Brook Floater

A Species of Greatest Conservation Need in the MA State Wildlife Action Plan

• Scientific name: Alasmidonta varicosa
• Species of Greatest Conservation Need (MA State Wildlife Action Plan)
• Endangered (MA Endangered Species Act)

Description

The brook floater is a small mussel that rarely exceeds 75mm (3 in) in length. The shape is trapezoidal to almost elliptical, and it has a prominent posterior ridge that gives it a “roman nose” lateral profile (1). The ventral margin (2) is usually flat or slightly indented. The valves are moderately inflated (3), giving it a swollen appearance in cross section. The periostracum (4) is yellowish-green in young animals to brownish-black in mature specimens and usually has prominent green rays (5). Rays are often obscure in heavily eroded or stained shells. The diagnostic feature for this species is a series of corrugations (or raised ridges) along the dorso-posterior slope (6), perpendicular to the growth lines; these corrugations are difficult to discern on shells that are young, eroded, stained, or covered with algae. Pseudocardinal teeth (7) are poorly developed, consisting of one small knob-like tooth on each valve. Lateral teeth (8) are absent. The color of the nacre (9) is variable, ranging from bluish-white to pinkish-white to a pale orange. The foot is usually the striking color of cantaloupe but the intensity of that color is variable. The brook floater has a unique habit of “gaping” (relaxing its adductor muscles and opening its valves) when removed from the water, exposing its cantaloupe-colored foot and mantle cavity.

Brook floater shells (dead animals) can be identified without difficulty, although sometimes are confused with the creeper (Special Concern), which has a similar shape and poorly developed pseudocardinal teeth. Accurate identification of live animals usually relies on the corrugations on the shell, shape of the animal, cantaloupe-colored foot, and its habit of gaping when removed from the water. Live juveniles or highly eroded adult brook floater, triangle floater, and creeper can sometimes be difficult to distinguish. Unlike the brook floater, the triangle floater has a triangular or slightly ovate shape, uneven and coarse sculpturing on the beak (10), and a ventral margin that is more rounded. The triangle floater also has well-developed pseudocardinal teeth. The creeper is more laterally compressed than the brook floater, its pseudocardinal teeth are more poorly developed, and its shell is considerably thinner and more fragile. An expert should be consulted to identify the brook floater because it is listed as endangered in Massachusetts and because the novice may confuse it with two other protected species.

Illustrations by Ethan Nedeau.

Life cycle and behavior

Brook floaters are essentially sedentary filter feeders that spend most of their lives partially burrowed into the bottoms of rivers and streams. Like all freshwater mussels, larvae (called glochidia) of the brook floater must attach to the gills or fins of a vertebrate host to develop into juveniles. Brook floater is considered a host generalist meaning they can metamorphosize on numerous distantly related fish species with varied success. Several fish hosts have been reported under laboratory conditions, including longnose dace, blacknose dace, slimy sculpin, golden shiner, pumpkinseed sunfish, yellow perch, margined madtom, brook trout, and tessellated darter (e.g., Wicklow 2017, Skorupa et al. 2022).

No studies have confirmed how well laboratory results relate to the availability of hosts or parasitism under natural conditions. Given its large number of widely distributed host fish, its rarity is likely due to aspects of their biology and ecology that are unrelated to host availability, such as sensitivity to water quality or habitat conditions.

Distribution and abundance

The brook floater occurs in Atlantic coastal rivers from South Carolina to Nova Scotia and New Brunswick. It is listed as Endangered in Massachusetts, Connecticut, and New Hampshire, and Threatened in Vermont and Maine (Nedeau 2008). It is extirpated in Rhode Island (Raithel and Hartenstine 2006). In Massachusetts, the species occurs in only four rivers in three different watersheds. Based on historic records and relatively recent surveys, it may have been eliminated from four watersheds in Massachusetts. Recent studies indicate that the extant populations in Massachusetts are significantly fragmented, low in density, and prone to mortality due to old age and poor condition. A few patches of brook floaters with densities high enough to be considered viable exist; however, they exhibit a high degree of spatial clustering and are significantly isolated from one another. There is growing concern that some populations have dwindled to the point where reproduction is unlikely and persistence beyond the life span of the remaining individuals is improbable. The persistence of brook floaters in Massachusetts seems to be closely tied to its survival and reproduction within isolated areas that are highly vulnerable to random events such as mortality related to floods, droughts, predators, poorly planned development or disturbance, pollution, or even trampling.

