- Scientific name: Margaritifera margaritifera
- Species of Greatest Conservation Need (MA State Wildlife Action Plan)
Description
Eastern pearlmussel
The eastern pearlshell is a medium- to large-bodied mussel in the family Margaritiferidae. It is the only representative of this family in Massachusetts and the Northeastern United States. In Massachusetts, specimens rarely exceed 14 cm (5.5 in) in total length. The periostracum is thick and durable. Adult shells are rough and nearly black, whereas juveniles can have smooth, brown-golden shells. Shells are laterally compressed, and the beaks barely extend above the hinge line. A broadly arching dorsal margin and a slightly indented ventral margin add to a kidney-shaped appearance of the shell. The left valves have two well-developed pseudocardinal teeth that pair with one on the right valve, and both valves lack lateral (hinge) teeth. The nacre is white to bluish-white, and the center of the nacre contains distinctive pits with a faint tail toward the anterior-dorsal surface. The mantle tissue is typically dark reddish-brown to black, and there is no separation between exhalant and inhalant siphon apertures (Smith 1991, Nedeau 2008).
Life cycle and behavior
The eastern pearlshell has the greatest fecundity and smallest glochidia of all freshwater mussels in North America. Individuals may take 20 years to reach sexual maturity, and reported lifespans have surpassed 100 years. Host fish species in Massachusetts are likely native brook trout (Salvelinus fontinalis), and introduced rainbow trout (Oncorynchus mykiss) and brown trout (Salmo trutta). Female mussels are gravid from late summer through October and release of glochidia is dependent on water temperatures. Glochidia are triangular with hooks for attachment to hosts, approximately 0.6 mm (0.02 in) in length and grow several times their size on fish hosts (Smith 1976). Metamorphosis is also dependent on water temperatures but may last up to 10 months (Nedeau 2008).
Distribution and abundance
Cluster of eastern pearlshell.
Eastern pearlshell is native to northern Europe, eastern North America, and Eurasia. European countries have witnessed extirpation or declines in distribution as high as 90%. Massachusetts appears to have a stable population of eastern pearlshell, however review of its status is warranted.
In Massachusetts, eastern pearlshell occur in the Farmington, Westfield, Middle and Lower Connecticut, Chicopee, Nashua, and Charles River watersheds. When present, species can be found at high densities exceeding 200 mussels/m2. Nedeau and Low (2008) studied eastern pearlshell from the Farmington River in Massachusetts and Connecticut, and reported length frequency distributions that suggest some populations are composed of primarily larger and older individuals. Given the long time to maturity in this species, evidence of a lack of recruitment is a concern for population persistence.
Distribution in Massachusetts.
1999-2024
Based on records in iNaturalist and the Natural Heritage Database.
Habitat
Eastern pearlshell inhabits cold water streams and rivers, but the best populations occur in small streams with intact riparian canopy, high dissolved oxygen and excellent water quality (Nedeau 2008). Eastern pearlshell habitat is consistent with high-quality trout habitat in lotic systems, and could benefit from protection of coldwater fishery resources throughout Massachusetts.
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Small stream with stable substrate habitat suitable for high density aggregations of eastern pearlshell.
Threats
Eastern pearlshell is sensitive to nutrient pollution and eutrophication; alterations in substrate type and stream channel morphology, increases in stream temperature, and increased acidity in streams (Nedeau 2008). Acid deposition, alterations of riparian and watershed land use and forest cover, increased nutrient pollution in streams, changes in in-stream and environmental flows, and loss or reduction of host fish habitat are likely threats to eastern pearlshell. Because of the species’ preference for high-quality cold-water habitats, could further restrict this species’ range. Increases in surface water temperature, and increased road salt use and metal contamination of streams from changes in winter weather patterns are concerning factors.
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). Survey efforts should continue to search for new populations particularly in coastal watersheds and expand our knowledge on species distribution in extant watersheds every 5 years or to the extent feasible. Establishment of long-term monitoring sites in extant watersheds are needed to acquire critical demographic and population trend data. Monitoring these sites should occur annually in multi-year blocks or as needed.
Management
Conservation and management recommendations include: maintain naturally variable river flows and limit water withdrawals; identify, mitigate, or eliminate sources of pollution to waterbodies; identify dispersal barriers for host fish, especially those that fragment the species range within a river or watershed, and seek options to improve fish passage or remove the barrier; maintain adequate vegetated riparian buffers along rivers and lakes; protect or acquire land at high priority sites. Careful management of dam removals is needed to prevent massive die-offs of eastern pearlshell found upstream or within impoundments.
Research needs
Effort is needed to evaluate the status of eastern pearlshell populations including identification of viable and aging populations with little to no recent recruitment, update of its distribution in historical and new locations, and evaluation of local and watershed-scale stressors. Further, since this species and its host fishes are likely to be impacted by climate change, projections for water temperature, streamflow, and host-fish occupancy are needed to identify watersheds under risk of extirpation.
Acknowledgements
MassWildlife thanks Peter Hazelton for providing significant support to this document.
References
Nedeau, E.J. 2008. Freshwater Mussels and the Connecticut River Watershed. Connecticut River Watershed Council, Greenfield, Massachusetts. xviii + 132pp.
Nedeau, E.J., and P. Low. 2008. Freshwater Mussels, Snails, and Crayfish of the Upper Farmington River Watershed. Report prepared for Farmington River Coordinating Committee and Massachusetts Natural Heritage and Endangered Species Program. Biodrawversity, LLC. Amherst, Massachusetts. 22pp.
Smith, D.G. 1976. Notes on the biology of Margaritifera margaritifera (Lin.) in central Massachusetts. The American Midland Naturalist 96:252-256.
Smith, D.G. 1991. Keys to the Freshwater Macroinvertebrates of Massachusetts: including the Porifera, Colonial Cnidaria, Entoprocta, Ectoprocta, Platyhelminthes, Nematophora, Nemertea, Mollusca (Mesogastropoda And Pelecypoda), and Crustacean (Branchiopoda and Malacostraca). Department of Zoology, University of Massachusetts, Amherst, Massachusetts. 236pp.
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| Date published: | March 31, 2025 |
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