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Epioblasma capsaeformis

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Taxonomy [top]

Kingdom Phylum Class Order Family
ANIMALIA MOLLUSCA BIVALVIA UNIONOIDA UNIONIDAE

Scientific Name: Epioblasma capsaeformis
Species Authority: (Lea, 1834)
Common Name/s:
English Oyster Mussel
Synonym/s:
Dysnomia capsaeformis I. Lea, 1834
Plagiola capsaeformis (I. Lea, 1834)
Taxonomic Notes: It has been suggested that this species may be a senior synonym of Epioblasma florentina walkeri (see Buhay et al. 2002), but molecular, morphological, and life history data from Jones (2004) and Jones et al. (2006) suggest they are distinct. Based on molecular, morphological, and life history data, the population from the Duck River, Tennessee, is proposed as a separate species from Epioblasma capsaeformis populations in the Clinch River, because of distinctiveness of molecular genetic markers, differences in mantle pad coloration and texture, greater height of marsupial expansion of the female shell, smaller glochidial size, differing host fish specificity, and behavioral differences in movement of micro-lures (Jones 2004, Jones et al. 2006). 
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: Endangered A2ac; B1ab(i,ii,iii,iv,v) ver 3.1
Year Published: 2012
Date Assessed: 2012-03-05
Assessor/s: Cummings, K. & Cordeiro, J.
Reviewer/s: Bohm, M., Collen, B. & Seddon, M.
Contributor/s: Richman, N., 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.
Justification:
Epioblasma capsaeformis has been assessed as Endangered under criterion A2ac and B1ab(i,ii,iii,iv,v). This is due to the species' limited extent of occurrence (250-1,000 km2) and the fact that surviving populations (at between 1-20 locations) are severely fragmented in nine tributaries; where it occurs, it is rare with evidence of decline. The great number of threats to this species are ongoing and are resulting in reduced area/quality of available habitat for the species, thus continuing to reduce extent of occurrence, area of occupancy, number of mature individuals and populations. In addition, its range has declined by 80% over the last 25-50 years to a few disjunct occurrences from what was a much more widespread historic distribution. This declines is highly likely to fall within three generations for this species, given what is known about mussel biology. How this relates to population declines is uncertain, but it is likely that 80% range declines lead to at least 50% population declines. There is potential for the discovery of new populations. Further research and the implementation of conservation strategies are suggested for this species.
History:
2000 Critically Endangered
1996 Critically Endangered
1994 Indeterminate (Groombridge 1994)
1990 Endangered (IUCN 1990)
1988 Endangered (IUCN Conservation Monitoring Centre 1988)
1986 Indeterminate (IUCN Conservation Monitoring Centre 1986)

Geographic Range [top]

Range Description: This species is endemic to the Tennessee and Cumberland River drainages. It historically occurred in the Cumberland River drainage downstream of Cumberland Falls. In the Tennessee River drainage, it historically occurred from the headwaters in southwestern Virginia downstream to Muscle Shoals (Williams et al. 2008), and in the Tennessee River across northern Alabama and in some tributaries.

The species is now extirpated from Alabama (the most recent dated record was from the Estill Fork of the Paint Rock River in 1980; Williams et al. 2003), Georgia (Wisniewski pers. comm. 2007) and Copper Creek, Virginia (Hanlon et al. 2009). This extirpation in two of its three range states and at some localities in Virginia represents a range reduction of over 80% over the last 25-50 years (A. Bogan, K. Cummings and J. Cordeiro pers. comm. 2010). The estimated extent of occurrence for the species is 250-1,000 km2, and it is limited to between 1 and 20 severely fragmented locations (NatureServe 2009). Consequently, our estimated extent of occurrence does not include these severe discontinuities within the species range and is therefore significantly smaller than the mapped range extent.

