- Scientific name: Limnadia lenticularis
- Species of Greatest Conservation Need (MA State Wildlife Action Plan)
- Special Concern (MA Endangered Species Act)
Description
Thorp, J.H., and Covich, A.P., eds. Ecology and Classification of North American Freshwater Invertebrates. 2nd Edition. Academic Press. 2001.
The American clam shrimp, also known as the Euroamerican clam shrimp, is a small crustacean from the class Branchiopoda that resembles a mollusk because of its bivalved structure called a carapace. The egg-shaped, laterally compressed carapace is transparent, consists of two shell-like valves that are connected by a fold, each with 7-18 growth lines. The valves enclose the American clam shrimp’s head and eyes, body, and feathery appendages. Massachusetts specimens average 10 mm (0.4 in) in length.
Two other species of clam shrimp are quite similar to the American clam shrimp. The more commonly encountered Holarctic clam shrimp (Lynceus brachyurus) from the order Laevicaudata is light orange in color, without growth lines on the carapace, and has a smaller, more rounded appearance. It is found in larger, more persistent ephemeral freshwater habitats. Agassiz’s clam shrimp (Eulimnadia agassizii) from the order Spinicaudata is also translucent in color, narrower and oval, with 4-5 growth lines, and is smaller averaging ~6 mm (0.24 in). It has been found occasionally to co-occur with Agassiz’s clam shrimp. Identification guides illustrate the differences between these three species (Smith 2000).
Life cycle and behavior
The American clam shrimp has a short life cycle, evolved to suit the ephemeral nature of its habitat (e.g., <1-2 months). When environmental conditions are met in their pool habitat, including sufficient water levels and temperature, American clam shrimp hatch from resting eggs or cysts. The eggs are actually developing embryos with a covering for protection from heat, freezing, and periodic dry conditions. Through successive molts, the young reach maturity in 4-11 days. Populations are composed of either all hermaphrodites, or a combination of males and hermaphrodites, although males are rare. “Females” (hermaphrodites) carry resting eggs between the body and the fold of the carapace, and they are shed as the adult molts.
Shortly before the pool dries, adults begin to die or become stranded and die. Typically, only one generation per wet period is produced before the pool dries. The resting eggs remain dormant in dry pool conditions until the appropriate environmental conditions are met, which could take years. The American clam shrimp is not found consistently year after year in the same shallow pool and its presence fluctuates depending on environmental conditions. American clam shrimp are active from May to September in Massachusetts.
Like all clam shrimps, this species swims with the fold of its carapace pointing up and its appendages pointing down to aid in locomotion, respiration, and feeding. The American Clam Shrimp swims, using a paddling motion created by the second antennae. If they cease to paddle, they tip on their side. They are most often found moving along the pool bottom in vegetation (Thorp and Covich 2001). This species is a filterer-collector and feeds by drawing water into the carapace using its feathery appendages to collect food particles.
Distribution and abundance
The American clam shrimp has a Holarctic range including North America and Europe. Within North America, this species is known from scattered locations in Massachusetts, the Atlantic coastal plain province of South Carolina, Florida, and southwest Georgia. In Massachusetts, American clam shrimp occurs in the Connecticut River watershed and several watersheds in northeastern and southeastern parts of the state including Cape Cod.
Little is known regarding the population status of the American clam shrimp in Massachusetts. Small to large abundances of American clam shrimp have been recorded from scattered locations in Massachusetts and potentially reflects the ephemeral nature of its habitat and availability for sampling. It is very rare in eastern North America and is listed under the Massachusetts Endangered Species Act as a Species of Special Concern. All listed species are protected from killing, collecting, possessing, or sale and from activities that would destroy habitat and thus directly or indirectly cause mortality or disrupt critical behaviors. In addition, listed animals are specifically protected from activities that disrupt nesting, breeding, feeding, or migration.
Distribution in Massachusetts.
1999-2024
Based on records in the Natural Heritage Database.
Habitat
The American Clam Shrimp inhabits ephemeral (vernal) pools, typically with shorter hydroperiods (weeks to months) and shallow depths. These depressions can hold water in the fall, late winter and spring, but also during the summer after precipitation events when American clam shrimp are active. Otherwise, these pools are dry. American clam shrimp have been recorded from flooded field depressions, unpaved road depressions (i.e., puddles), and a shoreline of an Atlantic white cedar swamp with aquatic vegetation. Biologists monitored the population in the hayfield depression, which fills with snowmelt and rainfall in the spring. The depression held water for about 3 weeks, was 1 acre in size, with a maximum depth of ~0.9 m (3 ft). Once dried, the habitat showed no evidence of the pool but clam shrimp bodies were found in the moist soil (Smith 1995). Elsewhere, this species has been found in small, shallow, isolated depressions in hardwood forests with dark-colored, acidic waters and short wet periods of 2-3 months (Battle and Golladay 2002; DiBiase and Taylor 2003).
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Threats
Pools that support the American clam shrimp are usually dry many months of the year, making these habitats easy to overlook. Losses of these pools to development, draining, filling, or contamination from pesticides or toxic substances have the potential to threaten this species. Hydrologic alterations (e.g., water withdrawals) may interfere with length and timing of habitat inundation and could cause local population extinction. Impacts to surrounding riparian areas, including development and colonization of invasive plant species, are also a threat to pool connectivity, water quality, and pool habitat degradation.
Conservation
Survey and monitoring
Standardized surveys are needed to update the population status of American clam shrimp at its historical sites. Survey effort should also target unsampled vernal pools and depressions with physical features that may support American clam shrimp – it’s likely the species occurs in other watersheds across the state. Surveys should target adults when pools are filled by precipitation events generally from May-September and likely requires multiple visits to increase detection. Monitoring efforts are recommended at least every 5-10 years or as needed to the extent practical (e.g., in response to potential disturbance or stressor events) at high-quality sites.
Management
Protection of American clam shrimp pool habitat is critical for its persistence in Massachusetts. Routine disturbances may benefit this species in some landscape contexts to maintain pool habitat and discourage succession of plant species encroaching from adjacent areas, particularly invasive species. Creation of new pool habitat may be warranted in suitable areas where depressions need to be filled (e.g., functional roadways).
Research needs
Surveys should continue to target historical but also identify new pools to better understand its distribution, relative abundance, and habitat requirements in Massachusetts. Other research needed for this species include identification of environmental thresholds that trigger cyst hatching including precipitation amounts and water temperature. Increased knowledge of the longevity and size of cyst banks, genetic isolation, and persistence and viability of pools composed of all hermaphrodites would inform population stability and risks to environmental stressors.
References
Battle, J.M., and S.W. Golladay. 2002. Aquatic invertebrates in hardwood depressions of southwest Georgia. Southeastern Naturalist 1(2): 149-158.
DiBiase, A.E., and B.E. Taylor. 2003. New reports of fairy shrimps (Crustacea: Anostraca) and clam shrimps (Crustacea: Laevicaudata and Spinicaudata) from South Carolina. Southeastern Naturalist 2(2): 207-216.
Smith, D.G. 2000. Keys to the Freshwater Macroinvertebrates of southern New England. Published by author. Sunderland, MA. 243 pp.
Smith, D.G. 1995. Notes on the status and natural history of limnadiid clam shrimp in southern New England. Anostracan News 3(2): 3-4.
Thorp, J.H., and A.P. Covich, eds. 2001. Ecology and Classification of North American Freshwater Invertebrates, Second Edition. Academic Press.
Contact
| Date published: | March 10, 2025 |
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