- Scientific name: Alasmidonta undulata
- Species of Greatest Conservation Need (MA State Wildlife Action Plan)
Description
Triangle Floater
The triangle floater is a small freshwater mussel that rarely exceeds 75mm (3 in) in length. The shape is subovate to almost triangular and has a somewhat “squat” appearance (it is short, wide, and fat (1)). The ventral margin is rounded (2), so that the shell rocks evenly when placed on a flat surface. Beaks (3) are prominent and raised above the hinge line (4). Sculpturing on the beak is uneven and coarse, although this feature is more readily observed in young animals with little shell erosion. The periostracum (5) is smooth and shiny, and ranges in color from yellowish-green to nearly black. The periostracum has green shell rays (6) that are prominent on all but very old, stained, or eroded animals. Pseudocardinal teeth (7) are a triangular shape and very prominent; the pseudocardinal teeth are buttressed by a thick portion of the nacre. Lateral teeth are absent (8). The nacre (9) is distinctively bicolored: the posterior half of the shell is thin and an iridescent bluish-pink color, and the anterior half of the shell is substantially thicker and a white or pinkish color. The foot is usually white.
The hinge teeth morphology, shell shape, and distinctly bicolored nacre make the shells (dead animals) of the triangle floater unmistakable from other species in Massachusetts. However, internal features cannot be used when identifying live animals. Live animals can often be confused with the brook floater, creeper, and dwarf wedgemussel. Greatest difficulty arises when trying to identify juveniles, animals with excessive shell erosion, or animals whose periostracum is darkly stained or covered with algae. The triangle floater is distinct from the brook floater because it lacks prominent ridges on the dorso-posterior slope and its ventral margin is curved rather than straight. In addition, triangle floater feet are white and brook floater feet are usually cantaloupe-colored. Compared to the creeper, the triangle floater is more laterally inflated (1), has prominent beaks (3), and has a stronger shell. Young triangle floaters can be confused with dwarf wedgemussels that are more ovate than the typical wedge shape; usually the coarse uneven beak sculpture of the triangle floater and the size and shape of the animals will enable accurate identification. An expert should be consulted to identify the species because it can be confused with three other state or federally protected species (brook floater, creeper, and dwarf wedgemussel).
Life cycle and behavior
Freshwater mussels are essentially sedentary filter feeders that spend most of their lives anchored to the bottoms of rivers, streams, lakes, and ponds by their muscular foot. Gills circulate water through their shells via incurrent and excurrent openings, siphoning nutrients to be absorbed by the digestive system. This filtering process is also critical for successful reproduction. Like all freshwater mussels, larvae (called glochidia) of the triangle floater must attach to the gills or fins of a vertebrate host (mainly fish) to develop into juveniles (for a review, see Nedeau et al. 2000). This parasitic phase is the only period during which mussels can disperse long distances. Fertilization occurs in the summer and glochidia are released the following spring. Studies have identified several hosts that are common in coldwater and warmwater environments in Massachusetts, such as the common shiner, blacknose dace, longnose dace, white sucker, pumpkinseed sunfish, fallfish, largemouth bass, slimy sculpin, and several species not found in the state (Nedeau et al. 2000, Wicklow 2004). The triangle floater uses a broader variety of host fish than the closely related dwarf wedgemussel. Following the parasitic period, juveniles drop to the bottom of the lake or river, burrow into the sediment, and spend the rest of their lives as free-living animals. Longevity is unknown, but given their size and the longevity of closely related species (dwarf wedgemussel and brook floater), the triangle floater likely lives for 8 to 20 years in Massachusetts. During that time, they may only move a few meters within a water body.
Distribution and abundance
The North American range of the triangle floater extends from North Carolina north to Nova Scotia and New Brunswick. It occurs in most Atlantic coastal drainages throughout that range. It also occurs in tributaries of the lower St. Lawrence River in Quebec. In Massachusetts, the triangle floater occurs in the Housatonic, Farmington, Westfield, Middle Connecticut, Chicopee, Quinebaug, Blackstone, Nashua, Concord, Charles, Taunton River watersheds and in coastal plain ponds of southeastern MA. Most occurrences have been recorded in central and western parts of the state. A careful status review is needed to assess the long-term viability of Triangle Floater populations in Massachusetts.
Distribution in Massachusetts.
1999-2024
Based on records in the Natural Heritage Database.
Habitat
In northeastern North America, the triangle floater inhabits small to large rivers and lakes. It is more commonly found in flowing water, where it occupies a wide range of substrate and flow conditions. Preferred habitats include low-gradient river reaches with sand and gravel substrates and low to moderate water velocities (Nedeau et al. 2000, Skorupa et al. 2024). It has been found in streams smaller than five meters wide and rivers wider than 100 meters. The triangle floater is the only species in the genus Alasmidonta that inhabits lakes; it occurs in both natural lakes and reservoirs occasionally, although at lower population densities than in rivers. Its ability to tolerate standing water makes this species less sensitive to the effects of dams than other species, such as the brook floater. In fact, at times it is as abundant in small impoundments of run-of-river dams as it is in free-flowing portions of rivers. Like most other mussel species, triangle floaters are sparse or absent in headwater streams and high-gradient river reaches. Because they are widespread in Massachusetts and inhabit a wide range of habitats, they share habitat with almost every other mussel species. However, they are most abundant in rivers that support eastern elliptio, eastern lampmussel, creeper, and brook floater.
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Stable run and pool habitat for triangle floater.
Threats
Because triangle 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, 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 removals, 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 triangle floaters populations. As local populations of creepers decline and/or become extirpated in response to these threats, dispersal distances between populations increase, weakening overall reproductive success and ultimately genetic diversity (Vaughn 1993).
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). Survey efforts should continue to search for new populations via physical and potentially eDNA surveys and revisit sites of relatively high viability every 5 to 10 years or to the extent feasible. At a subset of these sites long-term monitoring sites should continue or become established to acquire critical demographic (e.g. survival rates) and trend data. 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. 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; maintain adequate vegetated riparian buffer; protect or acquire land at high priority sites.
Research needs
Research needs for triangle floater include population-level data on survival rate, mortality rate, individual growth rates, population size trends, age at reproduction, sex ratio, and age structure. 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. Climate change projections for water temperature and streamflow in occupied watersheds is 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 triangle floater and other mussel species downstream needs further investigation.
References
Gray, E.V.N., B.A. Lellis, J.C. Cole, and C.S. Johnson. 2002. Host Identification for Strophitus undulatus (Bivalvia: Unionidae), the Creeper, in the Upper Susquehanna River Basin, Pennsylvania. The American Midland Naturalist 147(1): 153-161.
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.
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.
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.
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| Date published: | March 31, 2025 |
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