Green Floater

Lasmigona subviridis (Conrad, 1835)

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Class
Bivalvia (Bivalves)
Family
Unionidae (Unionid Mussels)
State Protection
Threatened
Listed as Threatened by New York State: likely to become Endangered in the foreseeable future. For animals, taking, importation, transportation, or possession is prohibited, except under license or permit. For plants, removal or damage without the consent of the landowner is prohibited.
Federal Protection
Proposed Threatened
Proposed for Listing as Threatened under the Federal Endangered Species Act
State Conservation Status Rank
S2
Imperiled in New York - Very vulnerable to disappearing from New York due to rarity or other factors; typically 6 to 20 populations or locations in New York, very few individuals, very restricted range, few remaining acres (or miles of stream), and/or steep declines.
Global Conservation Status Rank
G3
Vulnerable globally - At moderate risk of extinction due to rarity or other factors; typically 80 or fewer populations or locations in the world, few individuals, restricted range, few remaining acres (or miles of stream), and/or recent and widespread declines.
Contents
  1. Summary
  2. Conservation and Management
  3. Habitat
  4. Range
  5. Identification Comments
  6. Taxonomy
  7. Additional Resources
  8. About This Guide

Summary

Did you know?

Green Floater is one of the few freshwater bivalve mussels that do not require a host fish for larvae (glochidia) to mature. Typically, females release glochidia that then attach to the gills or skin of the host fish. After the juvenile metamorphose, they drop off the fish and settle in the sediment of the waterbody. However, Green Floater is one of the few mussel species that is also capable of holding glochidia until they metamorphose without the need for a host fish (Barfield and Watters 1998, Lellis and King 1998).

State Ranking Justification

Declines have been noted throughout the Green Floater range (Strayer and Jirka 1997), including in New York State. Surveys since 1970 resulted in finding live individuals or fresh shells in fifteen waterbodies in the Susuquehanna basin, Genesee basin, and Upper Seneca basin (New York State Department of Environmental Conservation 2020, Mahar and Landry 2013). It appears to be no longer found in the Hudson River, Mohawk River, or Erie Canal (New York State Department of Environmental Conservation 2020, Strayer and Jirka 1997).

Short-term Trends

Many mussel populations have been declining rangewide. However, in New York, it is difficult to determine using historical data because there are mostly opportunistic reports instead of statewide or regional surveys (Mahar and Landry 2013). Green Floater used to be more widespread in New York with this species appearing to be absent from the Hudson River, Mohawk River, and Erie Canal for many years (Strayer and Jirka 1997). There are still numerous extant locations in the Schoharie River basin; some populations have few individuals.

Conservation and Management

Threats

The single most important cause of the decline of freshwater mussels during the last century is the destruction of their habitat by siltation, dredging, channelization, impoundments, and pollution. A healthy fish assemblage is critical to viable mussel populations, and dams have resulted in heavy losses of mussels, mainly due to elimination of host fish species. Erosion due to deforestation, poor agricultural practices, and the destruction of riparian zones causes an increase in siltation and shifting substrates that can smother mussels. Domestic sewage, effluents from paper mills, tanneries, chemical industries and steel mills, acid mine runoff, heavy metals, and pesticides have all been implicated in the destruction of native mussel fauna (Mahar and Landry 2013, Strayer et al. 2004).

Conservation Strategies and Management Practices

Development, construction, and other activities can have deleterious impacts on freshwater mussel populations when they lead to increased sediments in the water channel; to alterations in substrate, water levels, flow velocity, or water temperature; or to disruptions to the populations and movements of host fish. Projects which could have these kinds of impacts on rivers and streams with known or potential populations of rare mussels should endeavor first to avoid these impacts. While guidelines and recommendations for specific sites or activities cannot be provided here, there are some general approaches to consider.

