This article was researched and written by a student at the University of Massachusetts, Amherst participating in the Encyclopedia of Earth's (EoE) Student Science Communication Project. The project encourages students in undergraduate and graduate programs to write about timely scientific issues under close faculty guidance. All articles have been reviewed by internal EoE editors, and by independent experts on each topic.
Freshwater mussels (Family Unionidae) populations are declining globally due to alteration in habitat, contamination, climate change and the introduction of exotic species. They are the most imperiled group of species in North America. From a group of almost three hundred species, 7% have become extinct in the last fifty years, and 65% are threatened or endangered. This significant loss of benthic biomass may result in large scale alterations of freshwater ecosystem processes and functions.
Importance of freshwaterm mussels to stream ecosystems
Freshwater mussels are important to food webs, water quality, nutrient cycling, and habitat quality of freshwater ecosystems. They spend their lives either fully or partially buried in sediment, usually only moving to seek conditions more favorable to survival. Species distributions depend on their biology and habitat preference, distribution of fish hosts, and environmental constraints. Freshwater mussels are found in permanent aquatic habitats such as streams, rivers, and lakes. Relatively stationary, these filter feeding mussels remove suspended algae, bacteria, zooplankton, and phytoplankton from the water column redistributing it in the form of feces and pseudofeces biodeposits. Freshwater mussels filter small particles that are largely unavailable to other organisms and convert them to larger particles that can be consumed by a greater diversity of animals. The cycling of particulate matter stimulates benthic productivity by providing a significant food source rich in dissolved nutrients that increases the abundance of benthic invertebrates and provides nutrients for primary producers. Filter-feeders can also have a positive effect on water quality.
Mussels are an important source of food for aquatic predators and land-based scavengers such as river otters (Lutra canadensis), muskrats (Ondatra zibethicus), raccoons (Procyon lotor), and skunks (Mephitidae). Juveniles are eaten by flatworms (Platyhelminthes), leeches (Hirudidae), and crayfish as well as an array of freshwater fish including carp (Cyprinidae), sturgeon (Acipenseridae), catfish (Ictaluridae), and sunfish (Centrarchidae). Gulls and shorebirds feed on mussels when water levels are low. Unionoidae shells provide a suitable substrate for epiphytic and epizoic colonization, and help stabilize fine-grained sediments that other organisms use for habitat.
Threats to Freshwater Mussels
Freshwater mussels are threatened by a number of factors including the loss of fish hosts, increased parasite loads, habitat loss and fragmentation, climate change, and the affects of introducted species
Loss of fish hosts
The life history of freshwater mussels is intimately linked with the life history of some stream fishes, so factors that decrease fish populations can have an adverse effect on freshwater mussels.
Reproduction and life history of freshwater mussels
Unionidae are dioecious, meaning they have separate sexes. Breeding success is higher in populations with high densities of males and females and when environmental conditions (e.g. flow and temperature) are optimal for the survival of sperm. Low-density populations often encounter chronic failure to breed due to low contact between sexes as well as vulnerability to stochastic events such as floods. Hermaphrodites are rare but present in low-density populations where males and females are less likely to encounter each other. Males release sperm into the water and females filter it from the water, to fertilize their eggs internally. Larvae, called glochidia, are released from the female mussel and complete their larval development by attaching to the gills or fins of aquatic vertebrates, typically fish, as obligate parasites. The high fecundity of mussels increases the likelihood for at least some glochidia finding a host. Glochidia become juvenile mussels over the course of a few days to a few months then release from the fish, burrow into the sediment, and spend the rest of their lives as free-living animals. Viability of freshwater mussel populations is intimately linked to the availability and health of fish hosts as well as variations in mussel density and flow conditions. Mussels can be categorized as either “generalists” with the ability to use a wide range of fish species as larval hosts, or “specialists” which use only a few closely related fish species as hosts.
Factors affecting fish populations
Fish populations may be affected by a variety of causes including physical impediments such as dams and roads, climate change, overfishing, increased predation, sea lice, pollution, and habitat degradation. Reproduction of specialists mussels relies upon host fish dynamics, adding more uncertainty to the reproductive process.