Still, several populations remain in isolated stretches of the Big South Fork (Scott County, Tennessee, and McCreary County, Kentucky) although the identity of these populations may be in question (USFWS 2003, 2004).  Populations are also extant in the Tennessee River system in the Clinch River (Russell and Scott Counties, Virginia and Hancock County, Tennessee), Powell River (Lee County, Virginia), North Fork Holston River (Scott County, Virginia - reintroduced), Nolichucky River (Cocke and Hamblen Counties, Tennessee), and Duck River (Maurey and Marshall Counties, Tennessee); while it may still be extant in Copper Creek (Jones et al. 2001, USFWS 2003, 2004). There is also high potential for the discovery of new populations of the species (J. Cordeiro pers. comm. 2012).
Countries:
Native:
United States (Alabama - Possibly Extinct, Georgia - Possibly Extinct, Virginia)
Range Map: Click here to open the map viewer and explore range.

Population [top]

Population: This species was previously widespread, however it is now confined to a few fragmented locations (K. Cummings pers. comm. 2010). Extirpation in two of its three range states and at some localities in Virginia represents a range reduction of over 80% over the last 25-50 years (A. Bogan, K. Cummings and J. Cordeiro pers. comm. 2010).
Population Trend: Decreasing

Habitat and Ecology [top]

Habitat and Ecology: This species occurs in shoal habitat in small to large rivers, where it occurs in silt-free gravel and sand substrates. It is a long-term brooder, gravid from late summer or autumn until the following summer. Fecundity of a Clinch River population has been reported to average 13,008 glochidia per female per year (n = 10) and to range from 7,780 to 16,876 glochidia per individual (Jones 2004). Males and females of this species have been reported to emerge from the substrate during May and June, when females display paired microlures against bluish white pads within their extrapallial swellings. This behavior in females is presumed to attract glochidial hosts. However, that does not explain the behavior in males, since long-term brooders generally spawn during autumn. The display has been described as rhythmic movements, with the microlure of the left mantle pad rotating in a clockwise, circular manner and that of the right mantle pad rotating counterclockwise (Jones 2004). Females have been observed to snap their valves together on darters that were investigating the lure and trap them between the valves (Jones 2004). This behaviour may facilitate infestation (Williams et al. 1993). The generation length is 5 years (Jones 2009) using age at first reproduction

Fishes reported to serve as glochidial hosts for the species in laboratory trials include Etheostoma blennioides (Greenside Darter), Etheostoma camurum (Bluebreast Darter), Etheostoma flabellare (Fantail Darter), Etheostoma rufilineatum (Redline Darter), Etheostoma simoterum (Snubnose Darter), Etheostoma vulneratum (Wounded Darter) and Percina sciera (Dusky Darter) (Percidae); and Cottus baileyi (Black Sculpin), Cottus bairdii (Mottled Sculpin) and Cottus carolinae (Banded Sculpin) (Cottidae) (Williams et al. 1993).

No specific longevity information is available for this species, but given the fact that thick-shelled mussel species are often very long-lived, that gravid females of this species were aged at around seven to nine years (USFWS 2004), and that the method used to age gravid females, growth ring counting, is likely to underestimate true age between three- to ten-fold (Anthony et al. 2001), we assume that three generations fall within the estimated range declines of 80% over a period of 25-50 years.
Systems: Freshwater

Threats [top]

Major Threat(s): The greatest threat to this species in the Cumberlandian Region is habitat alteration. Principal causes include impoundments, channelisation, pollution, and sedimentation that have altered or eliminated those habitats that are essential to the long-term viability of many riverine mussel populations. Impoundments result in the elimination of riffle and shoal habitats, disruption of a river's ecological processes, elimination of current and the covering of rocky and sand substrates by fine sediments, and alteration of downstream water quality and riverine habitat. Daily discharge fluctuations, bank sloughing, seasonal oxygen deficiencies, cold-water releases, turbulence, high silt loads, and altered host fish distribution have contributed to limited mussel recruitment and skewed demographics. Impoundments, as barriers to dispersal, contribute to the loss of local populations by blocking postextirpation recolonization. Population losses due to impoundments have probably contributed more to the decline of the Cumberlandian Combshell, Oyster Mussel, and Rough Rabbitsfoot and most other Cumberlandian Region mussels than any other single factor (as the Cumberland Elktoe and Purple Bean generally inhabit smaller rivers, impoundments have had less of an impact on them, although the impact is still significant) (USFWS 2004).

