The Nonindigenous Occurrences section of the NAS species profiles has a new structure. The section is now dynamically updated from the NAS database to ensure that it contains the most current and accurate information. Occurrences are summarized in Table 1, alphabetically by state, with years of earliest and most recent observations, and the tally and names of drainages where the species was observed. The table contains hyperlinks to collections tables of specimens based on the states, years, and drainages selected. References to specimens that were not obtained through sighting reports and personal communications are found through the hyperlink in the Table 1 caption or through the individual specimens linked in the collections tables.

Pisidium henslowanum
Pisidium henslowanum
(Henslow peaclam)

Copyright Info
Pisidium henslowanum Shepard, 1825

Common name: Henslow peaclam

Synonyms and Other Names: henslow(’s) pea/pill clam, pisidiid clam

Taxonomy: available through www.itis.govITIS logo

Identification: Henslow’s pea clam has a thin, triangle- to oval-shaped, relatively long bivalve shell with evenly spaced coarse striae. The beaks are located posterior to the centre and the joint at the dorsal margin is relatively pointed. The height to length ratio is 0.8–0.9. The 2nd cardinal tooth inside the shell is V-shaped and larger than the 4th cardinal, which begins above the 2nd cardinal and lies obliquely. The 3rd cardinal is relatively thick and located at the posterior margin. In live specimens, there is only an anal siphon (Clarke 1981, Herrington 1962, Mackie 2000, Mackie et al. 1980, Pennak 1989).

Size: reaches 4 to 4.6 mm in length (Clarke 1981, Herrington 1962, Holopainen 1979, Mackie 2000).

Native Range: Pisidium henslowanum is a Holarctic species found in Eurasia, in countries such as Iceland, Scandinavia, France, Germany, Belgium, the territory of the former USSR, and the United Kingdom (Clarke 1981, Mackie 2000).

Hydrologic Unit Codes (HUCs) Explained
Interactive maps: Point Distribution Maps

Nonindigenous Occurrences:

Table 1. States with nonindigenous occurrences, the earliest and latest observations in each state, and the tally and names of HUCs with observations†. Names and dates are hyperlinked to their relevant specimen records. The list of references for all nonindigenous occurrences of Pisidium henslowanum are found here.

StateFirst ObservedLast ObservedTotal HUCs with observations†HUCs with observations†
MN200520202Lake Superior; St. Louis
NY191620011Lake Ontario
TN201520151Upper Duck
WI201420203Beartrap-Nemadji; Lake Superior; St. Louis

Table last updated 6/16/2024

† Populations may not be currently present.

Ecology: In its native habitat P. henslowanum is most often found in rivers, canals, and streams of good water quality, while in North America it is mostly found in lakes and large rivers. Henslow’s pea clam can occur down to 20 m depth although it is more limited by lack of oxygen with increasing depth. It is typically associated with silt, mud, and sand assemblages. Densities in Europe have reached 330–9000 clams per m2 (Bishop and Hewitt 1976,Clarke 1981, Hinz et al. 1982, Holopainen 1979, Holopainen and Jonasson 1983, Holopainen and Ranta 1977, Jonasson 1984, Krieger 1984, Mackie et al. 1980, Smit et al. 1994,1995, Steiner and Turner 1998).            

Even though European authors typically indicate that P. henslowanum requires relatively hard, calcium rich water, it can survive in somewhat softer water in the Connecticut River system. In this system the pH is 7.0–7.7 and the CaCO3 concentration is 25–35 mg/l (Smith 1986). Pisidium henslowanum reproduces hermaphroditically and may self-fertilize. The number of embryos per gravid adult ranges from 1–40 and generally increases with parental size. Young are released depending on temperature and oxygen availability. Pisidium henslowanum may produce one or two broods per year. The breeding season occurs in spring and summer. Young become mature at age 2 and the lifespan is thought to be 4–5 years (Clarke 1981, Holopainen 1979, Holopainen and Hanski 1986, Holopainen and Jonasson 1983, Mackie 2000, Smith 1986). Pisidium henslowanum, like many other pea clams, filters food from the water column directly above the substrate and from the sediments in which it lives, including bacteria, diatoms, detritus and other algae (Holopainen 1979, Mackie 2000).

Means of Introduction: Pisidium henslowanum very likely arrived in ships to the Great Lakes basin. It was most likely delivered in solid ballast, which was in use at the turn of the century (Grigorovich et al. 2000, 2003).

Status: Established where recorded.

Impact of Introduction: The impacts of this species are currently unknown, as no studies have been done to determine how it has affected ecosystems in the invaded range. The absence of data does not equate to lack of effects. It does, however, mean that research is required to evaluate effects before conclusions can be made.

Remarks: There has been some debate over whether or not P. henslowanum is native to North America. Those who believe it is a native species cite discoveries of occurrences in central and western Canada as well as shell deposits from 7000 years ago (Harris 1973), or mention that the population in the Holyoke Canals would have had difficulty spreading from the Great Lakes, considering that this canal system is not commercially navigable (Smith 1986). However, the general consensus amongst most authors is that P. henslowanum is an introduced species.

References: (click for full references)

Bishop, M.J., and S.J. Hewitt. 1976. Assemblages of Pisidium spp. (Bivalvia: Sphaeriidae) from localities in eastern England. Freshwater Biology 6(2):177-182.

Clarke, A.H. 1981. French translation by A. La Rocque. Les Mollusques d’eau douce du Canada. Musée national des sciences naturelles, Musées nationaux du Canada. 447 pp.

