Purple Bankclimber

Elliptoideus sloatianus

Description

The Elliptoideus sloatianus (purple bankclimber) is a large, heavy-shelled, strongly sculptured mussel reaching lengths of 8 in (20 cm). A well-developed posterior ridge extends from the umbos to the posterior ventral margin of the shell. The posterior slope and the disk just anterior to the posterior ridge are sculptured by several irregular ridges that vary greatly in development. Umbos are low, extending just above the dorsal margin of the shell. Internally, there is one pseudocardinal tooth in the right valve and two in the left valve. The lateral teeth are very thick and slightly curved. Nacre color is whitish near the center of the shell becoming deep purple towards the margin, and very iridescent posteriorly. Glochidia for the purple bankclimber transformed on mosquitofish (Gambusia holbrooki ) and black-banded darter, but these species are not considered to be the primary hosts for this mussel. Centrarchids (the sunfish family) have been determined to be fish hosts for species of Elliptio, and may possibly also serve as host for the purple bankclimber, which is genetically very similar to Elliptio spp.

Modern taxonomy recognizes the nineteenth-century names Unio sloatianus Lea, Unio atromarginatus Lea, Unio aratus Conrad, and Unio plectophorus Conrad, 1850 as synonyms of Elliptoideus sloatianus. Elliptoideus sloatianus was included in the genus Elliptio until 1927. At which time the sub-genus Elliptoideus was erected based on the presence of glochidia in all four gills instead of two gills, a characteristic of the genus Elliptio. For a time in the 1950s, the species was still placed under Elliptio, but subsequent investigators have correctly assigned this species to the monotypic genus Elliptoideus.

Behavior

The purple bankclimber lives on the bottom of its riverine habitat. It filters its food of algae, tiny zooplankton, and organic detritus from water siphoned from the immediate environment. Sperm are shed into the water column, siphoned by the female, and used to fertilize her ova. The larvae are parasitic on species of fish, and later develop into the sedentary adult stage.

Habitat

The purple bankclimber inhabits small to large rivers with moderate current and a bottom substrate of sand, fine gravel, or muddy sand.

Distribution

The purple bankclimber was described from the Chattahoochee River in Georgia. The Purple bank-climber historically occurred in the Apalachicola-Chattahoochee-Flint (ACF), Chipola, and Ochlockonee River systems. The type locality was restricted in the 1950s to the Chattahoochee River at Columbus, Georgia. In the ACF River system, the purple bankclimber was historically found throughout the mainstem and in a few of the largest tributaries in the Flint River system, in the vicinity of Dead Lake on the lower Chipola River mainstem, and along the mainstems of the Apalachicola and Chattahoochee rivers. The species occurred in the lower two-thirds of the mainstem of the Ochlockonee River, and in the Little River.

During the status survey, 222 sites were sampled within the historic range of the purple bankclimber, including 14 of 27 (53%) known historic sites. Live individuals were found at 41 (18%) sites, with an average of 54 individuals per site. Having been extirpated from the Chipola and Chattahoochee rivers, no extant populations occur in Alabama. Its range in the Flint and Ochlockonee River systems also has been reduced. The Purple bankclimber currently occurs in the Apalachicola, Flint, and Ochlockonee rivers, with 41 sites known. The purple bankclimber was found at six of the 14 historical sites. The species was found at 17 mainstem sites and one tributary site on the lower two-thirds of the Flint River, at five sites in the Apalachicola River, and at 18 sites on the Ochlockonee River mainstem, mostly above Talquin Reservoir.

It is uncertain if purple bankclimber populations are successfully recruiting young. Two specimens less than 2.8 in (7.0 cm) in length were collected from the Ochlockonee River during the survey; they were 2.1 in (5.3 cm) and 2.3 in (5.9 cm) in length. Based upon the large size attained by this species, both were possibly juveniles. The smallest specimen found during the survey in the ACF River system was 3.0 in (7.6 cm) in length, a size that possibly represents a juvenile. A 1996 study took six 2.7 ft (0.25 m) square substratum quadrant samples at a site below Jim Woodruff Dam in the Apalachicola River where the purple bankclimber was the second most commonly encountered species. No specimens smaller than 5.2 in (13.3 cm) were found, indicating a lack of recruitment at this site.

Threats

Impoundments have altered about 29% of main-stem riverine habitat on the Flint River. Preimpoundment records from Seminole and Blackshear reservoirs exist for two sites for the purple bankclimber. 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 purple bankclimber at the upstream end of Talquin Reservoir; only occasional populations of the purple bankclimber have been found down stream of the dam, indicating the difficulty of potential host fish movements after impoundment. Populations of the purple bankclimber have been isolated due to major impoundments on the Apalachicola, Flint, and Ochlockonee rivers.

The threeridge (a relative of the fat threeridge) and the washboard (Megalonaias nervosa ), which is superficially similar to both the fat threeridge and purple bankclimber, are heavily utilized as sources of shell for nuclei in the cultured pearl industry. Commercial shell buyers generally regard shells from the ACF River system as poor quality; however, shell material from this area may be used as "filler" for higher quality material from elsewhere. In the 1980s, the price of shell increased, resulting in increased competition for the harvesting of shell beds in the Apalachicolan Region. This has possibly put the purple bankclimber under greater pressure.

Biological supply companies have used the Flint River and possibly the Ochlockonee River as sources for large mussel specimens, including the purple bankclimber, to sell to academic institutions for use in laboratory studies. The practice of dissecting mussels in introductory laboratory courses is no longer widespread, and the threat posed to large species such as the fat threeridge and purple bankclimber is probably decreasing. Nonetheless, harvest of the fat threeridge and purple bankclimber for these purposes could decimate their remaining populations. The increasing rarity of these mussels potentially makes them more appealing to shell collectors. Revealing specific stream reaches harboring these species could pose a threat from collectors.

Conservation and Recovery

Any further collection of the purple bankclimber must be strictly prohibited, and the ban enforced. This will include educating commercial shell and pearl collectors about how to identify the rare purple bankclimber, and of the need to avoid any harvest-related mortality. Its surviving critical habitat must be protected against damage caused by potential reservoir development or other activities causing changes to hydrology or water quality. Surveys should be undertaken to determine whether there are any unknown populations of the purple bankclimber. Its known populations should be monitored, and research undertaken to determine its specific habitat needs and management practices to increase its small, isolated populations. This could include beneficial modifications of water-release practices at existing reservoirs.

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

References

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.