The entire length of the Tennessee River and much of the Cumberland River is maintained as a navigation channel with a series of locks and dams - nine on the Tennessee River and four on the Cumberland River. Channel maintenance activities continue to cause substrate instability and alteration in these rivers and may serve to diminish what habitat remains for the recovery of riverine species.

Heavy metal-rich drainage from coal mining and associated sedimentation have adversely impacted many stream reaches, destroying mussel beds and preventing natural recolonisation. Acid mine runoff may be having local impacts on the recruitment of, particularly, the Cumberland Elktoe, since most of its range is within watersheds where coal mining is still occurring. Impacts associated with coal mining activities have particularly altered upper Cumberland River system streams with diverse historical mussel faunas and have been implicated in the decline of Epioblasma species, especially in the Big South Fork. Strip mining continues to threaten mussels in coal field drainages of the Cumberland Plateau with increased sedimentation loads and acid mine drainage, including Cumberland Elktoe and Cumberlandian Combshell populations. The Marsh Creek population of the Cumberland Elktoe has also been adversely affected and is still threatened by potential spills from oil exploration activities. Coal mining activities also occur in portions of the upper Powell and Clinch River systems, primarily in Virginia. Polycyclic aromatic compounds (PAHs) are indicative of coal fines in the bottom sediments of streams. Known to be toxic to mussels and fishes, PAHs have been found at relatively high levels in the upper portions of the Clinch and Powell Rivers in Virginia.

In-stream gravel mining has been implicated in the destruction of mussel populations. Negative impacts include riparian forest clearing (e.g., mine site establishment, access roads, lowered floodplain water table); stream channel modifications (e.g., geomorphic instability, altered habitat, disrupted flow patterns [including lowered elevation of stream flow], sediment transport); water quality modifications (e.g., increased turbidity, reduced light penetration, increased temperature); macroinvertebrate population changes (e.g., elimination, habitat disruption, increased sedimentation); and changes in fish populations (e.g., impacts to spawning and nursery habitat, food web disruptions). Gravel mining activities threaten the Cumberlandian Combshell populations in the Powell River and in Buck Creek, the latter stream representing one of only two remaining populations of this species in the entire Cumberland River system. Mining activities on the Elk River may have played a role in the extirpation of the oyster mussel and Cumberlandian Combshell from that river.

Contaminants contained in point and nonpoint discharges can degrade water and substrate quality and adversely impact, if not destroy, mussel populations. Although chemical spills and other point sources (e.g., ditch, swale, artificial channel, drainage pipe) of contaminants may directly result in mussel mortality, widespread decreases in density and diversity may result, in part, from the subtle, pervasive effects of chronic low-level contamination. Mussels appear to be among the most intolerant organisms to heavy metals, several of which are lethal, even at relatively low levels. Among other pollutants, ammonia has been shown to be lethal to mussels. Common contaminants associated with households and urban areas, particularly those from industrial and municipal effluents, may include heavy metals, ammonia, chlorine, phosphorus, and numerous organic compounds. Nonpoint-source runoff from urban areas tends to have the highest levels of many pollutants, such as phosphorus and ammonia, when compared to other catchments. Agricultural sources of chemical contaminants are considerable and include two broad categories--nutrients and pesticides. Nutrient enrichment generally occurs as a result of runoff from livestock farms and feedlots and from fertilizers used on row crops. Pesticide runoff that commonly ends up in streams may have effects (based on studies with laboratory-tested mussels) that are particularly profound.

Numerous Cumberlandian Region streams have experienced mussel kills from toxic chemical spills and other causes. The high number of jeopardised species in the upper Tennessee River system make accidental spills a particular concern to conservationists and resource managers.

