Adult Pearly Mussels are filter feeders, able to ingest a wide range of particle sizes; algae, detritus and bacteria are all important food sources (Strayer and Jirka 1997).
Formerly widespread in southeastern New York, this species has disappeared from many sites since the 1950's and is now extremely rare in the state. Populations in the Housatonic and Passaic basins have apparently disappeared and surveys of nearly a dozen historical populations throughout the Susquehanna River watershed in 1991 turned up only 1 living animal. Populations in the Shawangunk Kill and Delaware River basins (Lellis 2001) are sparse and limited in extent. Only the Neversink River population currently appears healthy although it also apparently declined by an estimated 38,000 individuals during the mid 1990's (Strayer and Jirka 1997).
The Neversink River, one of the healthiest populations in the entire range, declined by an estimated 38,000 individuals during the mid 1990's (Strayer and Fetterman 1999). In the Susquehanna River basin, they have declined since the 1960s (New York Natural Heritage Program 2019). New records from the Delaware River (Lellis 2001) most likely represent increased survey effort and not a range expansion.
Formerly widespread in southeastern New York, this species has disappeared from many sites since the 1950's and is now extremely rare (Strayer and Jirka 1997).
Freshwater unionid mussels may be North America's most imperiled animals (Lydeard et al. 2004, Strayer et al. 2004). Out of the 50 freshwater mussel species known to occur in New York, 13 have only historical records, 5 species are state-listed as endangered, 3 species are state-listed as threatened, and an additional 17 species have been designated Species of Greatest Conservation Need by New York State Department of Environmental Conservation. Populations of brook floaters have declined dramatically in New York State since the 1950's. The species has been extirpated from the Housatonic and Passaic basins and is nearly lost from the Susquehanna River (Lellis 2001, Strayer 2010). Limited populations still exist in tributaries of the Susquehanna and in the Delaware River basins. Recent surveys of the Shawangunk Kill failed to turn up any brook floaters where previous surveys had found them (Strayer 2001). The only substantial population remaining in the state exists in the Neversink River.
Freshwater mussels are particularly susceptible to threats due to their unique life history traits. They are essentially sedentary creatures that spend their lifetime burrowed part way into a stream or river bottom. They are filter-feeders that opportunistically consume what algae, detritus, and bacteria come their way (Nichols and Garling 2000). Because of this, adults are incapable of avoiding environmental or human impacts such as floods, contaminants, habitat destruction and invasive species.
The dispersal mechanism of the young is another unique factor that makes adapting to a quickly changing environment difficult. Unionid mussels are obligate fish parasites and in order for the young to disperse they must latch on to a fish host to complete their development cycle before dropping off in a new location. Different species have varying adaptations enabling this critical transfer from female mussel to fish to occur. Some species simply broadcast larvae into the water, others release conglutinates, a larval form attractive to fish, that appear as food. In many species, the adult female has lures that appear as a prey item or as another fish and in some species the female mussel may close on the fish to capture it prior to releasing the glochidia to complete the larval transfer. Availability and abundance of host fish species, which may also be in decline, may limit dispersal capabilities and juvenile recruitment, especially for mussel species that are limited to a single or few host fish species. The brook floater may use a variety of host fish species including blacknose dace (Rhinichthys atratulus), longnose dace (Rhinichthys cataractae), golden shiner (Notemigonus crysoleucas), pumpkinseed (Lepomis gibbosus), yellow perch (Perca flavescens), slimy sculpin (Cottus cognatus), and the margined madtom (Noturus insignis). Although there is variety and widespread distribution of host fish species for brook floaters, darter and sculpin species tend to be sensitive to pollutants and their disappearance from a waterbody without other host species present could limit dispersal and juvenile recruitment of brook floaters. More research is needed to determine the most frequently used fish host species in nature to determine whether or not brook floaters are limited by fish host availability (Strayer pers. comm).
