Oval Pigtoe

Pleurobema pyriforme

Description

The Pleurobema pyriforme (oval pigtoe) is a small to medium-sized species that attains a length of about 2.4 in (6.0 cm). The shell is suboviform compressed, with a shiny smooth epidermis. The periostracum is yellowish, chestnut, or dark brown, rayless, and with distinct growth lines. The posterior slope is biangulate and forms a blunt point on the posterior margin. The umbos are slightly elevated above the hingeline. As is typical of the genus, no sexual dimorphism is displayed in shell characters. Internally, the pseudocardinal teeth are fairly large, crenulate, and double in both valves. The lateral teeth are somewhat shortened, arcuate, and double in each valve. Nacre color varies from salmon to bluish white and is iridescent posteriorly. The sailfin shiner (Pteronotropis hypselopterus ) serves as the host fish for the oval pigtoe.

Variation in this species has led to the description of various nominal species. Modern taxonomy currently recognizes the nineteenth-century namesUnio pyriformis Lea, Unio modicus Lea, Unio bulbosus Lea, Unio amabilis Lea, Unio reclusum Wright, and Unio harperi Wright, as well as the twentieth-century name Pleurobema simpsoni Vanatta as synonyms of Pleurobema pyriforme.

Behavior

See the Upland Combshell (Epioblasma metastriata ) entry.

Habitat

The oval pigtoe inhabits streams and small rivers with clean, flowing water.

Distribution

The oval pigtoe was described from the Chattahoochee River, near Columbus, Georgia. This species was historically one of the most widely distributed and common mussels endemic to the Apalachicolan Region. It occurred throughout the mainstems and several tributaries of both the Flint and Chipola River systems, in the lower Chattahoochee River mainstem and several of its tributaries, in the Apalachicola River mainstem, and in the upper portion of the Ochlockonee River system. The oval pigtoe was also known from a single Suwannee River mainstem site and the confluent Santa Fe River system, and in Econfina Creek. Once a species of localized abundance, oval pigtoe populations sometimes numbered in the hundreds, as reported by a study in the 1940s. This study reported 470 specimens from nine sites (an average of 52.2 per site) in the Chopola River system. It has been extirpated from the mainstem of the Chattahoochee River, representing a significant portion of its historical range; occurrences in the Flint and Suwannee River systems have decreased from 32 to 12.

During the status survey, 410 sites were sampled within the historic range of this species, including 20 of 50 (40%) known historical sites. The oval pigtoe was found at 24 (6%) of the sample sites, including seven of the historic sites, with an average of 5.2 live individuals per site. The oval pigtoe has apparently been extirpated from the Chattahoochee River system in Alabama and much of the Chipola River system.

The oval pigtoe is currently known to occur at 26 sites, with no evidence of recruitment. The species was found at one mainstem site and seven tributary sites in the Flint River system, six mainstem Chipola River sites, six mainstem sites and one tributary site in the upper Ochlockonee River system, one site in the New River (upper Santa Fe River system), and two sites in Econfina Creek.

Five new occurrences of the oval pigtoe were subsequently located in three Apalachicola-Chattahoochee-Flint (ACF) River system tributaries. One occurrence was from a stream in the Chipola River system not previously known to have harbored this species. The other four occurrences were in two streams (two sites in each stream), that are tributaries to the Chattahoochee and Flint Rivers where the species had been recorded during the status survey.

Oval pigtoe density at the five new sites never exceeded 0.04 specimens per sq ft (0.4 specimens per sq m). The smallest individual collected during or subsequent to the status survey was 1.0 in (2.5 cm) in length, indicating that juveniles were not present in these collections.

Threats

Impoundments have altered about 29% of mainstem riverine habitat on the Flint River. Preimpoundment records from Seminole and Blackshear Reservoirs exist for one site for the oval pigtoe. Talquin Reservoir flooded about 12% of the riverine habitat in the middle portion of the Ochlokonee river and the lower end of its largest tributary (the Little River). Preimpoundment records exist for the oval pigtoe, now also absent downstream of the dam. This indicates that potential host fish movements may havebeen blocked. Future impoundments to satisfy expanding urban and surburban demand, particularly in the metropolitan Atlanta area, could damage stream habitats where small populations of the oval pigtoe exist.

Although muskrats are not common within the range of these species, Piedmont populations of the oval pigtoe in the upper Flint River system may be subject to some degree of muskrat predation.

Conservation and Recovery

The oval pigtoe survives at about 26 sites, almost all of which are in streams running through privately owned land. Its conservation requires the protection of the stream reaches where it persists in small, isolated populations. These habitats must be protected from proposed impoundment, and from pollution associated with poor land-use practices in the watershed that cause erosion and inputs of nutrients and other chemicals. The populations of the oval pigtoe must be monitored, and research undertaken into its basic biology and ecological requirements.

Contacts

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
http://southeast.fws.gov/

U.S. Fish and Wildlife Service
Wildlife and Habitat Management
6620 Southpoint Drive South, Suite 310
Jacksonville, Florida 32216
Telephone: (904) 232-2580
Fax: (904) 232-2404

Reference

U.S. Fish and Wildlife Service. 16 March 1998. "Endangered and Threatened Wildlife and Plants; Determination of Endangered Status for Five Freshwater Mussels and Threatened Status for Two Freshwater Mussels From the Eastern Gulf Slope Drainages of Alabama, Florida, and Georgia." Federal Register 63(50): 12664-12687.