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Fusconaia masoni

(Conrad, 1834)

Atlantic Pigtoe

G1Critically Imperiled Found in 4 roadless areas NatureServe Explorer →
G1Critically ImperiledGlobal Rank
EndangeredIUCN
Very highThreat Impact
Identity
Unique IDELEMENT_GLOBAL.2.1066291
Element CodeIMBIV40050
Record TypeSPECIES
ClassificationSpecies
Classification StatusStandard
Name CategoryInvertebrate Animal
IUCNEndangered
Endemicendemic to a single nation
KingdomAnimalia
PhylumMollusca
ClassBivalvia
OrderUnionoida
FamilyUnionidae
GenusFusconaia
Other Common Names
Atlantic pigtoe (EN)
Concept Reference
Williams, J. D., A. E. Bogan, R. S. Butler, K. S. Cummings, J. T. Garner, J. L. Harris, N. A. Johnson, and G. T. Watters. 2017. A revised list of the freshwater mussels (Mollusca: Bivalvia: Unionida) of the United States and Canada. Freshwater Mollusk Biology and Conservation 20:33-58.
Taxonomic Comments
Fusconaia masoni now includes Lexingtonia subplana based on Johnson (1970a) and Bogan et al. (2003) (Williams et al. 2017).

Taxonomic confusion exists regarding this species and Lexingtonia subplana. Lexingtonia is diagnosed on the basis of using only the outer demibranchs as marsupia versus all four demibranchs in Fusconaia (Ortmann, 1914). Pleuronaia barnesiana has historically been treated as a member of the genus Fusconaia despite differences from other members of the genus (Ortmann 1917; 1918). Preliminary genetic analyses suggest it is more closely related to Pleuronaia dolabelloides than to species of Fusconaia (Campbell et al. 2005). Based on DNA analyses, Unio subplana Conrad, 1837, was found to be genetically indistinguishable from Fusconaia masoni (Conrad, 1834) (Bogan et al., unpublished data, in Williams et al. 2008). Since U. subplana is the type species of Lexingtonia, the genus Lexingtonia is considered a junior synonym of Fusconaia (Williams et al. 2008). The next available generic name for barnesiana and dolabelloides is Pleuronaia Frierson, 1927. Pleurobema gibberum was also found to belong to the Pleuronaia clade (see Williams et al. 2008).
Conservation Status
Rank MethodLegacy Rank calculation - Excel v3.1x
Review Date2018-10-11
Change Date2018-10-11
Edition Date2018-10-11
Edition AuthorsStagliano, D. (2018), Cordeiro, J. (2007); Stevenson, P.; Keferl, E. P. [1998 version]
Threat ImpactVery high
Range Extent20,000-200,000 square km (about 8000-80,000 square miles)
Number of Occurrences21 - 80
Rank Reasons
This species is in critical danger of becoming extinct because of low population numbers and minimal viability with very high threats.
Range Extent Comments
Fusconaia masoni has a moderate sized geographic range (95,770 sq km calculated via geocat) across Georgia, South Carolina, North Carolina, and Virginia; however, it is now highly reduced in the number of known extant occurrences. Many populations from the mainstream of larger rivers have disappeared. The general pattern of distribution now is that this species is limited to the headwater areas of drainages in which it is still present.
Occurrences Comments
40-50 occurrences have been recently confirmed to be extant; but only 13-14 of these are considered viable.
Threat Impact Comments
Passive impacts on water quality, non-point source siltation and eutrophication, as damaging as major point source. The conversion of low intensity land use to high intensity uses continues to increase and compromise freshwater habitats. Impoundments and/or alteration of the rivers for navigation significantly alter the habitat for this species. A recent study determined that the glochidia (larvae) of Fusconaia masoni are extremely sensitive to pollution (Augspurger et al. 2003).
Ecology & Habitat