Conservation of New York's freshwater mussels will hinge on the abatement of current threats such as those affecting water quality and dispersal. Measures that reduce nonpoint source pollution such as stream and river buffers, sediment control, and limits on lawn chemicals and fertilizer use, as well as, measures enabling host fish movement such as dam removal or installing well-designed fish passages may provide a starting point for recovery. Further inventory of known populations and less surveyed parts of rivers and stream systems is needed, as well as research into the habitat needs and thermal and water quality tolerances of individual species. If populations are unable to recover on their own due to fish host decline, impassable barriers, and poor habitat quality, a captive-release program may be a last option to assist certain species, provided proper rearing techniques, population monitoring and additional research into survival of released individuals are implemented. However, captive-release programs are expensive, can be difficult to implement and won't work for all species. Therefore, implementing strong conservation and management practices while the species still has a chance to recover is the optimal strategy.
Summary: Threats to Brook Floater in New York include altered river flows and loss and fragmentation of habitat, especially from dams; siltation and sedimentation from dams and surface run-off; water pollution; invasive non-native mussels and clams; and hybridization with another mussel species.
The threats to freshwater mussel species in New York are numerous and daunting. Past changes to stream and river hydrology due to dams, channelization, and impoundments have isolated, fragmented, and eliminated historical populations and species. Dams turn free-flowing rivers and streams into reservoirs. These altered habitats have unnatural flow cycles, higher sedimentation, lower oxygen levels, and lower flow rates resulting in lower food availability and often loss of host fish. Altered flow rates from dams or large water withdrawals from hydrofracking could alter water temperatures; these thermal changes can mask temperature cues inhibiting reproduction in some species (Galbraith and Vaughn 2011, Helfrich et al. 2009). More sensitive species in particular are hardest hit and often die out of the impounded sections of their range within a watershed. Changes to the flow rates and sediment budgets in streams and rivers from dams and changes in land use may also destabilize the streambed which is likely very bad for mussels (Strayer pers. comm.).
The past impacts of dams and poor land use practices are still affecting mussel populations and species today. Current and future fragmentation is a major threat. Strayer et al. (2004), in an article discussing the current thinking in mussel conservation, wrote that "negative density dependence can cause sparse populations to continue to decline even after the original cause of decline is removed." This means that long-lived species, such as mussels, that are sedentary and need to be in proximity to reproduce, may persist in decline for many decades beyond the end of the disturbance which will ultimately cause their extinction. Smaller populations of mussels, such as the brook floater in New York, which have become isolated and fragmented over time, may not retain enough genetic diversity to withstand a disease or other random event.
Threats to the future viability of mussel species and populations still remain and require attention. High nutrient loads and contaminants from nonpoint source pollution, particularly from agriculture, are thought to be a threat to mussel survival and growth (Richter et al. 1997). Although research demonstrating a direct link between nonpoint source pollution, water quality, and mussel decline is rare and difficult to document (Strayer and Fetterman 1999), researchers still believe past and present water quality may be a major cause of decline. Bauer (1988, 1992) demonstrated that Eastern Pearlshell (Margaritifera margaritifera) growth and survival was reduced in nutrient-rich streams in Europe. Strayer and Fetterman (1999) found a weak, but perhaps biologically significant, correlation between mussel species richness and water clarity in the Upper Susquehanna River Basin. Water pollution and impoundments appear to be primary threats to brook floaters as well. This species requires a low silt environment with a slow to moderate current, a situation that dams alter both upstream and downstream of the impoundment.
Sedimentation occurs when intensive land use exposes soil that subsequently washes into a nearby waterbody. Increased sedimentation may occur from agriculture, storm water discharge, and construction projects such as bridge and dam work, dredging, and bank clearing (Lisa Holst pers. comm.). The most obvious effect of sedimentation on mussels is that they may actually become buried by the sediment. Increased sediment in the water column can clog the gills of mussels suffocating them (Lisa Holst pers. comm.). An entire mussel bed could be threatened by a single rain event or flood that washes a large amount of exposed soil into an aquatic system. Particular species of mussels are also more susceptible to increased sediment in the water column or on the stream bottom. Brook Floaters require a low silt environment so increased sedimentation is a significant threat to this species.
