only found in one particular country or geographic area.

Invasive

Describes species introduced deliberately or unintentionally outside their natural habitats where they have the ability to establish themselves, invade, outcompete natives and take over the new environments.

Mantle

In molluscs, a fold of skin that encloses a space known as the mantle cavity, which contains the gills. The mantle is responsible for the secretion of the shell.

Molluscs

A diverse group of invertebrates, mainly marine, that have one or all of the following: a horny, toothed ribbon in the mouth (the radula), a shell covering the upper surface of the body, and a mantle or mantle cavity with a type of gill. Includes snails, slugs, shellfish, octopuses and squid.

Operculum

In some snail species, a horny or calcareous plate that is attached to the foot and used to close the shell aperture when the soft parts of the body are retracted.

1. Species 2000 and ITIS Catalogue of Life (April, 2014)
   http://www.catalogueoflife.org/
2. Tiemann, J.S. and Cummings, K.S. (2009) Status of Three Species of Freshwater Snails (Gastropoda: Pleuroceridae) in the Lower Ohio River Basin, Illinois. Illinois Natural History Survey, Institute of Natural Resource Sustainability, University of Illinois. Available at:
   https://www.ideals.illinois.edu/bitstream/handle/2142/18200/INHS2009_35.pdf?sequence=1
3. NatureServe Explorer - Shawnee rocksnail (April, 2014) 
   http://explorer.natureserve.org/servlet/NatureServe?searchName=Lithasia+obovata+
4. Greenwood, K.S. and Thorp, J.H. (2001) Aspects of ecology and conservation of sympatric, prosobranch snails in a large river. Hydrobiologia, 455: 229-236.
5. Thorp, J.H. and Covich, A.P. (2010) Ecology and Classification of North American Freshwater Invertebrates. Academic Press, Waltham, Massachusetts.
6. Greenwood, K.S., Thorp, J.H., Brent Summers, R. and Guelda, D.L. (2001) Effects of an exotic bivalve mollusc on benthic invertebrates and food quality in the Ohio River. Hydrobiologia, 462: 169-172.