The brook floater is one of the most endangered mussels in northeastern North America. It is listed as Endangered in Massachusetts and protected under the Massachusetts Endangered Species Act (MG.L. c.131A) and its implementing regulations (321 CMR 10.00). 

Distribution in Massachusetts. 
1999-2024
Based on records in the Natural Heritage Database. 

Habitat

The brook floater inhabits streams and rivers of varying sizes that contain low to moderate flow velocities and stable substrates (Nedeau 2008). In fast water, they often will be found clustered in protected areas such as behind boulders and near banks. The brook floater never occurs in lakes or reservoirs but may inhabit the upstream end of small impoundments created by run-of-river dams. Like most other mussel species, the brook floater is sparse or absent in headwater streams and high-gradient river reaches that are prone to scour. It is frequently found in streams that have low calcium levels, low nutrients, cool water temperatures, and good water quality. They are often found in the center portions of a reach and flow transition areas with relatively high and stable sediment size heterogeneity (Skorupa et al. 2024). They usually occur with the triangle floater, eastern pearlshell, creeper, and eastern elliptio.

Healthy habitats are vital for supporting native wildlife and plants. Explore habitats and learn about conservation and restoration in Massachusetts.

Stable run habitat for brook floater.

Threats

Because brook floaters are essentially sedentary filter feeders, they are unable to flee from degraded environments and are vulnerable to the alterations of waterways. Some of the many threats to the brook floater and its habitat in Massachusetts include: nutrient enrichment, sedimentation, point-source pollution, road salts (Skorupa et al. 2024), alteration of natural flow regimes, water withdrawal, encroachment of river corridors by development, non-native and invasive species, habitat fragmentation caused by dams and road-stream crossings, sudden release of water and sediment from dam blowouts, and a legacy of land use that has greatly altered the natural dynamics of river corridors (Nedeau 2008). In addition, the long-term effects of regional or global problems such as acidic precipitation, mercury, and climate change are considered severe but little empirical data relates these stressors to mussel populations. Effects from climate change including extreme flows and sustained droughts, threaten current brook floater populations. As local populations of brook floaters decline and/or become extirpated in response to these threats, dispersal distances between populations increase, weakening overall reproductive success, and ultimately genetic diversity.

Conservation

Survey and monitoring

Standardized surveys are critically needed to monitor known populations, evaluate habitat, locate new populations, and assess population viability at various spatial scales (e.g., stream, watershed, state; Sterrett et al. 2018). Standardized survey efforts in the past 10 years have redocumented the species in known extant watersheds and have failed to find new or historical populations. Survey efforts should continue to search for new populations via physical and potentially eDNA surveys and expand our knowledge on the species distribution in extant watersheds every 5 years. Established long-term monitoring sites (Sterrett et al. 2022) in extant watersheds have acquired critical demographic and trend data suggesting high survival rates. Monitoring these sites should continue to occur annually in multi-year blocks or as needed.

Management

Discovery and protection of viable mussel populations is critical for the long-term conservation of freshwater mussels. Currently, much of the available mussel occurrence data are the result of limited presence/absence surveys conducted at road crossings or other easily accessed points of entry. In addition, regulatory protection under MESA only applies to rare species occurrences that are less than 25 years old. Surveys are critically needed to monitor known populations, evaluate habitat, locate new populations, and assess population viability at various spatial scales (e.g., river, watershed, state) so that conservation and restoration efforts, as well as regulatory protection, can be effectively targeted. Other conservation and management recommendations include: maintain naturally variable river flow and limit water withdrawals; identify, mitigate, or eliminate sources of pollution to rivers; identify dispersal barriers (e.g., dams, impassable culverts) for host fish, especially those that fragment the species range within a river or watershed; identify dispersal barriers (e.g., dams, impassable culverts) for host fish, especially those that fragment the species range within a river or watershed; maintain adequate vegetated riparian buffer; protect or acquire land at high priority sites; restore populations through propagation and reintroduction at historical or suitable streams identified from recent (Skorupa 2022, Skorupa et al. 2024) and future efforts. 

