Freshwater Mussels of Virginia (Bivalvia: Unionidae): An Introduction to Their Life History, Status and Conservation

With 77 species, the mussel fauna of Virginia is one of the most diverse in the United States. Fifty-four species or ~70% of the state’s mussel fauna occurs in the rivers of the upper Tennessee River basin, especially in the Clinch and Powell rivers of southwestern Virginia. An additional 23 species reside in rivers of the Atlantic Slope, including the Potomac, Rappahannock, York, James and Chowan basins, and in the New River, a major tributary to the Ohio River. A total of 39 species or 51% of Virginia’s mussel fauna is listed as federally endangered, state endangered or state threatened. Excess sediment, nutrients and various types of pollutants entering streams from agriculture and industries are the main drivers of imperilment. Freshwater mussels reproduce in a specialized way, one that requires a fish to serve as a host to their larvae, called glochidia, allowing the larvae to metamorphose to the juvenile stage. This extra step in their life cycle uniquely defines mussels among bivalve mollusks worldwide, in freshwater or marine environments, and adds significant complexity to their reproductive biology. Further, they utilize “lures” that mimic prey of fishes to attract their host. Mussels rely on their fish host to provide them with long-distance dispersal and nutrition while they are glochidia, which are small (<0.5 mm) ecto-parasites that attach and encyst on the gills and fins of fishes, typically taking weeks to months to metamorphose, excyst and then drop-away as similar-sized juveniles to the stream bottom where they grow into adults. Adult mussels are mostly sedentary animals living in the benthos, i.e., the bottom of streams and lakes, typically in mixed substrates of sand, gravel and fine sediments. Mussels generally filter suspended organic particles <20 μm from the water column but can also filter deposited particles through the shell-gap when burrowed in the benthos. Further, the adults of most species are long-lived, regularly living 25-50 years or longer in freshwater environments throughout North America. Conservation of freshwater mussels in Virginia will require citizens, nongovernmental organizations, local, county, state and federal governments to apply their resources to five main areas: (1) water quality monitoring and 310 VIRGINIA JOURNAL OF SCIENCE regulation enforcement, (2) restoration of stream habitat, (3) restoration of mussel populations, (4) educating the public about the importance and status of mussels, and (5) monitoring and research to understand why mussels are declining and what are the best ways to protect them. Sustained long-term efforts in these five areas offers the greatest potential to conserve freshwater mussels throughout Virginia. INTRODUCTION With 77 documented species, the mussel fauna of Virginia is one of the most diverse in the United States — only the states of Alabama (178 species), Tennessee (129 species), Georgia (123 species), Kentucky (104 species) and Mississippi (84) have more species than Virginia (Neves et al. 1997; Paramalee and Bogan 1998; Williams et al. 2008). Virginia’s mussel fauna spans two major geographic regions, the southwest region where rivers drain to the Mississippi River and ultimately to the Gulf of Mexico, and the eastern region where rivers drain to the Chesapeake Bay and ultimately to the Atlantic Ocean (Figure 1). The species occurring in these two regions generally are restricted to the major river basins of these areas. Hence, their distributions do not overlap and distinct morphological and biological differences exist between the regional faunas. These differences are in part due to the varied ecological and geological conditions that exist throughout Virginia, and the long-term separation of the Atlantic Slope and Mississippi River basin faunas. Nationally, freshwater mussels are considered one of the most imperiled groups of animals in the country, with 213 species (72 %) listed as endangered, threatened, or of special concern (Williams et al. 1993). Virginia’s fauna is no exception, with more than 50% of its species listed at the federal or state level (Figure 2) (Terwilliger 1991). Most of the endangerment is caused by habitat loss and destruction due to sedimentation, water pollution, dredging, and other anthropogenic factors (Neves et al. 1997). Many of these listed species occur in southwestern Virginia in the Clinch, Powell and Holston rivers, headwater tributaries to the Tennessee River (Figure 1). However, nearly all river systems in the