Sedimentation, including siltation runoff, has been implicated as the number one factor in water quality impairment in the United States. Specific biological impacts on mussels from excessive sediment include reduced feeding and respiratory efficiency from clogged gills, disrupted metabolic processes, reduced growth rates, increased substrate instability, limited burrowing activity, and physical smothering. Host fish / mussel interactions may be indirectly impacted by changes in stream sediment regimes through three mechanisms: fish abundance, diversity, and reproduction reduced; impedes host fish attractant mechanisms; interfere with the ability of some species' adhesive conglutinates to adhere to rock particles. Waterborne sediment is produced by the erosion of stream banks, channels, plowed fields, unpaved roads, roadside ditches, upland gullies, and other soil disturbance sites. Agricultural activities produce the most significant amount of sediment that enters streams. Silvicultural sedimentation impacts are more the result of logging roads than the actual harvesting of timber.

Developmental activities associated with urbanisation (e.g., highways, building construction, infrastructure creation, recreational facilities) may contribute significant amounts of sediment and other pollutants in quantities that may be detrimental to stream habitats. With development, watersheds become more impervious, resulting in increased storm-water runoff into streams and a doubling in annual flow rates in completely urbanised streams. Impervious surfaces may reduce sediment input into streams but result in channel instability by accelerating storm-water runoff, which increases bank erosion and bed scouring. Water withdrawals for agricultural irrigation and municipal and industrial water supplies are an increasing concern for all aquatic resources and are directly correlated with expanding human populations. This impact has the potential to be a particular problem for the Cumberland Elktoe population in the Big South Fork system and the Oyster Mussel population in the Duck River.

The alien Asian Clam (Corbicula flumminea) was first reported from the Cumberlandian Region around 1959. This species has been implicated as a competitor with native mussels for resources such as food, nutrients, and space, particularly as juveniles. Densities of Asian Clams are sometimes heavy in Cumberlandian Region streams, making competition with populations of some of these five species likely. Paradoxically, large, seemingly healthy, populations of unionids may coexist with Asian Clams. The invasion of the nonnative Zebra Mussel (Dreissena polymorpha) poses a threat to the mussel fauna of the Cumberlandian Region. Although Zebra Mussels are now in the Tennessee and Cumberland River systems, the extent to which they will impact native mussels is unknown. However, as Zebra Mussels are likely to reach higher densities in the main stems, large tributaries, and below infested reservoirs, native mussels in these areas will likely be more heavily impacted than mussels in smaller streams without upstream reservoirs. Mussel extinctions are expected as a result of the continued spread of Zebra Mussels in the Eastern United States. Other potential threats from alien species on native mussels include the Black Carp (Mylopharyngodon piceus), a native of China. If these species invade Cumberlandian Region streams, they could wreak havoc on already stressed native mussel populations. The Round Goby (Neogobius melanostomus) is another alien invader fish species released in the 1980s into the Great Lakes in ballast waters originating in southeastern Europe. The arrival of Round Gobies may therefore have important indirect effects on unionid communities through negative impacts to their host fishes.

The overall threat to this species, posed by piscine and invertebrate predators, in most instances is not thought to be significant. Although parasitism is not thought to be a significant problem in mussels, excessive trematode infestations in their gonads have been implicated in inducing mussel senescence. The harvest of Cumberlandian Region mussel species for commercial purposes is well documented (Anthony and Downing 2001). It is doubtful, however, that this species has ever been overly exploited for pearling, pearl buttons, cultured pearls, or any other exploitative activity (USFWS 2004).