Effects of dams and roads
In the Connecticut River watershed, the alewife floater (Anodonta implicata) relies on anadromous fishes, American shad (Alosa sapidissima), alewife (Alosa pseudoharengus), and blueback herring (Alosa aestivalis), as hosts. The loss of these species upstream due to impassable dams will also eliminate the alewife floater in these areas. Stocking of fish in river systems may help sustain populations of mussels where streams have become too warm and degraded to support self-sustaining fish populations. The host fish populations of the dwarf wedgemussel (Alasmidonta heterodon), a federally endangered species, rely on seasonal movement into small streams that are often impeded by road-stream crossings.
Effects of climate change
Host fish availability might decline as a result of climate change,. Host fish include salmonids which are sensitive to temperature rise, as well as the decrease in dissolved oxygen that comes with it. Increased precipitation may have a negative impact on gravel spawning ground which can be completely destroyed by large floods reducing egg survival. With the decline in host salmon and trout stocks, low host densities may be limiting recruitment of some mussel species.
Increased parasite load
Increased parasite densities are associated with reduced mussel reproductive output and physiological condition especially in impounded and nutrient rich streams. Parasites of unionoids feed on the host’s gill tissue which is used not only for respiratory function but also for feeding and reproduction output. Digentic (host-castrating) trematodes target gonadal tissue and also affect growth rate and larval production in freshwater mussels. It has been observed that physiologically compromised mussels are more likely to be susceptible to parasitism.
Habitat loss and fragmentation
Habitat loss and fragmentation have greatly contributed to the loss of freshwater mussels. There are many mussel populations composed of exclusively old age individuals due to human induced changes to habitat which have made it unfavorable for juvenile survival. Important causes of habitat loss are building dams and surrounding land use.
Habitat disturbance by river engineering is often seen as the biggest culprit. The Nature Conservancy estimates that there are 2,622 dams in the Connecticut River watershed alone. All dams, regardless of size, have an effect on freshwater mussel survival because dams may affect hydrology, water temperature, water quality, and sediment transport. Fragmentation of river systems by multiple dams leads to isolated mussel populations and decreased reproductive success which will eventually lead to higher risk of extirpation. Dams impede or block the movement of native and anadromous fish which will also lower reproductive rates and survival. Habitat loss and ultimately species loss will occur downstream due to unnaturally high flow variations on short time scales causing the loss of fine sediments. Drawdowns of impoundments, if unmanaged, will also cause high mortality of mussels inhabiting the impoundment due to the drying of their habitat. The increased production of electricity by hydropower dams during periods of high demand causes rapid changes in low and high flow, producing near flood or drought conditions for mussels. Other contributors to habitat loss and fragmentation include road-stream crossings, poorly planned land use and development, and industrialization. Flood prevention and post-flood infrastructure has also caused a considerable amount of habitat degradation and high mussel mortality.
Surrounding land use
The quality of both water and sediment in river systems is affected by land conversion, agriculture, industries, urbanization, and industrialization. Nonpoint-source pollution stemming from a variety of land based sources reaches waterways by surface runoff, groundwater, or atmospheric deposition. Primary pollutants include bacteria, sediment, road salt, pesticides, herbicides, hydrocarbons, nutrients including nitrogen and phosphorus, as well as a number of other chemicals. Some species of freshwater mussels are affected by eutrophication more than others. Nitrogen in the form of ammonia and nitrates can be toxic to freshwater mussels and other aquatic organisms. Sediment pollution contributes to the loss of mussel habitat, decreased channel stability, loss of fish species, and causes a disruption of mussel feeding and respiration.
Changes in temperature
Changes in temperature have the potential to affect individual growth, longevity, and reproductive success. It is predicted that by around 2050 there will be a 1-2 degree centigrade increase in mean air surface temperature, which will affect surface water temperature. Although elevated surface water temperature has been shown to enhance recruitment (post-settlement survival) and increase the growth rates of glochidia, this is primarily observed in instances where mussels have a chance to acclimate themselves to the temperature, however, extreme thermal events may be detrimental to their survival. Temperature rise will also play a role in the timing of spawning, causing females to release glochidia into the water column earlier, thus uncoupling the timing of mussel and fish reproduction cycles, especially in anadromous fish.