Exotic freshwater bivalves such as the Zebra Mussel (Dreissena polymorpha), the Quagga Mussel (Dreissena rostriformis bugensis), and the Asian Clam (Corbicula fluminea) may pose significant threats primarily due to their capacity to outcompete native species for oxygen and food resources. Zebra Mussels may also attach to and physically suffocate native mussels. Since Zebra Mussels have been introduced into the Hudson and Mohawk Rivers in New York, they have heavily impacted native mussel populations as well as other invertebrate species. Once they have infested a river system, Zebra Mussels are nearly impossible to stop. Both the Asian Clam and Zebra Mussel are expected to eventually invade all of New York's watersheds although some streams may be too cold or not have enough calcium for them to take hold (Strayer and Ralley 1993, David Strayer pers. comm.).
Climate change may pose a significant threat to certain mussel species. Although some mussels have been negatively affected by cold water releases from dams causing a year of low reproduction, and warming stream temperatures have been shown to benefit some species to a degree, ultimately there is a temperature threshold where species may begin to be impacted by increased water temperature. Mussels may adapt to gradually warming streams but extreme temperatures at either spectrum is what causes large die offs (Hastie et al. 2003). Pandolfo et al. (2010) studied acute lethal thermal tolerances among several mussel species and found that even mildly increasing temperatures can lead to significantly reduced survival in some species; especially affecting glochidia. Although streams rarely reach the temperatures capable of causing large dieoffs in New York, increased temperatures have been shown to cause changes in filtration rate (Loayza-Muro and Elias-Letts 2007), immune response (Chen et al. 2007), and may cause asynchrony in the timing of life history events such as reproduction (Barnett 1972). Increased temperatures in extremes pose a risk directly to survival and minimal increases are an added stressor to declining populations. Pandolfo et al. 2010 also found temperature sensitivity among Brook Floaters in particular. Survival of Brook Floater glochidia was significantly lower at 36C then in the 20C control and significantly lower at 39C and 42C then in all other treatments (Pandolfo et al. 2010).
Brook Floaters may have an additional unique threat to their numbers. Hybridization with A. marginata, a close relative, was suggested by Strayer and Fetterman (1999) as a primary threat in the Susquehanna River basin because many intermediate individuals, not assignable to either species, were found in the mid 1990's. They suggested that anthropogenic habitat changes may have promoted higher levels of hybridization between these formerly distinct species. Long-term studies show that human-caused stream fragmentation disrupts Alasmidonta life cycles, prevents host fish migration, blocks gene flow, and prohibits recolonization, resulting in reduced recruitment rates, decreased population densities, and a higher probability of localized extinctions (Wicklow 2004). Further research into whether hybridization may be impacting Brook Floater populations is a priority research need for this species in New York (Strayer pers. comm.).
Threats must first be recognized and eliminated before populations may even begin to recover. Issues pertaining to water quality, sedimentation, altered hydrology, water temperature, invasive species, and pollution must be identified and dealt with, both in the immediate area, and the entire stream system, perhaps especially including areas upstream from a mussel bed.
Dams can be removed or flow regimes altered to benefit mussels and fish passages can be placed to allow juvenile dispersal by their fish hosts. Dam removal or adapting with well-engineered fish passages would help larval dispersal of Brook Floaters by their fish hosts. More research is also needed regarding the capabilities and frequency by which fish (and Brook Floater fish hosts) are able to cross fish passages.
Increasing and restoring vegetated stream/river buffers will have a triple benefit to New York's most imperiled animal taxa. It will help limit sedimentation, reduce runoff from nonpoint source pollution, such as agricultural waste and lawn chemicals, and provide increased shade to help offset increasing water temperatures due to global climate change. Brook Floaters have particularly limited thermal tolerances and a high sensitivity to warm temperature extremes (Pandolfo et al. 2010). The increased shade provided by stream buffers may help offset rising temperatures due to global climate change. Broadmeadow et al. (2011) found that as little as 20-40% shade may keep summer stream temperatures below lethal limits for brown trout in England. Stream shading may have similar benefits for Brook Floaters and fish host species. Some species of host fish for Brook Floaters, including sculpins and darters, may be especially intolerant of pollutants and so increased stream buffers may ensure the continued availability of these species by helping to protect water quality.