Without the level of genetic interchange these species experienced historically, many small and isolated populations that are now comprised predominantly of adult specimens may be slowly dying out due to various factors. This may, in part, account for the relatively recent demise of numerous tributary populations. Even given the improbable absence of the impacts from current and existing threats, smaller isolated populations of this species may be lost to the devastating consequences of below-threshold effective population size (EPS). Once-sizable populations of many Cumberlandian mussel species occurred throughout significant portions of the main stems of the large rivers and tributary systems comprising the Cumberlandian Region. This was particularly true for the Cumberlandian Combshell and Oyster Mussel. Historically, there were no natural absolute barriers to genetic interchange among their tributary subpopulations and those of their host fishes (with the notable exception of Cumberland Falls). Without the level of genetic interchange these species experienced historically (because of anthropogenic threats), many small and isolated populations that are now comprised predominantly of adult specimens may be slowly dying out due to various factors (USFWS 2004).

Conservation Actions [top]

Conservation Actions: This species is listed as Endangered throughout its range, except in the free-flowing reach of the Tennessee River from the base of Wilson Dam downstream to the backwaters of Pickwick Reservoir and the lower 5 RM of all tributaries to this reach in Colbert and Lauderdale Counties, Alabama (USFWS 2001). It has also been assigned a NatureServe Global Heritage Status Rank of G1 - Critically Imperiled, due to the historical decline in its range and the fact that it now occurs in only a few disjunct locations (NatureServe 2009).

The USFWS, in cooperation with the State of Tennessee and Conservation Fisheries, Inc., proposes to reintroduce this species into its historical habitat in the free-flowing reach of the French Broad River below Douglas Dam to its confluence with the Holston River, Knox County Tennessee, and in the free-flowing reach of the Holston River below Cherokee Dam to its confluence with the French Broad River (USFWS 2006).

Recently, critical habitat was designated for the Duck River (74 river km occupied, 0 river km unoccupied habitat) in Tennessee, Bear Creek (0 river km occupied, 43 river km unoccupied habitat) in Alabama and Mississippi, Powell River (0 river km occupied, 154 river km unoccupied habitat) in Tennessee and Virginia, Clinch River (242 river km occupied, 0 river km unoccupied habitat) in Tennessee and Virginia, Copper Creek (0 river km occupied, 21 river km unoccupied habitat) in Virginia, Nolichucky River (8 river km occupied, 0 river km unoccupied habitat) in Tennessee, Big South Fork (0 river km occupied, 43 river km unoccupied habitat) in Tennessee and Kentucky, and Buck Creek (0 river km occupied, 58 river km unoccupied habitat) in Kentucky (USFWS 2004).

A recovery plan has been published (USFWS 2004) with the recovery objective of delisting. The Recovery Strategy includes the preservation of extant populations and the occupied habitats of these five species is the most immediate and important recovery priority for these mussels. Preservation and protection of these populations will be achieved by continuing to use existing regulatory mechanisms, establishing partnerships with various stakeholders, using best management practices, and minimising or eliminating threats to the species. Each extant population must also be viable to achieve recovery. Unless a previously unknown population is found, other viable populations within the historic range of each of the five species must be reestablished and protected to effect recovery. Reestablishing new viable populations will require close coordination with and concurrence of the State(s) involved and with other partners that have interests at any potential reintroduction sites. Additional research into the life history and ecological requirements of these mussels as defined in this plan will help formulate the biological information necessary for the preservation of existing or reestablishment and maintenance of other viable populations. Knowledge of the effective population size is particularly critical for determining the size and demographic makeup of a viable population for these species. Due to the rarity of extant populations, propagation of laboratory or hatchery-reared progeny is the most logical means of providing individuals for the establishment of new populations. Facilities that attempt to propagate these mussels should follow the Service's established controlled propagation policy. Priorities for recovery efforts for the five species via propagation should be to develop propagation technology, augment and expand the ranges of extant populations to ensure their viability, and reestablish viable populations in other streams within their historical range that have suitable habitat and water quality. Pursuing and implementing these efforts will enable the recovery of the five species. The Recovery Criteria: Downlisting from endangered to threatened status will occur when the following criteria are met for the protection of extant stream populations, discovery of currently unknown stream populations, and/or reestablishment of historical stream populations: (1) five streams with distinct viable populations of the Cumberland Elktoe, six streams with distinct viable populations of the Oyster Mussel and Cumberlandian Combshell, four streams with distinct viable populations of the Purple Bean, and three streams with distinct viable populations of the Rough Rabbitsfoot have been established; (2) one distinct naturally reproduced year class exists within each of the viable populations; (3) research studies of the mussels' biological and ecological requirements have been completed and any required recovery measures developed and implemented from these studies are beginning to be successful, as evidenced by an increase in population density of approximately 20 percent and/or an increase in the length of the river reach of approximately 10 percent inhabited by the species as determined through biennial monitoring; (4) no foreseeable threats exist that would likely impact the survival of the species over a significant portion of their ranges; (5) within larger streams the species are distributed over a long enough reach that a single catastrophic event is not likely to eliminate or significantly reduce the entire population in that stream to a status of nonviable; and (6) biennial monitoring of the five species yields the results outlined in criterion (1) above over a 10-year period.

