Vallisneria L. is a genus of submersed macrophytes that have often been a major target for phytoremediation and restoration efforts (Cao and Ruan Reference Cao and Ruan2015; Korschgen and Green Reference Korschgen and Green1988; Les et al. Reference Les, Jacobs, Tippery, Chen, Moody and Wilstermann-Hildebrand2008). Often, American eelgrass or Vallisneria americana Michx. is a primary species for restoration efforts in the United States due to the natural habitat, forage, and structure it can provide to many ecosystems; however, there has been a steady decline of the species throughout many aquatic systems (Engelhardt et al. Reference Engelhardt, Lloyd and Neel2014). Simultaneously, recent research has determined that there are two native species of Vallisneria in the United States, V. americana and Vallisneria neotropicalis Vict. (southern eelgrass; Les et al. Reference Les, Jacobs, Tippery, Chen, Moody and Wilstermann-Hildebrand2008; Martin and Mort Reference Martin and Mort2023). Thus, most of the current research on Vallisneria has focused on how to restore V. americana to the landscape and reasons why it may be disappearing. However, there has been a sudden and rapid invasion by shortstemmed eelgrass, V. × pseudorosulata S. Fujii & M. Maki (= Vallisneria spiralis L. [tapegrass] × Vallisneria denseserrulata Makino [toothed eelgrass]) which has received increasing attention in the past 5 yr. Vallisneria × pseudorosulata is thought to have been introduced from the aquarium trade as an ornamental Vallisneria plant for sale but was later dumped into a water body and has spread rapidly throughout the U.S. Southeast (Gorham et al. Reference Gorham, Seyoum, Furman, Darnell, Reynolds and Tringali2021; Wasekura et al. Reference Wasekura, Horie, Fujii and Maki2016). It is also thought to have been used in native Vallisneria restoration under the guise of being a native hybrid (King’s Bay Restoration Project, Crystal River, FL). Both parental taxa of V. × pseudorosulata, V. spiralis and V. denseserrulata, are nonnative to the United States and originate from the Eurasian continent (Chen et al. Reference Chen, Chen, Gituru and Wang2012; Gorham et al. Reference Gorham, Seyoum, Furman, Darnell, Reynolds and Tringali2021; Les et al. Reference Les, Jacobs, Tippery, Chen, Moody and Wilstermann-Hildebrand2008; Mesterházy et al. Reference Mesterházy, Somogyi, Efremov and Verloove2021; Wasekura et al. Reference Wasekura, Horie, Fujii and Maki2016). Vallisneria × pseudorosulata has infested the Tennessee Valley Authority (TVA) system; this population is thought to have established sometime between 2018 and 2019 (Gorham et al. Reference Gorham, Seyoum, Furman, Darnell, Reynolds and Tringali2021).

The introduction into the TVA system is not an isolated event, as V. × pseudorosulata has been found in multiple systems in Florida, Alabama, Tennessee, and recently Mississippi. Internationally, V. × pseudorosulata was initially reported in 2016 as present in multiple water bodies throughout Japan (Wasekura et al. Reference Wasekura, Horie, Fujii and Maki2016). The cross is unlikely to have occurred naturally, as both parent species are completely disjunct (Gorham et al. Reference Gorham, Seyoum, Furman, Darnell, Reynolds and Tringali2021; Wasekura et al. Reference Wasekura, Horie, Fujii and Maki2016). Since this hybridization, V. × pseudorosulata has been distributed widely through the aquarium trade under the names V. spiralis, Vallisneria ‘Rock Star’, and potentially other common names (Martin and Mort Reference Martin and Mort2023; Padilla and Williams Reference Padilla and Williams2004; Wasekura et al. Reference Wasekura, Horie, Fujii and Maki2016). Gorham et al. (Reference Gorham, Seyoum, Furman, Darnell, Reynolds and Tringali2021) confirmed V. × pseudorosulata primarily in Florida, with one site in Alabama, the present report confirms several other sites where V. × pseudorosulata is found in the U.S. Southeast. There is little understanding about V. × pseudorosulata and its ecological interactions with other organisms. However, there is anecdotal evidence that V. × pseudorosulata is rapidly displacing the invasive species hydrilla [Hydrilla verticillata (L. f.) Royle], prompting major concern from resource managers (Gorham et al. Reference Gorham, Seyoum, Furman, Darnell, Reynolds and Tringali2021; Wetzel Reference Wetzel2020). Whether or not H. verticillata is being displaced by V. × pseudorosulata, it is incredibly clear that V. × pseudorosulata can rapidly become the dominant plant within a water body (Gorham et al. Reference Gorham, Seyoum, Furman, Darnell, Reynolds and Tringali2021). It is currently hypothesized that V. × pseudorosulata grows as an evergreen perennial and can form large floating mats of propagules, upward of 16 km (10 miles), which has led to its rapid spread (Anonymous 2023; Plotka Reference Plotka2023; Sapp Reference Sapp2024). The propagules are capable of floating for long stretches and may be why Mississippi was recently invaded by V. × pseudorosulata. The population of V. × pseudorosulata found in Mississippi was discovered in June 2024 in Pool E (Prentiss County, near Belmont) of the Tennessee-Tombigbee Waterway, a system that is connected to the TVA system, flows into Mobile River, and ultimately drains into the Gulf of Mexico at Mobile Bay (Schmid and Magandy 353 MISSA039771; Table 1). This is the first report to document the newest population of V. × pseudorosulata as well as the distance of spread from the sampling location Gorham et al. (Reference Gorham, Seyoum, Furman, Darnell, Reynolds and Tringali2021) reported in Lake Guntersville.

