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Pyganodon gibbosa

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Taxonomy [top]

Kingdom Phylum Class Order Family
ANIMALIA MOLLUSCA BIVALVIA UNIONOIDA UNIONIDAE

Scientific Name: Pyganodon gibbosa
Species Authority: (Say, 1824)
Common Name(s):
English Inflated Floater
Taxonomic Notes: This species was previously known as Anodonta gibbosa until 1990 (NatureServe 2009).

This species was placed in the newly elevated genus Pyganodon by Hoeh (1990).  Recently, Zanatta et al. (2007) supported the monophyly of both Pyganodon and Utterbackia using mutation coding of allozyme data, but also resolved the Eurasian Anodonta cygnea to Pyganodon, Utterbackia, and North American Anodonta; indicating futher phylogenetic analysis of the Anodontinae is required including both North American and Eurasian species. Clench and Turner (1956) considered many populations of inflated Anodonta to be Anodonta (= Pyganodon) gibbosa. The definition of this species has changed over the years and it now is considered endemic to the Altamaha River system. This question of its validity can only be answered by comparing allozymes, DNA, or RNA from various populations and determining their relationships.

A list of synonyms for this species can be found on The MUSSEL project web site (Graf and Cummings 2011).

Assessment Information [top]

Red List Category & Criteria: Near Threatened ver 3.1
Year Published: 2012
Date Assessed: 2012-04-23
Assessor(s): Cummings, K. & Cordeiro, J.
Reviewer(s): Bohm, M., Seddon, M. & Collen, B.
Contributor(s): Bogan, A., Dyer, E., Soulsby, A.-M., Whitton, F., Kasthala, G., McGuinness, S., Milligan, HT, De Silva, R., Herdson, R., Thorley, J., McMillan, K., Collins, A., Offord, S., Duncan, C. & Richman, N.
Justification:
Pyganodon gibbosa has been assessed as Near Threatened. This species is endemic to the Altamaha River basin and is known to be declining. However, these declines are not yet severe enough to warrant a listing in a more threatened category, as they are not assumed to exceed 20-25%. Hence it falls just outside Vulnerable under criterion A2. If any of the threats impacting the species in parts of its range increase, this may require a higher threat category in future. Further survey work is needed to determine the population status of this species to assess whether population declines become more severe in future.

The species was assessed in 1996 as Near Threatened, again as result of presumed slow decline, not exceeding 20% (Bogan and Seddon, pers. comm., 2012).

Geographic Range [top]

Range Description: This species is endemic to the Altamaha River basin, Georgia (Wisniewski 2005, NatureServe 2009).
Countries:
Native:
United States (Georgia)
Range Map: Click here to open the map viewer and explore range.

Population [top]

Population: This species estimated to have declined in site occupancy and was present at 17% of sites (40 of 241 sites) prior to 2000 and at 6% of sites (7 out of 120 sites) after 2000 (Wisniewski 2005). However, it has to be noted that not all known occurrences were sampled. It is estimated that declines are not in excess of 30% and more in the region of 20-25% since just before 2000 (a period of 12-15 years).
Population Trend: Decreasing

Habitat and Ecology [top]

Habitat and Ecology: This species has previously been found in soft substrates such as mud, silts or fine sands (Wisniewski 2005, NatureServe, 2009). It is presumed that in the future, more extensive surveys of more remote areas of the Altamaha River to include backwaters and oxbow lakes, will reveal evidence that this species is more common than thought at present (Wisniewski 2005, NatureServe 2009).

Direct life-history data are not available for this species. Freshwater mussels are highly variable in their longevity from species to species (e.g. Haag and Rypel 2010). Studies have shown longevity of species in the genus Pyganodon to range from 9 to 12 years (from three populations of Pyganodon grandis; Haag and Rypel 2010). As a result, generation length is likely to be relatively short on the basis of these data, so that declines should be assessed over a period of between 18 to 21 years (assuming individuals reach maturity at 2 years [the shortest estimate for mussel species from Haag and Staton (2003)]. However, this may represent a vast underestimate of generation length, as it has been suggested that growth ring counts may underestimate age by a factor of between three and ten (Anthony et al. 2001).
Systems: Freshwater

Use and Trade [top]

Use and Trade: This species is not utilized.

Threats [top]

Major Threat(s): Any threats are only inferred and not well delineated plus these only apply to a  portion known of populations (part of Altamaha, not all of it). The species is believed to have been secure historically but some recent declines have been observed in the lower Altamaha River drainage (the principal range of the species). It was lost from more sites than it colonized after 2000, but this was not statistically significant (Wisniewski et al. 2005). Also the extent ot its range at the upstream end decreased by 37 linear km and simlar declines were noted for the Ocmulgee River (Wisniewski et al. 2005). Note also that this species prefers slackwater habitats which tend to be less impacted by habitat alterations.

At present this species occupies areas of the Altamaha River that are not exposed to heavy recreational use and have a low human residential population (NatureServe 2009). Any changes to the habitat from building, dredging, increased sediments or fluvial adjustments from timber removal, water extraction or land development could threaten this species (Master et al. 1998, The Nature Conservancy 2005). For example, forestry activities may contribute sediments and herbicides or pesticides to the rivers, whereas water demand in the Ocmulgee River basin has increased over the last two decades, due to agricultural practices (Georgia Department Natural Resources 2003, The Nature Conservancy 2005). This is expected to increase significantly in the future, posing a threat from potential domestic waste water requirements and impoundments (Georgia Department Natural Resources 2003, The Nature Conservancy 2005).

Additional threats to this species from agricultural practices include increased bacterica, pathogens and nutrient loads in water systems from animal waste and fertilizers and additional sedimentation and pollution from herbicides and pesticides (Georgia Department Natural Resources 2003). The Altamaha River appears to be comparatively healthy, although there is the realistic threat from cumulative pollution sources (The Nature Conservancy 2005). It is thought that many of the larval hosts for Altamaha mussels have not been identified yet, and the known host fish are at risk from degraded habitat and predation from introduced non-native Flathead Catfish Pylodictis olivaris (The Nature Conservancy 2005).

Conservation Actions [top]

Conservation Actions: A 'system-wide' conservation plan was created by The Nature Conservancy to protect the Altamaha River basin by identifying the stresses and sources of threats to the health of the river and producing action plans in response (The Nature Conservancy 2005). These include a directive to improve the understanding about the various sources of threats to this species by comparing the current distribution to historic records through conducting surveys and developing a research program, which assessed impacts of land use changes such as forestry practices (The Nature Conservancy 2005).

Williams et al. (2010) lists this species as vulnerable according to the American Fisheries Society assessment. Research is recommended into the population status and distribution of this species (it may be found in other undisturbed areas of the Altamaha River) and the threats which are affecting it, in order to effectively assess the efficiency of conservation actions. Monitoring of populations and habitat is recommended.

Citation: Cummings, K. & Cordeiro, J. 2012. Pyganodon gibbosa. The IUCN Red List of Threatened Species. Version 2014.2. <www.iucnredlist.org>. Downloaded on 18 September 2014.
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