GREEN FLOATER (Lasmigona Subviridis)

GREEN FLOATER (Lasmigona subviridis) Green Floater, credit USGS The green floater is a freshwater mussel that has declined precipitously in population size and distribution over the last 100 years, and is now vulnerable to extinction. Land use changes have increased erosion and sediment runoff, reducing water quality, and harming mussels. The green floater was petitioned for Federal listing under the Endangered Species Act in 2010 and the American Fisheries Society classified it as threatened in 1993. Historically, they were present in 10 states (New York, Pennsylvania, New Jersey, Maryland, West Virginia, Virginia, Tennessee, North Carolina, Georgia, and Alabama) and the District of Columbia (Figure 1). They are now extirpated in two states and the District of Columbia and have had significant declines in abundance and presence in other states (NatureServe 2020). In Maryland, this species is present in only a few watersheds. The green floater is small, typically less than 55 millimeters (mm) in length. It has a subovate or trapezoidal shape and a thin yellowish-brown shell covered in varying amounts of green rays. Younger specimens are typically greener (PNHP 2012). The green floater is also distinct for its interior shell color of whitish to blue, and an interdental tooth. The small size and low abundance of this species can make it more difficult to find during surveys. More research is needed to obtain additional information regarding ecology, genetics, and life history (USGS 2019). 1 Figure 1. This 2020 map demonstrates the distribution of the green floater mussel from Alabama to New York (NatureServe 2020). LIFE HISTORY Like other freshwater mussels, sexual reproduction occurs through the water column. Male specimens discharge sperm into the water, and females passively filter water through the gills where eggs are fertilized and develop into the larval stage called glochidia. This fertilization process typically occurs in mid-summer to fall. However, unlike most freshwater mussel species, green floaters are also known to be hermaphroditic, meaning that female mussels can self- fertilize. For many freshwater mussels, the glochidia are released the following spring-summer back into the water column, where they attach to a host fish for dispersal. The larvae transform on the host fish’s gills into the juvenile stage, before dropping to the river bottom, where they then grow to full-sized adults. Green floaters are thought to use this strategy but also have another strategy, unique among Unionid mussels for reproduction. Green floaters have the ability to transform glochidia to juveniles in their gills without a host fish. Juvenile green floaters are released from the female’s gills fully formed as juvenile mussels. While direct transformation is thought to be used more commonly by green floaters and allows them to reproduce with no need of a host fish, it only allows for downstream dispersal. Attachment of larvae to host fish is the sole method for upstream dispersal. More research is needed to determine the frequency of self- fertilization, juvenile transformation in the gills, and use of host fish. The lifespan of green floaters is quite short, ranging from 3 to 7 years, with an average of 4 years (DECNY 2014). HOST FISHES For most other freshwater mussel species, host fishes are necessary to complete the reproductive cycle of the mussel for dispersal of larvae. However, the green floater is unique in that it does not 2 require a host fish. Juveniles are instead able to form in the adult female mussel. However, in a host fish study conducted by Jones (USFWS, unpublished data), green floater larvae successfully metamorphosed to juveniles while attached to five of the six species tested: mottled sculpin (Cottus bairdii), rock bass (Ambloplites rupestris), central stoneroller (Campostoma anomalum), blacknose dace (Rhinichthys atratulus), and margined madtom (Noturus insignis). Research is still ongoing to identify if their self-sufficient reproductive cycle is typical or if they occasionally use host fish in the wild (USGS 2019). HABITAT Green floaters occupy slow running streams and rivers and can sometimes be found in pool habitat. The green floater is not often found in strong currents. They can be found in a variety of different flowing-water systems, but are more likely to be found where there is stable, fine substrate particles including gravel, sand, and silt (USGS 2019). They are found in backwater or side channels in larger rivers. They require well oxygenated water systems with clean silt, little pollution, and of depths between 1 to 4 feet. They are therefore particularly susceptible to changes in land use that may cause increases in water flow or sediment (DECNY 2014). One challenge with this species is