state have mussel species of conservation concern. The rate of mussel imperilment in Virginia and nationally is increasing over time as populations of many species continue to decline and as additional species are listed as endangered by the federal government and state governments. Population declines and the listing of many mussel species has prompted interest in their conservation (Freshwater Mollusk Conservation Society 2016). State and federal natural resource management agencies, including Virginia Department of Game and Inland Fisheries (VDGIF) and U.S. Fish and Wildlife Service (USFWS), various non-governmental organizations and universities are involved in improving water quality, stream habitat, and increasing abundance and distribution of mussels using population management techniques, such as out-planting hatchery-reared mussels back to native streams, and monitoring populations to determine their status and trends. For example, Virginia Tech, VDGIF and USFWS have been working together to raise mussels in hatcheries and release them to their native streams to build-up populations. Since 2004, this program has released thousands of mussels of numerous species to population restoration sites throughout Virginia. Most mussels rely on fishes as hosts to metamorphose their larvae to juveniles, and therefore to complete their life cycle. This parasitic relationship uniquely defines FRESHWATER MUSSELS OF VIRGINIA 311 FIGURE 1. Major river drainages of Virginia. Map created by T. Lane, Virginia Tech. freshwater mussels among bivalve mollusks worldwide, both in freshwater and marine environments. The larvae and newly metamorphosed juveniles are very small, typically less than 0.5 mm long. Hence, these stages are considered weak links in the mussel life cycle, as they are susceptible to loss of host fishes, contaminants in streams, and physical disturbance of stream habitats. However, it is this interaction with fishes that makes mussels unique, and evolutionarily has given rise to some of the most complex and striking mimicry known in the natural world. For students of all ages, mussels are a fascinating portal to understanding streams and the incredible organisms that they contain. Thus, the purpose of this paper is to provide an introduction to the life history, status and conservation of freshwater mussels in Virginia. METHODS Occurrence of mussel species in the major river basins of Virginia was determined from publications, reports and personal communications with biologists. However, because mussel surveys and records from the Albemarle, Big Sandy, Eastern Shore and Yadkin basins are sparse to non-existent, species occurrences for these basins were not determined. A mussel species was considered extant in a basin if a live individual was recorded from 1985 to the present. Otherwise, it was considered extirpated or extinct. Species occurrences in the upper Tennessee River basin were determined for the Powell River from Ortmann (1918), Johnson et al. (2012), and Ahlstedt et al. (2016), for the Clinch River from Ortmann (1918), Jones et al. (2014), and Ahlstedt et al. (2016), for the North Fork Holston River from Ortmann (1918), Henley and Neves (1999), and Jones and Neves (2007), for the Middle Fork Holston River from Ortmann (1918), 312 VIRGINIA JOURNAL OF SCIENCE FIGURE 2. Number of species per major aquatic taxon in Virginia. Number of listed species includes species listed as federally endangered, federally threatened, state endangered, and state threatened. Henley et al. (1999), and Henley et al. (2013), and for the South Fork Holston River from Ortmann (1918) and Pinder and Ferraro (2012). Species occurrences in the New River basin were determined from Pinder et al. (2002). Species occurrences in the major Atlantic Slope river basins were determined for the Roanoke, Chowan, James, York, Rappahannock, and Potomac (including its major tributary the Shenandoah River) river basins from Johnson (1970) and personal communication with VDGIF state malacologist Brian Watson. The legal status of listed species, including federally endangered (FE), federally threatened (FT), federal candidate species (FC), state endangered (SE), state threatened (ST) were accessed from VDGIF’s database (last u p d a t e d o n J u l y 1 8 , 2 0 1 4 ) a n d a v a i l a b l e o n l i n e a t : http://www.dgif.virginia.gov/wildlife/virginiatescspecies.pdf. The number and status of fishes in Virginia was obtained from Jenkins and Burkhead (1993), for snails from Johnson et al. (2013) and for crayfishes based on personal communication with B. Watson. The common and scientific