Management of invasive species poses a significant challenge. Implementing an early detection program of Zebra and Quagga Mussels and research into natural predators and other control methods would be highly beneficial, as well as an early detection program and method of stopping early infestations of new species. Of particular concern are the Black Carp (Mylopharyngodon piceus), a fish that eats mussels, and the Golden Mussel (Limnoperna fortunei) an exotic mussel (Strayer pers. comm.). Further research into the findings of Strayer et al. (2010) to determine the cause of the recent stabilization of native mussels in the Hudson River many years after Zebra Mussels were introduced is also important to advancing future control efforts.
Inventory of less accessible parts of streams and rivers is needed and may identify new populations of species of concern. Thorough inventory, including documenting age classes, is essential to monitoring and determining population status. Sampling the whole water body may be necessary to accurately assess status due to the establishment of new populations, metapopulation dynamics, and previously overlooked populations (Strayer and Fetterman 1999). Determining population status and trends of the remaining brook floater populations in New York is a priority research need for this species (Strayer pers. comm.).
Once threats have been abated and stream systems begin to recover, if fish hosts and quality habitat are available, dispersal is unimpeded, and mussel populations have not declined to critical levels, populations may reestablish and a species may begin to recover on its own. However, if this is not the case, then human intervention may be necessary. Recently, research into life history and rearing techniques of some species have made it possible to produce the large number of juveniles necessary for a successful captive-release program, but the implementation of such programs are in early stages of development (Heinricher and Layzer 1999, Henley et al. 2001, Strayer et al. 2004).
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.
There is still much to learn about this species, including diet, age and growth, and mortality factors. Details about habitat requirements (current speed, water depth, substrate grain size, substrate stability, water temperature, and water quality factors) also need further study. However, Strayer and Ralley (1993) found that the distribution of this species was not related to these typical physical habitat qualities, but instead to long-term stability of the substrate (i.e., flow refuges). Both large and smaller scale forces promoting the patchy occurrence of Unionid mussel beds is an active area of research (Strayer et al. 2004).
In the Comprehensive Wildlife Conservation Strategy (CWCS), NYSDEC has reviewed the priority conservation needs in the state and developed goals and action items to this end. The overarching goal for the conservation of freshwater unionids is to: "maintain healthy populations of all native species of freshwater bivalves throughout their historic ranges in New York (New York State 2006)." This is not likely achievable in the near future, considering the impacts of past land use, current threats, and the numerous species that were historically present but are now extirpated from the state. However, this is a strong goal to approach over the long-term.
Important objectives that would benefit freshwater bivalves from the CWCS include developing a management and monitoring program to assess the status of all species of concern throughout the state and to keep up to date with current mussel research. Other important objectives are to understand the current distributions of species across the state, to establish a baseline for trend data, as well as to understand the causes of decline. For species affected by dramatic declines such as the brook floater, continued monitoring to assess population status is warranted and essential to the species persistence.
Actions items in the CWCS developed towards these objectives include research into the life history and habitat requirements of species of concern as well as developing methods to improve and restore habitat. More specifically, research and monitoring of flow requirements, temperature tolerances, fish hosts and food sources is needed. Implementing control measures to limit nonpoint source pollution from agricultural, residential, and industrial areas is essential as is to diminish other degradation factors. To this end, controlling livestock access to creeks and flow alteration were mentioned specifically. Also called for, are an early detection program for invasives, research on exotic bivalve control through natural predators, land acquisition and/or acquisition of development rights in key locations for listed species, and research into ammonia and chemical pollutants that may pose a threat. Continued population monitoring and distribution surveys of at-risk species, and where appropriate, reintroduction efforts, are essential actions to conserving mussel diversity in New York.
Other action items relating to public education include developing a curriculum and fact sheets for public education about mussels, developing an outreach program for private landowners through the Landowner Incentive Program, and educating boaters to slow the transfer of exotic bivalves.