lack of reliability in some historical records. The estimates of historic range are most likely quite conservative, and it is likely that there were historically additional populations in states not mentioned prior such as Delaware, Kentucky, South Carolina, and Ohio. However, unconfirmed records have been debated as misidentifications, possibly Carolina heelsplitter (Lasmigona decorata) or creek heelsplitter (Lasmigona compressa), which are similar in appearance. Additionally, some creeks and streams were not sampled until after declines were noted, making it possible they were extirpated instead of absent (NatureServe 2020). Today over 50 percent of confirmed populations are extirpated. Populations that are extant are dwindling in size and have poor viability. Most sites have on average 1 to 5 live individuals. It is also important to note that biannual surveying of all sites would be beneficial to continue documenting presence, as many sites are not surveyed regularly enough to maintain an understanding of their population size. Two years is frequent enough to gain knowledge, while preventing disturbance from survey efforts (NatureServe 2020). MARYLAND POPULATIONS Maryland green floater populations were historically prominent throughout the Upper, Middle, and Washington Metro area of the Potomac River, as well as tributaries in the larger Potomac River watershed throughout Allegany, Washington, Fredrick, Montgomery, and Carroll Counties (Figure 2). It is likely that this species was once prominent in other creeks nearby that were never surveyed, making its historical range larger than previously thought (CBWP 2016). Survey efforts in the early 2000s depicted population declines and extirpation or presumed extirpation. The few extant populations remaining were small, isolated, and scattered. Most recently, the green floater has been documented in the Potomac River in both Washington and Montgomery Counties, however only one specimen was found at each site. Outside the Potomac River, there are only two recently documented extant populations remaining, Sideling Hill and Licking Creek (USFWS SSA 2020). 3 In September of 2019, Sideling Hill Creek and Licking Creek were surveyed. In Sideling Hill Creek watershed, 25 sites were surveyed where green floaters have been found in the past. Of those 25 sites, only 2 sites had green floaters, and only 11 total specimens were found. This is down from 56 total specimens found in 2006 across 6 sites, though it is worth noting that not all sites were resampled, there were some new sites, and some landowners did not give permission to survey (McCann and Feller 2020). In Licking Creek 13 sites were sampled. Results demonstrated small populations with just three live specimens found at two sites. This is up from 2006, when no specimens were found across nine sites, though worth mentioning that there was again variance in site sampling due to landowners not giving permission and the addition of new sites in 2019 (McCann and Feller 2020). At sites where green floaters were found in 2019, there was overall higher mussel abundance and species richness in comparison to sites where green floaters were not found. Though a onetime observation is not a trend, this could be an interesting focus for future research. Threats to existence in these two watersheds are similar to the many broad threats: impoundments, sediment deposition, pollution, and land use changes. Asian clams (Corbicula fluminea) are widespread throughout both watersheds, though not in pointedly high concentrations (McCann and Feller 2020). Sideling Hill Creek experiences many seasonal changes in hydrology: high flows during storm seasons and snow melts and low flows or no flows during droughts. Changes in hydrology can impact sedimentation and change flow patterns, impacting depth temperature, and more. The report points out that these events are happening more intensively and more frequently due to climate change which may be harmful to green floaters. Another note, was the presence of habitat changes due to beavers. Beavers have impounded at least two green floater sites, potentially leading to their extirpation in these sites. The impact of beavers is inconclusive though as, in two other sites, green floaters were found near recently impounded areas (McCann and Feller 2020). Licking Creek does not have these same seasonal changes, though is still impacted by heavy rains and droughts. Storm events can increase nutrient loads during high flows due to runoff from surrounding livestock and agriculture, reducing water quality, and making green floater habitat unsuitable (McCann and Feller 2020). As a wide track of land in Sideling Hill Creek is now preserved through The Nature Conservancy as a bioreserve, efforts

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