Fat Threeridge Mussel

Amblema neislerii

Description

The Amblema neislerii (fat threeridge) is a mediumsized to large, subquadrate, inflated, solid, and heavy shelled mussel that reaches a length of 4 in (10.2 cm). Older, larger individuals are so inflated that their width approximates their height. The umbos are in the anterior quarter of the shell. The dark brown to black shell is strongly sculptured with seven to eight prominent horizontal parallel ridges. Internally, there are two subequal pseudo-cardinal teeth in the left valve and typically one large and one small tooth in the right valve. The nacre is bluish white to light purplish and very iri-descent. The host fish for the fat threeridge is unknown at this time. Several host fish families have been identified for the threeridge, a congener of the fat threeridge, and include eight species of centrarchids (the sunfish family). Minnows (Cyprinidae) may serve as hosts for the fat three-ridge. The nineteenth-century name Unio neislerii Lea is now believed to be a synonym of Amblema neislerii. This taxon was incorrectly assigned to the genera Quadrula and Crenodonta earlier this century, but modern taxonomy correctly assigned the fat threeridge to the genus Amblema in 1988.

Behavior

Adult fat threeridge mussels are sedentary as adults. They siphon streamwater and filter phytoplankton and organic detritus as food. The female mussels siphon water containing sperm from the water to fertilize their eggs. The eggs hatch into larvae that are parasitic on fish. The larvae later meta-morphose into sedentary adults.

Habitat

The fat threeridge inhabits relatively stable reaches of sandy and gravelly substrates in medium-sized streams to large rivers, often in areas swept free of silt by the current.

Distribution

The fat threeridge was described from the Flint River in Macon County, Georgia. This species was historically endemic in the mainstems of the Flint, Apalachicola, and lower Chipola rivers, where it was generally rare over its range but locally abundant. It has been extirpated from the Flint River, which included most of its historic range. It has disappeared from most of the historical sites where it was formerly found, and only 7% of sampled sites within the historic range still have live individuals. It is currently known from six sites on the latter two rivers.

For the status survey, 86 sites were sampled within the historical range of the fat threeridge, including eight of the 12 (67%) known historical sites. Only one of the eight (13%) historical sites still had live individuals. No live fat threeridge mussels have been found since 1981 in the Flint River; the species is apparently extirpated from Georgia. This species was apparently common in Dead Lake in 1967, but no living specimens were found there in 1974 or in the status survey. The status survey found fat three-ridge at six of the 86 (7%) sampled sites, three each on the Apalachicola and lower Chipola rivers. An average of 6.4 live individuals were found per site.

A 1956 study conducted in the Chipola River system reported 17 specimens from two sites (average of 8.5 per site). The same study documented 0.9 to 1.4 mussels per sq ft (10-15 mussels per sq m) over a 656 ft (200 m) stretch of Dead Lake (Chipola River) shoreline.

The smallest live fat threeridge found during the survey was 1.7 in (4.3 cm) long. A 1956 study found evidence of juvenile fat threeridge at a site in the lower Apalachicola River thought to have the best extant population of this species, where it was the second most common mussel species encountered. Three fat threeridges under 2.0 in (5.0 cm) in length were found employing total substratum removal from six 2.7 ft (0.25 m) square quadrants. The smallest specimens had fewer than the five presumed annual growth rings that might be indicative of juveniles. A fresh dead individual measured 0.9 in (2.4 cm) in length and had two to three growth rings. In 1996, three live specimens ranging from 1.6 to 2.0 in (4.0-5.0 cm) in length were located in the same bed. These data indicate that the fat threeridge is experiencing limited recruitment at the site representing its best known population.

Threats

Impoundments have altered about 29% of mainstream riverine habitat on the Flint River. Preimpoundment records from Seminole and Blackshear reservoirs exist showing one site for the flat three-ridge.

A large population of the fat threeridge has been extirpated in Dead Lake, possibly from chromium contamination from an abandoned battery salvage operation. The threeridge (a relative of the fat three-ridge) and the washboard (Megalonaias nervosa ), superficially similar to both the fat threeridge and purple bankclimber, are heavily utilized as sources of shell for nuclei in the cultured pearl industry. Although commercial shell buyers generally regard shells from the Apalachicola-Chattahoochee-Flint (ACF) River system as are of poor quality, shell material from this area may be used as "filler" for higher quality material from elsewhere. In the 1980s, the price of shell increased in the 1980s, resulting in increased competition for the harvesting of shell beds in the Apalachicolan Region. This has possibly put additional pressure on the fat three-ridge. Biological supply companies have used the Flint River and possibly the Ochlockonee River as sources for large mussel specimens, including the fat threeridge, 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 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

The fat threeridge mussel only survives in a few critical habitats, and its reproductive success appears to be quite limited. Its surviving areas of critical habitat must be protected from impoundment and other damages, such as pollution. The rare mussel must also be strictly protected from any collection by naturalists or for commercial purposes. Its known populations should be monitored and additional ones searched for. Research should be undertaken into its ecological needs, with a view to developing management practices to maintain and improve its habitat.

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