For activities outside the stream channel, prevent any discharge, runoff or erosion from the project site from adding sediments or chemicals to the stream, both during construction and after the project is completed. Maintain pre-project volumes and patterns of surface water runoff after the project is completed. To protect stream habitats and water quality from the cumulative effects of development, maintain and restore floodplains, riparian corridors, and forested watersheds as much as possible; minimize impervious surfaces; and locate land uses requiring applications of pesticides and fertilizers away from streams and rivers.

For activities which require work in the stream channel itself, such as work on bridge supports, maintain water flow and water levels over any mussels. For example, coffer dams to wall off the construction zone from the stream current should be deployed to wall off as small an area as possible and to keep water flowing over mussels.

Often, mussels may have been recorded upstream and downstream from a project site, but not from the stretch of stream or river where the project is located. That particular stretch often has not been surveyed for mussels, but may support rare mussels if the substrate and other habitat parameters are suitable. Project sponsors can either proceed assuming there are rare mussels present, or they can sponsor surveys. If surveys are conducted to determine the presence and distribution of mussels in the area potentially affected by a project, the surveys should be conducted by qualified biologists using standardized methods, and the area surveyed should include downstream and upstream of the project site and its impact zone (e.g., 200 meters downstream and 100 meters upstream).

If the mussel bed itself must be uncovered, then moving the mussels can be considered. However, the success rate of mussel translocation is low, often leading to high mortality, and this approach should be reserved as a last resort. Such efforts require that a suitable target site with appropriate habitat and water conditions be identified; areas where other individuals already occur are good candidates. There are no standardized transfer methods. Post-move monitoring is necessary to evaluate the success of the translocation. Moving mussels has generally been attempted with only a few individuals at a time. If a large mussel bed or many individuals will be impacted by a project, redesigning the project to avoid impacts is preferable to attempting a large-scale translocation with uncertain results.

Research Needs

The host fish speices are unknown (Strayer and Jirka 1997, Mahar and Landry 2013). Additional research is needed to determine which fish species are important to the Green Floater life cycle.

Habitat

Habitat

Green Floater is found in calm waters of large creeks and small rivers with good water quality in New York (NatureServe 2022, Strayer and Jirka 1997). Waters that are less likely to be affected by flooding or droughts are preferred (NatureServe 2022, Strayer 1993). NatureServe (2022) states that this species is found in water depths ranging from 1 to 4 feet in substrates of gravel or sand.

Associated Ecological Communities

  • Confined river (guide)
    The aquatic community of relatively large, fast flowing sections of streams with a moderate to gentle gradient.
  • Unconfined river* (guide)
    The aquatic community of large, quiet, base level sections of streams with a very low gradient.

* probable association but not confirmed.

Associated Species

  • Elktoe (Alasmidonta marginata)
  • Yellow Lampmussel (Lampsilis cariosa) (guide)
  • Creeper (Strophitus undulatus)

Range

New York State Distribution

Green Floater is currently confined to the Susquehanna, Genesee, and Upper Seneca (Finger Lakes) basins (New York State Department of Environmental Conservation 2020, Strayer and Jirka 1997). Since 1970, this species has been found in the following waterbodies: Susquehanna River, Unadilla River, Chenango River, Tioughnioga River, Chemung River, Sangerfield River, Catatonk Creek, Fivemile Creek, Cohocton River, Genesee River, Black Creek, Honeoye Creek, Oatka Creek (New York State Department of Environmental Conservation 2020, Mahar and Landry 2013), Fall Creek, and Crane Brook (New York State Department of Environmental Conservation 2020). Historically, it was found in the Mohawk River, Hudson River, and Erie Canal (Strayer and Jirka 1997).