Table 1. Collection and location details of Vallisneria × pseudorosulata collected in the United States in 2023 and 2024 ^a .

^a All specimens have been submitted and digitized in the Mississippi State University herbarium (MISSA).

Morphologically, many Vallisneria species are incredibly similar with few diagnostic characteristics; this crypticity equally applies to V. × pseudorosulata, which can impede early detection (Les et al. Reference Les, Jacobs, Tippery, Chen, Moody and Wilstermann-Hildebrand2008). Vallisneria × pseudorosulata looks similar to V. americana, growing as a rosette with long, ribbon-like leaves and reduced floral characteristics that can make in-field identification difficult (Les et al. Reference Les, Jacobs, Tippery, Chen, Moody and Wilstermann-Hildebrand2008; Martin and Mort Reference Martin and Mort2023). Often, V. × pseudorosulata can be identified by a bright green lacunal band running up the midvein to the leaf tip; however, this can be difficult to determine on young leaves or if water clarity is an issue (Figures 1D and 2). Another characteristic, and the most diagnostic, is a partially lignified stem that V. × pseudorosulata will grow just above the soil line (Wasekura et al. Reference Wasekura, Horie, Fujii and Maki2016; MGG, personal observation). This stem structure was described by Wasekura et al. (Reference Wasekura, Horie, Fujii and Maki2016) that grows on mature plants, but not every V. × pseudorosulata individual has shown this in the field (MGG, personal observation). This stem can be 1 to 12 cm in length and typically has small, but apparent internodes with adventitious roots growing from the nodes (Figure 1G and 1H). Currently it is highly recommended that if a Vallisneria population should look similar to V. × pseudorosulata, then genetic testing should be done for confirmation. Genetic testing is highly accurate, and assays have been developed to distinguish V. × pseudorosulata from V. americana and V. neotropicalis (Martin and Mort Reference Martin and Mort2023; Tringali et al. Reference Tringali, Gorham, Seyoum, Puchulutegui, Bass, Furman and Mallison2023). Many Vallisneria species typically reproduce via runners that have been observed growing above and below the soil line, often with upward of 10 to 20 daughter plants (Korschgen and Green Reference Korschgen and Green1988; Martin and Mort Reference Martin and Mort2023; McFarland and Shafer Reference McFarland and Shafer2008). Anecdotally, these runners have been observed detaching from the parent plant and floating long distances before settling in the sediment again. Vallisneria spp. are dioecious, but V. × pseudorosulata has only been observed with pistillate flowers, leading researchers to hypothesize it does not reproduce sexually (Gorham et al. Reference Gorham, Seyoum, Furman, Darnell, Reynolds and Tringali2021; Martin and Mort Reference Martin and Mort2023; Figure 1I). Preliminary observations have also shown that V. × pseudorosulata can handle low to freezing temperatures, allowing rapid establishment in the spring (MGG, personal observation). Vallisneria × pseudorosulata presents not only a unique challenge in the field with identification, but the entire genus of Vallisneria has presented multiple complexities for systematists, taxonomists, and evolutionary biologists. Particularly, species like ribbonweed, Vallisneria australis S.W.L. Jacobs & Les and V. neotropicalis are still debated in many circles and often get lumped under the name of a congener (Les et al. Reference Les, Jacobs, Tippery, Chen, Moody and Wilstermann-Hildebrand2008; Jacobs and Frank Reference Jacobs and Frank1997).