names of freshwater mussels generally follow Turgeon et al. (1998). RESULTS A total of 77 mussel species are known from the major river basins of Virginia. Of these, three species (Epioblasma haysiana, E. lenior, and Lexingtonia subplana) and one sub-species (E. torulosa gubernaculum) are considered extinct range-wide, and FRESHWATER MUSSELS OF VIRGINIA 313 four species (Anodontoides ferrusacianus, Leptodea fragilis, L. leptodon, and Villosa fabalis) are considered extirpated from the state, bringing the total extant species in Virginia to 69. From the total species known from the state, 25 are listed as FE, 32 as SE, and six as ST. Since most of the species listed as FE also are listed as SE, the total number of listed mussel species in Virginia is 39, or approximately 51% of the fauna (Figure 2). The Powell, Clinch and forks of the Holston rivers form part of the upper Tennessee River basin (UTRB), and collectively contain a total of 54 mussel species known from the Virginia sections of these rivers (Table 1). This basin contains the highest diversity of mussel species in the state, especially the faunas of the Clinch and Powell rivers, with 53 and 47 known species, respectively. In the Virginia sections of the Holston, a total of 36 species are known from the North Fork, 22 species from the Middle Fork, and 14 species from the South Fork. Due to the extinction or extirpation of 7 species, a total of 47 species remain extant in the UTRB of Virginia. Again, most of these species occur in the Clinch and Powell rivers, with 46 and 37 extant species, respectively. From the total species known from the UTRB in Virginia, 23 are listed as FE, 29 as SE, and 3 as ST. The New River flows northwest from North Carolina, through southwestern Virginia, and into West Virginia, where it becomes the Kanawha River just upstream of Charleston, WV. This large, ancient river system has a depauperate mussel fauna of just 12 species (Table 2). Most of the fauna is derived from the Ohio River drainage system, with similarities to the UTRB. However, the pistogrip (Tritogonia verucossa), while widespread throughout its range, only occurs in Virginia in the New River. No species that occur in the basin are listed as FE but one species is listed as SE (Lasmigona holstonia) and two others as ST (Lasmigona subviridis and T. verucossa). Further, there are no known mussel species extinctions or extirpations from the basin. The rivers of the Atlantic Slope of Virginia collectively contain a total of 24 mussel species (Table 3). All species known from the region remain extant, except L. subplana, which has not been collected alive in the upper James River basin for decades. The Chowan River basin, specifically its tributary the Nottoway River of Virginia, contains the highest diversity with 20 species, followed by the James River with 19 species. The Roanoke River system has 14 recorded species based on collections in the Virginia section of the Dan River. However, at least five additional species (Alasmidonta varicosa, Elliptio congarea, E. fisheriana, E. lanceolata, Uniomerus carolinianus) are known from the nearby section of the river and its tributaries in North Carolina. Thus, additional species may occur in the Virginia section of the river. Two species listed as FE occur in Atlantic Slope rivers of Virginia, Alasmidonta heterodon remains extant in the Po River of the upper York River basin and in the Nottoway River, and Pleurobema collina is extant in several tributaries to the James River basin and in the Dan and Mayo rivers of the upper Roanoke River basin. Additionally, Alasmidonta varicosa (SE) occurs in Broad Run of the Potomac River basin, while Fusconaia masoni (ST) occurs in the James River and several river systems to the south and L. subviridis (ST) is more broadly distributed, known from all major Atlantic Slope river basins in the state. 314 VIRGINIA JOURNAL OF SCIENCE T A B L E 1 . S ci en ti fi c an d c o m m o n n am es o f fr es h w at er m u ss el s pe ci es o cc u rr in g i n m aj o r tr ib u ta ri es t o t h e u p p er T en n es se e R iv er b as in i n V ir g in ia , w h er e F E = fe d er al ly e n d an g er ed , S E = st at e en d an g er ed , S T = st at e th re at en ed a n d = n o fe d er al o r st at e st at u s, T = ex ta n t, * = v er y r ar e in r iv er , X = k n w n f ro m t h e sy st em b u t p o ss ib ly e x ti n ct , E X = k n o w n f ro m s y st em b u t p o ss ib ly e x ti rp at ed , an d N R = n o r ec o rd s o f sp ec ie s fr o m r iv er s y st em . H o ls to n S ci en ti fi c N am e C o m m o n N am e S ta tu s P o w el l C li n h N o rt h