Global Distribution

Green Floater occurs in the Atlantic Slope from North Carolina and Tennessee north to New York (Clarke 1985). The southern edge of range has been indistinct as Johnson (1970) previously reported specimens from South Carolina and North Carolina as Lasmigona subviridis which are now recognized as Lasmigona decorata (Clarke 1985). It is also known from the Interior Basin, specifically the Kanawha drainage system above Kanawha Falls in Virginia, West Virginia, and North Carolina (Clarke 1985). Note a discontinuity exists in the genetic population structure between populations inhabiting the Susquehanna and Potomac Rivers and more southern populations (King et al. 1999). Historical 19th century records are known from Alabama and Georgia in the Chattahoochee River (Mirarchi et al. 2004; Williams et al., 2008) but this different variant is now extirpated in the Apalachicola basin (Brim Box and Williams 2000). There is some question as to whether historical records from Kentucky are actually Leptodea fragilis.

Best Places to See

  • Susquehanna River (Broome, Chenango, Tioga Counties)

Identification Comments

Identifying Characteristics

Green Floater is a somewhat small (less than 65 mm long), fragile mussel with an ovate to subtrapezoidal-shaped shell (Strayer and Jirka 1997). The periostracum (outer shell) is a yellowish-brown to greenish with many dark green rays (Pennsylvania Natural Heritate Program 2022, Strayer and Jirka 1997). The narce (inner shell) is white (Strayer and Jirka 1997) with the posterior being iridescent and bluish or greenish (NatureServe 2022). Individuals have delicate hinge teeth (Strayer and Jirka 1997). The beak is described as "conspicuous and double-looped" by Strayer and Jirka (1997).

Characters Most Useful for Identification

The shape (ovate to trapezoidal), size, and color (yellowish-brown with green rays) of the outer shell are most helpful in identification.

Best Life Stage for Proper Identification

Adult.

Behavior

Green Floater adults are sessile with only limited movement in the substrate. Passive downstream movement may occur when they are displaced from the substrate during floods. More major dispersal occurs while glochidia are encysted on their fish hosts. Being ectothermic, activity levels of mussels are thought to be greatly reduced during colder months of the year.

Diet

Adult Pearly Mussels are filter feeders, able to ingest a wide range of particle sizes; algae, detritus and bacteria are all important food sources. Mussels in turn are eaten by muskrats, raccoons, fish, and birds (Strayer and Jirka 1997). Larvae (glochidia) of this species are parasitic on the gills or skin of unknown host fish species.

Best Time to See

Little is known about the activity periods of Unionid mussels but they are presumed to be greatly reduced during cold times of the year. Freshwater mussels are typically easier to locate when water levels are lowest (i.e., late summer).

  • Present

The time of year you would expect to find Green Floater present in New York.

Similar Species

  • Creek Heelsplitter (Lasmigona compressa)
    Green Floater is smaller, more ovate, and has a thinner shell than Creek Heelsplitter (Strayer and Jirka 1997). The interdental tooth is also smaller (Strayer and Jirka 1997). Note that there is some range overlap of these two species in New York State (New York State Department of Environmental Conservation 2020).

Taxonomy

Green Floater
Lasmigona subviridis (Conrad, 1835)

  • Kingdom Animalia
    • Phylum Mollusca (Mollusks)
      • Class Bivalvia (Bivalves)
        • Order Unionoida (Freshwater Mussels)
          • Family Unionidae (Unionid Mussels)

Additional Resources

References

Barfield, M.L. and E.T. Watters. 1998. Non-parasitic life cycle in the green floater, Lasmigona subviridis (Conrad, 1835). Triannual Unionid Report, 16: 22.

Brim Box, J. and J.D. Williams. 2000. Unionid mollusks of the Apalachicola Basin in Alabama, Florida, and Georgia. Alabama Museum of Natural History Bulletin, 21: 1-143.

Burch, J.B. 1975. Freshwater unionacean clams (mollusca: pelecypoda) of North America. Malcological Publications. Hamburg, Michigan. 204 pp.

Clarke, A.H. 1985. The tribe Alasmidontini (Unionidae: Anodontinae) part II: Lasmigona and Simpsonaias. Smithsonian Contributions to Zoology, No. 399. 75 pp.