Changes in precipitation
There have been recent increases in cloud cover as well as precipitation, evidenced as greater storm events, altering the habitat structure of mussel beds. Mussels have appeared to recruit well during wet years and recruitment may even increase as a result of increased precipitation. Since mussels require clean well aerated sand, higher river flows associated with wet years may be able to increase habitat. However, rainfall may also negatively influence mussel habitat availability by increasing high flow and runoff thereby changing patterns of erosion and deposition that degrade the river bed. Effects from increased precipitation on recruitment success will therefore vary due to the size and hydraulic characteristics of each river. In contrast, changes in seasonal pattern may be detrimental to mussel populations if summers continue to become drier. Mussel beds would be at risk of drying out and silt deposits, algal growth, and organic debris would increase. Periodic floods could serve to clean sediments and reduce debris, but mortality would occur when mussels are removed from the sediment and washed downstream.
Changes in sea level
Melting of ice caps may lead to a rise in sea level. Mussel populations in the lower reaches of rivers are at greater risk of immersion in sea water. Most freshwater mussels cannot tolerate saline conditions and a few species would be killed by permanent immersion in salt water. Also, many populations would be affected by sporadic intrusion by saltwater or brackish water caused by storm events, spring tides, storm surges, and onshore winds.
The zebra mussel (Dreissena polymorpha) was introduced to North America in the early 1990s and has been spreading throughout the Mississippi river basin, which contains the largest number of endemic mussels in the world. Following the introduction of zebra mussels, native mussel populations have been extirpated within 4-8 years. The invasion of zebra mussels has increased the extinction rate of native species from 4% to 12% per decade. D. polymorpha is a biofouling organism that smothers other mollusks and competes with other suspension feeders for food. They attach to solid surfaces using adhesive byssal fibers and possess a planktonic larvae stage that stays in the water column before settlement. No native freshwater mussel has these characteristics. Unionid mussels have a complex life cycle and spend most of their lives buried halfway into the sediment providing a suitable surface for zebra mussels to colonize. D. polymorpha impairs unionid metabolic activity and locomotion resulting in depleted energy and effectively starve them to death. Zebra mussels are also known to harm other suspension feeders though massive filtration, depleting all food sources. Most river systems in North America will be colonized with zebra mussels in the near future, substantially reducing the species richness and abundance of native mussels. Populations that had survived several decades of environmental degradation were wiped out within a few years of the D. polymorpha invasion in the Mississippi river basin. The zebra mussel invasion reduces populations into small fragmented assemblages which become prone to extinction by other anthropogenic threats.
In North America, freshwater mussels (Unionidae) are declining at a catastrophic rate. Many factors including habitat degradation, pollution, climate change, and the introduction of exotic species point toward impending mass extinction. The significant loss of biodiversity may permanently alter ecosystem functioning in rivers and lakes as well as alter the rate of ecological processes. Recent developments in management such as statutory protection, habitat restoration, and captive breeding, provide some optimism, but it is clear more research is needed in order to assess the extent to which populations are affected and what measures should be taken in the future.
References and Further Reading
- Gangloff, M.M., Lenertz, K.K., & Feminella J.W. “Parasitic mite and trematode abundance are associated with reduced reproductive output and physiological condition of freshwater mussels.” Hydrobiologia 610(2008):25-31.
- Hastie, L.C., Cosgrove, P.J., Ellis, N., & Gaywood, M.J. “The Threat of Climate Change to Freshwater Pearl Mussel Populations.” Ambio 32(2003):40-43.
- Nedeau, E.J. Freshwater Mussels and the Conneticut River Watershed. Conneticut River Watershed Council, Greenfield, MA. 2008: xvii+132 pp.
- Ricciardi, A., Neves, R.J., & Rasmussen, J.B. “Impending extinctions of North American freshwater mussels (Unionida) following the zebra mussel (Dreissena polymorpha) invasion.” Journal of Animal Ecology 67(1998):613-619.
- Vaughn, C.C., & Hakenkamp, C.C. “The functional role of burrowing bivalves in freshwater ecosystems.” Freshwater Biology 46(2001):1431-1446.