First Record of the Invasive Orconectes rusticus (Rusty Crayfish) from the Potomac River, Maryland Jay V. Kilian1.* and Patrick Ciccotto2 Abstract - We report the first record of invasive Orconectes rusticus (Rusty Crayfish) in the Potomac River, MD. Four specimens were collected in June 2010 in the Potomac River downstream from the confluence of Antietam Creek near Sharpsburg, MD. Subsequent surveys in the Potomac River mainstem and three other tributaries from Brunswick to Hancock, MD indicate that this invasive species is not yet widespread in the river. Further spread of Rusty Crayfish in the Potomac River may displace congeneric species including two natives, Orconectes obscurus (Allegheny Crayfish) and Orconectes limosus (Spinycheek Crayfish), and the non-native Orconectes virilis (Virile Crayfish) in Maryland, Virginia, and West Virginia. Orconectes rusticus (Girard) (Rusty Crayfish), a species native to portions of the Ohio River basin, has been widely introduced outside of its native range primarily through its use as bait (Taylor et al. 2007). Now established in at least 18 states as well as Canada, Rusty Crayfish has gained notoriety as an aggressive invasive species (Hobbs et al 1989, Lodge et al. 2000, Olden et al. 2009). Rusty Crayfish has caused local displacement of native crayfishes (Capelli 1982, Lodge et al. 1986) and hybridized with congeneric species (Capelli and Capelli 1980, Perry et al. 2001, Smith 1981). Impacts of Rusty Crayfish on aquatic vegetation, benthic macroinvertebrates, snails, and fishes have also been documented (Hobbs et al. 1989, Houghton et al. 1998, Olsen et al. 1991, Wilson et al. 2004). On 9 June 2010, we conducted surveys to document the distributions of crayfishes in large (4th order and higher) streams in the Valley and Ridge physiographic province, a portion of Maryland that has not been surveyed for crayfishes in nearly 50 years (Kilian et al. 2010). We surveyed a total of 10 sites in the Potomac River and in four tributaries, including Licking, Conococheague, Little Conococheague, and Antietam creeks. During this effort, we documented the first record of the invasive Rusty Crayfish in the Potomac River (Fig. 1). Using a 3-m × 1-m kick seine, we collected four individuals (3 Form II males, 1 female) in a gravelly riffle among emergent vegetation on the Maryland shoreline approximately 100 m downstream of the confluence of Antietam Creek south of Sharpsburg, MD (39.41708 N, 77.74670 W). The four specimens collected from this location ranged in size from 26 to 31mm (mean = 26.8 mm carapace length). These specimens are currently stored at the Maryland Department of Natural Resources in Annapolis, MD. Rusty Crayfish was also collected in high abundance at three sites in the lower reaches of Antietam Creek (Fig. 1). Prior to this survey, the known distribution of Rusty Crayfish in the Maryland portion of the Potomac River basin included the upper Monocacy River near Emmitsburg, where it was discovered in 2007 (Kilian et al. 2010) and Antietam Creek north of Sharpsburg, where it was discovered in 2008 (Ellen Friedman and Neal Dziepak, MDNR Resource Assessment Service, Annapolis, MD, 2010 pers. comm.). Our findings extend the known range of Rusty Crayfish in the basin to include the lower reaches of Antietam Creek south of Sharpsburg and adjacent portions of the mainstem Potomac River. Its invasion of the 1Maryland Department of Natural Resources, Monitoring and Non-tidal Assessment, 580 Taylor Avenue, Annapolis, MD 21401. 2University of Maryland Baltimore County, Department of Biological Sciences, 1000 Hilltop Circle, Baltimore, MD 21250. *Corresponding author - jkilian@dnr. state.md.us. Notes of the Southeastern Nat u ral ist, Issue 10/3, 2011 553 554 Southeastern Naturalist Vol. 10, No.3 Potomac River likely occurred through its downstream dispersal from Antietam Creek, where it has been established for at least several years. We did not collect Rusty Crayfish at other sites sampled in the Potomac River or in the lower reaches of Licking, Conococheague, and Little Conococheague creeks, suggesting that this invasive species is not yet widely dispersed in the river (Fig. 1). The possession of Rusty Crayfish is prohibited in Maryland (Code of Maryland Regulations 08.02.19.04), and the use and possession of all species of live crayfish is currently banned in the portion of the Potomac River basin where Rusty Crayfish is now established. These regulations are intended to slow the spread of this invasive species via Figure 1. Distribution of Rusty Crayfish in the upper Potomac River based on 10 sites sampled in June 2010. 2011 Southeastern Naturalist Notes 555 bait-bucket transfer. However, the Potomac River is now a conduit through which Rusty Crayfish can spread naturally into Virginia, West Virginia, and other Maryland tributaries where it may compete with and displace congeneric species, including the native Orconectes obscurus (Hagen) (Allegheny Crayfish) and Orconectes limosus (Rafinesque) (Spinycheek Crayfish), and the non-native Orconectes virilis (Hagen) (Virile Crayfish). The impacts of Rusty Crayfish in the Potomac River may reach beyond congeneric species. As a top predator and consumer of a wide variety of prey, Rusty Crayfish has the potential to alter the entire trophic web (Charlebois and Lamberti 1996, Hobbs et al. 1989). It has reduced the diversity and abundance of benthic macroinvertebrates (Charlebois and Lamberti 1996, Houghton et al. 1998, McCarthy et al. 2006, Olsen et al. 1991) and aquatic vegetation (Olsen et al. 1991, Peters et al. 2008) in other invaded areas. Impacts on populations of snails (Wilson et al. 2004), unionid mussels (Klocker and Strayer 2004), and fishes (Hobbs et al. 1989) are also possible. Further surveys are necessary to monitor the dispersal of this invasive species and to document its effects on crayfish populations and other biological communities in the Potomac River. Acknowledgments. We thank John Schuster and Adam Eshleman for assistance with field sampling. 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