Clarke, A.H. and C.O. Berg. 1959. The freshwater mussels of central New York. Cornell University Agricultural Experiment Station Memoir 367.

Johnson, R.I. 1970. The systematics and zoogeography of the Unionidae (Mollusca: Bivalvia) of the southern Atlantic slope region. Bulletin of the Museum of Comparative Zoology, Harvard University 140(6):263-449.

Kerfel, Eugene A. 1990. Lasmigona subviridis (Conrad, 1835) Green Floater. pages 63-65. [in] A Report on the conservation status of North Carolina's freshwater and terrestrial molluscan fauna. Scientific Council on Freshwater and Terrestrial Mollusks. Raleigh, NC. 283 pages.

King, T.L., M.S. Eackles, B. Gjetvaj, and W.R. Hoeh. 1999. Intraspecific phylogeography of Lasmigona subviridis (Bivalvia: Unionidae): conservation implications of range discontinuity. Molecular Ecology 8: 565-578.

Kitchel, Lisie 1991. Green Floater, Lasmigona subviridis (Conrad, 1835). pages 269-270. [in:] J.N. McDonald and T. Skwara, eds. Virginia's Endangered Species: Proceedings of a Symposium/Coordinated by Karen Terwilliger. The McDonald and Woodward Publishing Company, Blacksburg, Virginia.

Lellis, W.A. and T.L. King. 1998. Release of metamorphosed juveniles by the green floater, Lasmigona subviridis. Triannual Unionid Report, 16: 23.

Letson, E. J. 1905. Checklist of the Mollusca of New York. Bulletin. No. 88. New York State Museum, Albany, NY.

Mahar, Amy and Jenny Landry. 2013. New York State Department of Environmental Conservation species status assessment for Lasmigona subviridis (Green Floater).

Mirarchi, R.E., J.T. Garner, M.F. Mettee, and P.E. O'Neil. 2004b. Alabama wildlife. Volume 2. Imperiled aquatic mollusks and fishes. University of Alabama Press, Tuscaloosa, Alabama. xii + 255 pp.

NatureServe. 2022. NatureServe Explorer [web application]. NatureServe, Arlington, Virginia. Available https://explorer.natureserve.org/. (Accessed: March 1, 2022).

New York Natural Heritage Program. 2023. New York Natural Heritage Program Databases. Albany, NY.

New York State Department of Environmental Conservation. 2020. ArcGIS layer package on mussel records in New York State.

Ortmann, A.E. 1919. Monograph of the naiades of Pennsylvania. Part III. Systematic account of the genera and species. Memoirs of the Carnegie Museum 8(1):1-385.

Strayer, David L. 1993. Macrohabitats of freshwater mussels (Bivalvia: Unionacea) in streams of the northern Atlantic Slope. Journal of the North American Benthological Society 12: 236-246.

Strayer, David L. and K.J. Jirka. 1997. The Pearly Mussels (Bivalva: Unionoidea) of New York State. New York State Museum Memoir 26. The New York State Education Department.

Strayer, David L., J.A. Dowling, W.R. Haag, T.L. King, J.B. Layzer, T.J. Newton and S.J. Nichols. 2004. Changing perspectives on Pearly Mussels, North America's most Imperiled Animals. BioScience 54:429-439.

Williams, J.D., A.E. Bogan, and J.T. Garner. 2008. Freshwater Mussels of Alabama & the Mobile Basin in Georgia, Mississippi & Tennessee. University of Alabama Press: Tuscaloosa, Alabama. 908 pp.

Links

About This Guide

This guide was authored by: Shaw, Hollie Y.

Information for this guide was last updated on: June 30, 2022

Please cite this page as:
New York Natural Heritage Program. 2023. Online Conservation Guide for Lasmigona subviridis. Available from: https://guides.nynhp.org/green-floater/. Accessed December 8, 2023.