Research needs

Research needs for brook floater include population-level data on survival rate, mortality rate, individual growth rates, population size trends, age at reproduction, sex ratio, and age structure. This data will help develop population viability models to identify when populations may need active restoration via introduction of propagated mussels. Reintroduction of brook floater will require habitat suitability evaluation of historically occupied watersheds and potentially new sites (Skorupa et al. 2022). Additional research is needed within occupied watersheds to address potential current factors limiting distribution and abundance including road salts, ammonia concentrations, beaver activity, and host-fish use (Skorupa et al. 2024). Climate change projections for water temperature and streamflow in occupied and potentially watersheds for brook floater introduction are also needed to assess current and future population risks and identify potential flow refuges during drought and flooding conditions. Further, the role of run-of-river dams supporting relatively high abundances of brook floater and other mussel species downstream needs further investigation.

References

Lefevre, G., and W.C. Curtis. 1911. Metamorphosis without parasitism in the Unionidae.  Science 33: 863-865.

Nedeau, E.J. 2008. Freshwater Mussels and the Connecticut River Watershed. Connecticut River Watershed Council, Greenfield, Massachusetts. xviii+ 132 pp.

Nedeau, E.J., and J. Victoria. 2003. A Field Guide to the Freshwater Mussels of Connecticut. Connecticut Department of Environmental Protection, Hartford, CT.

Nedeau, E.J., M.A. McCollough, and B.I. Swartz. 2000. The Freshwater Mussels of Maine. Maine Department of Inland Fisheries and Wildlife, Augusta, Maine.

Raithel, C.J., and R.H. Hartenstine. 2006. The Status of Freshwater Mussels in Rhode Island. Northeastern Naturalist 13(1): 103-116.

Skorupa, Ayla, "Developing a restoration strategy for Brook Floater (Alasmidonta varicosa) in Massachusetts" (2022). Doctoral Dissertations. 2475. https://doi.org/10.7275/26894339.0 https://scholarworks.umass.edu/dissertations_2/2475

Skorupa, A.J., A.H. Roy, P.D. Hazelton, D. Perkins, T. Warren. 2024. Food, water quality, and the growth of a freshwater mussel: implications for population restoration. Freshwater Science 43(2):107-123.

Skorupa, A.J., A.H. Roy, P.D. Hazelton, D. Perkins, T. Warren, A. Fisk. 2024. Abundance of five sympatric stream dwelling mussels varies with physical habitat. Aquatic Conservation Marine and Freshwater Ecosystems 34(2):e4069.

Sterrett, S. Roy, A., Hazelton P., Watson, B., Swartz, B., Russ T.R., Holst, L., Marchand, M., Wisniewski, J., Ashton, M. and Wicklow, B. 2018. Brook Floater Rapid Assessment Monitoring Protocol. U.S. Department of Interior, Fish and Wildlife Service, Cooperator Science Series FWS/CSS-132-2018, Washington, D.C.

Sterrett, S.C., A.H. Roy, P. Hazelton, B. Swartz, E. Nedeau, J. Carmignani, and A. Skorupa. 2022. Standard Operating Protocol for Mark and Recapture Monitoring of Brook Floater in Streams. U.S. Department of Interior, Fish and Wildlife Service, Cooperator Science Series FWS/CSS-142-2022, Washington, D. C. https://doi.org/10.3996/css67282137

Vaughn, C. 1993. Can biogeographic models be used to predict the persistence of mussel populations in rivers? pp.117-122 in K.S Cummings, A.C. Buchanan and L.M. Koch (eds)., Conservation and Management of Freshwater Mussels: proceedings of a UMRCC symposium, 12-14 October 1992, St. Louis, Missouri. Upper Mississippi River Cons. Com., Rock Island, Illinois. 189 pp.

Date published:	March 28, 2025