Actions Needed:
1. Utilise existing legislation/regulations to protect current and newly discovered populations.
2. Determine the species' life history requirements and threats and reduce or alleviate those threats which threaten the species.
3. Develop and use an information/education program to solicit the assistance of local landowners, communities, and others to recover the species.
4. Search for additional populations, and through propagation activities, pursue augmentations or reintroductions in order to establish viable populations.
5. Conduct anatomical and molecular genetic analysis of the species to determine the potential occurrence of species complexes or hidden biodiversity.
6. Develop and implement a monitoring program, and annually assess the recovery program where needed.

This species occurs in very low numbers within the jurisdiction of the US Park Service in the Big South Fork National River and Recreation Area. It has also been found in the vicinity of TNC's Pendleton Island Preserve, but only dead shells have been reported from the preserve. Survival of populations in such "protected" areas is largely dependent on the continuation of high environmental quality in the watershed upstream from such sites. Such conditions do not exist at either site (USFWS 2004). Nonessessential Experimental Populations (NEPs) have been established in the Tennessee River below Wilson Dam, Colbert and Lauderdale Cos., Alabama, extending13.4 km and including the lower 8 km of all tributaries that enter the Wilson Dam tailwaters (USFWS 2001). Nonessessential Experimental Populations (NEPs) have been proposed for reintroduction into the free-flowing reach of the French Broad River below Douglas Dam (Knox and Sevier Cos., Tennessee) to its confluence with the Holston River, Knox Co., Tennessee, and in the free-flowing reach of the Holston River below Cherokee Dam to its confluence with the French Broad River (Knox, Grainger, and Jefferson Cos., Tennessee), where this species currently does not exist (USFWS 2006). Recently, critical habitat was designated for the Duck River (74 river km occupied, 0 river km unoccupied habitat) in Tennessee, Bear Creek (0 river km occupied, 43 river km unoccupied habitat) in Alabama and Mississippi, Powell River (0 river km occupied, 154 river km unoccupied habitat) in Tennessee and Virginia, Clinch River (242 river km occupied, 0 river km unoccupied habitat) in Tennessee and Virginia, Copper Creek (0 river km occupied, 21 river km unoccupied habitat) in Virginia, Nolichucky River (8 river km occupied, 0 river km unoccupied habitat) in Tennessee, Big South Fork (0 river km occupied, 43 river km unoccupied habitat) in Tennessee and Kentucky, and Buck Creek (0 river km occupied, 58 river km unoccupied habitat) in Kentucky (USFWS 2004).

Above statements referring to the Duck River basin apply to a new undescribed species (Jones et al. 2010).

Further research is required on the taxonomy, population status, ecology and life history of the species, as well as the effectiveness of recovery measures, in order to safeguard this species in the wild. Continued monitoring is recommended.
Citation: Cummings, K. & Cordeiro, J. 2012. Epioblasma capsaeformis. In: IUCN 2013. IUCN Red List of Threatened Species. Version 2013.2. <www.iucnredlist.org>. Downloaded on 20 April 2014.
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