Figure 1. (A) The typical growth form of Vallisneria × pseudorosulata as a rosette with long, thin leaves. (B) Asexual reproduction usually takes form of stolons that extend from the rosette base. (C) When established, stolons with many small ramets can extend more than 30 cm. (D) V. × pseudorosulata leaves are denoted by a bright green lacunal band that follows the midvein. (E) Leaf tips for V. × pseudorosulata are obtusely angled and appear rounded. (F) Serrations are small and dense along leaf margins. (G) Stem formation by V. × pseudorosulata during experimentation in mesocosms. (H) Chains of ramets can form in both still and moving water, which can allow for new establishment once the ramets sink. (I) Pistillate flowers of V. × pseudorosulata, which are the only flower type currently found.

Figure 2. (A) A comparison between Vallisneria × pseudorosulata (left), Vallisneria americana (center), and Vallisneria neotropicalis (right). (B) A comparison of the leaf colorations between V. × pseudorosulata (left), V. americana (center), and V. neotropicalis (right). Both V. americana and V. neotropicalis leaves can have different colorations based on the growing depth, but may be typically darker than V. × pseudorosulata leaves.

The evolutionary and taxonomic history of Vallisneria is thought to be another major contributor to the confusion surrounding the invasive V. × pseudorosulata. Lowden (Reference Lowden1982) sought to create a taxonomic tree using floral characteristics specifically, thus leading to only two species of Vallisneria, V. spiralis and V. americana. However, reevaluations of the genus in both 2008 and 2023 determined genetically that there are most likely 12 to 16 species scattered throughout the world (Les et al. Reference Les, Jacobs, Tippery, Chen, Moody and Wilstermann-Hildebrand2008; Martin and Mort Reference Martin and Mort2023). In both 2008 and 2023, arguments were made for the distinction of two native Vallisneria species in North America, V. americana and V. neotropicalis (Les et al. Reference Les, Jacobs, Tippery, Chen, Moody and Wilstermann-Hildebrand2008; Marie-Victorin Reference Marie-Victorin1943; Martin and Mort Reference Martin and Mort2023). Vallisneria americana is well understood and has presence throughout the United States. However, there is little knowledge about the range and biology of V. neotropicalis. Both native species have been observed co-occurring with V. × pseudorosulata, which again can make in situ identification difficult (Figures 2 and 3). These co-occurrences have been genetically confirmed through ITS sequencing with the sampled populations found in Figure 3. We recommend that systems connected to known populations of V. × pseudorosulata be monitored for invasion, and when new populations of Vallisneria are observed, they should be examined for an elongated stem and, ideally, genetically confirmed. Current invasions have taken over impressively large areas of the TVA system, most recently in Mississippi, suggesting further spread throughout the Tennessee River system.

Figure 3. A map displaying known locations of Vallisneria × pseudorosulata in the U.S. Southeast. The circles represent approximate sampling locations from Gorham et al. (Reference Gorham, Seyoum, Furman, Darnell, Reynolds and Tringali2021), and the squares represent the population extent being reported here.

The current observations of V. × pseudorosulata tied with how little information is available at present raise great concern for further spread throughout the U.S. Southeast. Vallisneria × pseudorosulata was first recorded in the United States recently; however, the rate of its invasion and spread is of major concern. Gorham et al. (Reference Gorham, Seyoum, Furman, Darnell, Reynolds and Tringali2021) previously reported only one geographic point in Lake Guntersville where V. × pseudorosulata is found; however, our report shows spread both upstream and downstream. Our report is also the first to have visual representations of the structures that make V. × pseudorosulata unique for identification and to describe problems with in-field identification. There is a dearth of cohesive research for many of the species in Vallisneria, including V. × pseudorosulata, therefore, future studies should strive toward creating comprehensive information focused on ecological impacts, biological traits, and management of V. × pseudorosulata.