Regulatory and legislative recommendations in the NYS Comprehensive Wildlife Strategy benefiting native freshwater bivalves include a ban on importing fish that feed on native mussels such as the black carp and the inclusion of all life stages of mussels in testing for approval of new pesticides in NYS. Recommendations of stronger water quality regulations and enforcement that would benefit mussels include affording protected stream status to some non-navigable stream segments that include Species of Greatest Conservation Need (SGCN), exploring the issuance of general permits for regulated activities on navigable streams that provide habitat for SGCN, and working to strengthen existing support programs for local government planning and zoning boards to incorporate water quality and land side habitat protections into local regulations (New York State 2006). In addition, many of the watersheds have called for enhanced implementation and enforcement of existing water quality protections including stream buffers and other best management practices (New York State 2006).
There is regional work, as well, that would be highly beneficial to the brook floater. It is imperative to protect the large population of brook floaters occurring in the Neversink River. Land acquisition or easements along the banks of the Neversink has been identified as a highly beneficial priority land protection recommendation in the Delaware watershed by DEC. The Susquehanna Basin also lists land acquisition for freshwater bivalves as a basin-wide priority to protect water quality. Research on population dynamics and flow requirements are listed as high priority data collection needs for freshwater bivalves, especially for the high priority species such as the brook floater in the Susquehanna watershed. Both the Delaware and Susquehanna watersheds prioritize restoration and research into the restoration of degraded habitat to allow for recolonization and reintroduction of imperiled freshwater bivalves. (New York State 2006)
The Brook Floater is strictly a running water species favoring gravelly riffles in creeks and small rivers (Strayer and Jirka 1997).
Formerly widespread in southeastern New York, this species has disappeared from many creeks and rivers and is now extremely rare. In NY, populations in the Housatonic and Passaic basins have mostly disappeared with only a few old dead individual or spent shells found in recent years. In the Susquehanna River basin they have declined. Populations in the Shawangunk Kill and Delaware basin are sparse and limited in extent. Only the Neversink River population appears healthy (Strayer and Jirka 1997).
Brook Floaters were historically found from Nova Scotia to South Carolina in Atlantic drainages. Their present distribution is spotty, especially in the south, including the Potomac drainage in Virginia, and small populations in North and South Carolina. Farther north there are some larger populations (e.g., Maine, New York) and much smaller populations in Massachusetts, Connecticut, and New Hampshire. Although A. varicosa was reported in Rhode Island over 100 years ago, there has not been a documented occurrence in the state since (Wicklow 2004).
The thin shell is subtrapezoidal to subovate and small (<70mm), with a prominent, rounded posterior ridge. The posterior slope has fine, well-defined ridges running across the growth lines. The beak sculpture is coarse and the shape is variable. The periostracum is greenish with dark green color rays that are often continuous. The pseudocardinals are thin and lamellar, with smooth surfaces. The laterals are absent or represented as low, rounded ridges along the hinge line. The nacre is bluish white (Strayer and Jirka 1997).
This species can be recognized by its unique shape, a prominent posterior ridge, and the fine corrugations on the posterior slope (Strayer and Jirka 1997).
Adults of this species 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.
Larvae (glochidia) of this species are parasitic on the gills of several fish species including Blacknose and Longnose Dace, Golden Shiners, Pumpkinseeds, Yellow Perch, Slimy Sculpins, and the Margined Madtom. 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).
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 easiest to locate during late summer when water levels are lowest. This species is a long-term breeder (August to April) (Clarke 1981) and larvae (glochidia) are released into the water from mid-April to May (Wicklow 2004).
The time of year you would expect to find Brook Floater active, reproducing, and larvae present and active in New York.
Alasmidonta varicosa (Lamarck, 1819)
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This guide was authored by: Kelly A. Perkins
Information for this guide was last updated on: April 22, 2019
Please cite this page as:
New York Natural Heritage Program. 2022. Online Conservation Guide for Alasmidonta varicosa. Available from: https://guides.nynhp.org/brook-floater/. Accessed July 1, 2022.