Description

Shell subrhomboidal. Anterior margin rounded. Ventral margin slightly convex. Posterior margin sharply joins the ventral margin and is truncated. The dorsal margin is straight to slightly convex. Beaks located slightly anterior of dorsal midpoint. Shell is subsolid and subcompressed. Periostracum has been described as clothlike (Johnson, 1970) or parchmentlike (Adams et al., 1990). Periostracum is yellow to dark brown, frequently having some fine raying posteriorly. Ligament is thick and short. Nacre is white or bluish white. Dentition is well-developed. Typically two pseudocardinals in each valve, although anterior one in right valve is rudimentary. Lateral teeth are well developed. The musculature is typically orange in this species with the visceral mass yellowed white (Fuller, 1974). See Fuller (1974) or Johnson (1970) for complete descriptions and illustrations.

Diagnostic Characteristics

Pleurobema collina of James River is similar but typically possesses spines. As compared to unspined P. collina, Fusconaia masoni possesses a more vertically truncate posterior margin, stronger raying, and has a clothlike periostracum generally brownish color in subadults, whereas P. collina has a more obliquely truncate posterior margin, only indistinct raying, and a smooth periostracum with a generally yellowish color in subadults (Johnson, 1970). Lexingtonia subplana is a highly questionable taxa possibly derived from F. masoni on the basis of observations of incompletely gravid females. Lexingtonia is defined as using only outer two demibranchs as marsupia versus all four demibranchs possessing marsupia for Fusconaia.

Habitat

This is considered to be a species of relatively fast waters with high quality riverine/large creek habitat. It is typically found in headwaters or rural watersheds. The preferred habitat of the Atlantic pigtoe is coarse sand and gravel at the downstream edge of riffles. It is less common in sand, cobble and mixtures of sand, silt and detritus (Bogan and Alderman, 2004). The Atlantic pigtoe requires fast flowing, well oxygenated streams and is restricted to fairly pristine habitats. Adams et al. (1990) state that Fusconaia masoni prefers yielding substrates of sands or gravel below riffles.

Ecology

No definitive studies. Mussel literature consists primarily of taxonomy, systematics, distribution, and life history of the taxa. No ecological studies of this species exist. It appears to be a species negatively affected by eutrophication and siltation.

Reproduction

Reproductive biology of Fusconaia masoni has not been extensively studied. The species is typical of Fusconaia in using all four demibranchs as marsupia for brooding eggs/glochidia. Adams et al. (1990) reported that females are fully gravid in the first week of July. Fuller (1974) reported females to have discharged glochidia by September 19. F. masoni is a short term brooder. Period of time for required for glochidia to complete metamorphosis to juvenile varies according to species. Time period generally between 30-60 days. Laboratory studies by Watters and O'Dee (1997) and O'Dee and Watters (2000) identified the bluegill (Lepomis cyanellus) and shield darter (Percina peltata) as potential fish hosts.
Other Nations (1)
United StatesN1
ProvinceRankNative
GeorgiaS1Yes
South CarolinaSHYes
VirginiaS2Yes
North CarolinaS3Yes
Threat Assessments
ThreatScopeSeverityTiming
1 - Residential & commercial developmentPervasive (71-100%)Extreme or 71-100% pop. declineHigh (continuing)
1.1 - Housing & urban areasPervasive (71-100%)Extreme or 71-100% pop. declineHigh (continuing)
7 - Natural system modificationsLarge (31-70%)Serious or 31-70% pop. declineHigh (continuing)
7.2 - Dams & water management/useLarge (31-70%)Serious or 31-70% pop. declineHigh (continuing)
9 - PollutionPervasive (71-100%)Extreme or 71-100% pop. declineHigh (continuing)
9.1 - Domestic & urban waste waterPervasive (71-100%)Extreme or 71-100% pop. declineHigh (continuing)
9.2 - Industrial & military effluentsLarge (31-70%)Serious or 31-70% pop. declineHigh (continuing)

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Roadless Areas (4)
Virginia (4)
AreaForestAcres
Broad RunJefferson National Forest10,971
Hoop HoleJefferson National Forest4,652
Patterson MountainJefferson National Forest4,865
Price MountainJefferson National Forest9,119
References (38)
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