Gulf Moccasinshell

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Gulf Moccasinshell

Medionidus penicillatus

ListedMarch 16, 1998
DescriptionSmall mussel with yellowish to greenish-brown shell.
HabitatStable sandy and gravelly substrates in medium-sized streams to large rivers, often in areas swept free of silt by the current
FoodFilter-feeder of phytoplankton and organic detritus
ReproductionFemale siphons sperm from the water to fertilize her eggs, which hatch into parasitic larvae, which metamorphose into sedentary adults.
ThreatsImpoundments, water pollution, possibly predation by muskrats.
RangeAlabama, Florida, Georgia


The Medionidus penicillatus (Gulf moccasinshell) is a small mussel that reaches a length of about 2.2 in (55 mm), is elongate-elliptical or rhomboidal and fairly inflated, and has relatively thin valves. The ventral margin is nearly straight or slightly rounded. The posterior ridge is rounded to slightly angled and intersects the end of the shell at the base line. Females tend to have the posterior point above the ventral margin and are somewhat more inflated. Sculpturing consists of a series of thin, radially-oriented plications (folds) along the length of the posterior slope. The remainder of the surface is smooth and yellowish to greenish brown with fine, typically interrupted green rays. The left valve has two stubby pseudocardinal and two arcuate lateral teeth. The right valve has one pseudocardinal and one lateral tooth. Nacre color is smokey purple or greenish and slightly iridescent at the posterior end. The Gulf moccasinshell utilizes the brown darter (Etheostoma edwini ) and black-banded darter (E. nigrofasciata ) as host fishes.

Modern taxonomy recognizes the nineteenth-century names Unio penicillatus and Unio kingi as synonyms of Medionidus penicillatus. The recent taxonomic history of Medionidus species in the Apalachicolan Region is complex. Two species of Medionidus M. kingi and M. penicillatus were recorded in the Chipola river system in 1940. In 1956, two scientists synonymized M. kingi and two other nominal species, the Ochlockonee moccasinshell and Suwannee moccasinshell (M. walkeri ) under the Gulf moccasinshell, an arrangement also followed by another scientist in 1975.

It was erroneously reported in 1970 that the Gulf moccasinshell and Suwannee moccasinshell from the Apalachicola-Chattahoochee-Flint (ACF) River system and the Suwannee moccasinshell from the Ochlockonee and Suwannee Rivers fit as well. It was not until 1977 that the Gulf moccasinshell, Ochlockonee moccasinshell, and Suwannee moccasinshell from Apalachicolan Region streams were recognized as valid and distinct based on their shell characteristics. The validity of the three allopatrically distributed Apalachicolan Region Medionidus species is now generally accepted.


Adult Gulf moccasinshell 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.


The Gulf moccasinshell mussel 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.


The Gulf moccasinshell was described from three sites in the ACF River system in Georgiathe Chattahoochee River near Columbus and near Atlanta, and the Flint River near Albany. This species historically occurred in the mainstems and larger tributaries of the ACF, Chipola, Choctawhatchee, and Yellow River systems, as well as in the more west-erly confines of Econfina Creek (Bay County, northwest Florida). The species was considered rare over its range but locally abundant. A 1940 study reported 166 specimens from 11 sites, including 130 from two sites in the Chipola River system, an average of 15.1 per site. It is no longer present at most of the historical sites sampled, and is apparently extirpated from the Apalachicola, Choctawhatchee, and Yellow Rivers. There are 13 known sites, none showing evidence of recruitment. During the status survey, 330 sites within the historic range of the Gulf moccasinshell were sampled, including 13 of 31 (42%) known historical sites. This species was found at eight sites (2%), including only one of the historical sites. All Alabama populations of the Gulf moccasinshell appear to be extirpated, and no specimens were found in the Chipola River system during the status survey. The species has not been collected in the Choctawhatchee River system since the early 1930s and in the Yellow River since 1963. The status survey found the species at seven sites (including one mainstem site) in the middle Flint River system, and at one Econfina Creek site. An average of 1.4 live individuals was found per site. Six new sites for the Gulf moccasinshell from tributaries of the ACF River system were found subsequent to the status survey. Three sites were streams in which this species had never been found (one tributary each in the Chattahoochee, Flint, and Chipola Rivers), two were streams (both Flint River system) where this species was found live during the status survey, and one site was a stream in the Chattahoochee River system where a single dead shell had been located during the status survey. Densities of Gulf moccasinshells at two sites where quantitative work was conducted were under 0.04 specimens per sq ft (0.4 specimens per sq m). All specimens located during and subsequent to the status survey were adults; no specimens less than 2.2 in (50 mm) were located.


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 Gulf moccasinshell.

Populations of the Gulf moccasinshell have been isolated due to major impoundments on the Apalachicola, Flint, and Ochlockonee Rivers. Future impoundments to satisfy expanding urban and suburban demand, particularly in the metropolitan Atlanta area, could damage stream habitats where small populations of the Gulf moccasinshell exist. Although muskrats are not common within the range of these species, Piedmont populations of the Gulf moccasinshell in the upper Flint River system may be subject to some degree of muskrat predation.

Conservation and Recovery

The Gulf moccasinshell 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. 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.


U.S. Fish and Wildlife Service
Jacksonville Ecological Services Field Office
6620 Southpoint Drive South, Suite 310
Jacksonville, Florida 32216-0958
Telephone: (904) 232-2580
Fax: (904) 232-2404

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345


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.

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Gulf Moccasinshell

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