Grigorovich, I.A., A.V. Korniushin, and H.J. MacIsaac. 2000. Moitessier’s pea clam Pisidium moitessierianum (Bivalvia, Sphaeriidae): a cryptogenic mollusk in the Great Lakes. Hydrobiologia 435:153-165.

Grigorovich, I.A., R.I. Colautti, E.L. Mills, K. Holeck, A.G. Ballert, and H.J. MacIsaac. 2003. Ballast-mediated animal introductions in the Laurentian Great Lakes: retrospective and prospective analysis. Canadian Journal of Fisheries and Aquatic Sciences 60:740-756.

Harris, S.A. 1973. Pisidium henslowanum in western Canada. Nautilus 87(3):86-87.

Heard, W.H. 1961. Pisidium henslowanum (Sheppard) in Lake Michigan. Nautilus 74(3):123.

Heard, W.H. 1962. The Sphaeriidae (Mollusca: Pelecypoda) of the North American Great Lakes. American Midland Naturalist 67(1):194-198.

Herrington, H.B. 1962. A revision of the Sphaeriidae of North America (Mollusca: Pelecypoda). Museum of Zoology, University of Michigan, Miscellaneous Publications 118. 74 pp 7 plates.

Hinz, W., H. Daub, C. Marx, C. Tinz, and W. Wiersch. 1982. The mollusk fauna of the Issel system at the time of the 1976-77 expansion. Decheniana. Bonn. 134:176-177.

Holopainen, I.J. 1979. Population dynamics and production of Pisidium species (Bivalvia, Sphaeriidae) in the oligotrophic and mesohumic Lake Paajarvi, southern Finland. Archiv für Hydrobiologie Supplement 54(4):466-508.

Holopainen, I.J., and E. Ranta. 1977. Carbon dioxide output in the respiration of three Pisidium species (Bivalvia, Sphaeriidae). Oecologia (Berlin) 30:1-8.

Holopainen, I.J., and I. Hanski. 1986. Life history variation in Pisidium (Bivalvia: Pisidiidae). Holarctic Ecology 9:85-98.

Holopainen, I.J., and P.M. Jonasson. 1983. Long-term population dynamics and production of Pisidium (Bivalvia) in the profundal of Lake Esrom, Denmark. Oikos 41(1):99-117.

Johnson, M.G., and D.H. Matheson. 1968. Macroinvertebrate communities of the sediments of Hamilton bay and adjacent Lake Ontario. Limnology and Oceanography 13(1):99-111.

Jonasson, P.M. 1984. Oxygen demand and long term changes of profundal zoobenthos. Proceedings of the Second International Symposium on Aquatic Oligochaete Biology. Hydrobiologia 115:121-126.

Krieger, K.A. 1984. Benthic macroinvertebrates as indicators of environmental degradation in the southern nearshore zone of the central basin of Lake Erie. Journal of Great Lakes Research 10(2):197-209.

Mackie, G.L. 2000. Ballast water introductions of Mollusca. Pp. 219-254 in R. Claudi and J. H. Leach, eds. Nonindigenous Freshwater Organisms: Vectors, Biology and Impacts. CRC Press LLC, Boca Raton, Florida. 464 pp.

Mackie, G.L., D.S. White, and T.W. Zdeba. 1980. A guide to freshwater mollusks of the Laurentian Great Lakes with special emphasis on the genus Pisidium. Environmental Research Laboratory, Office of Research and Development, U. S. Environmental Protection Agency, Duluth, Minnesota 55804. 144 pp.

Pennak, R. 1989. Fresh-water Invertebrates of the Unites States, 3rd ed. Protozoa to Mollusca. John Wiley & Sons, Inc., New York, New York State. 628 pp.

Ricciardi, A. 2001. Facilitative interactions among aquatic invaders: is an “invasional meltdown” occurring in the Great Lakes? Canadian Journal of Fisheries and Aquatic Sciences 58:2513-2525.

Smit, H., J.A. Van Der Velden, and A. Klinik. 1994. Macrozoobenthic assemblages in littoral sediments in the enclosed Rhine-Meuse Delta. Netherlands Journal of Aquatic Ecology 28(2):199-212.

Smit, H., H.C. Reinhold-Dudok Van Heel, and S.M. Wiersma. 1995. Sublittoral macrozoobenthic assemblages in the enclosed sediment-polluted Rhine-Meuse Delta; their relationship to environmental conditions. Netherlands Journal of Aquatic Ecology 29(1):31-47.

Smith, D.G. 1986. Pisidium henslowanum in the Connecticut River, Massachusetts, USA (Bivalvia: Pisidiidae). Nautilus 100(4):124-126.

Steiner, P., and H. Turner. 1998. The Mollusca of Lake Greifensee: inventory and abundance of species in relation to site factors. Vierteljahrsschrift der Naturforschenden Gesellschaft in Zuerich 143(3):85-96.

Trebitz, A.S., C.W. West, J.C. Hoffman, J.R. Kelly, G.S. Peterson, and I.A. Grigorovich. 2010. Status of non-indigenous benthic invertebrates in the Duluth-Superior Harbor and the role of sampling methods in their detection. Journal of Great Lakes Research 36:747-756.

US Army Corp of Engineers (USACE).  2012.  Molluscicides.  Accessed 10/28/13.  http://glmris.anl.gov/documents/docs/anscontrol/Molluscicides.pdf

Wolfert, D.R., and J.K. Hiltunen. 1968. Distribution and abundance of the Japanese snail, Viviparus japonicus, and associated macrobenthos in Sandusky Bay, Ohio. The Ohio Journal of Science 68(1):32-40.

Other Resources:
Great Lakes Water Life

Author: Kipp, R.M., J. Larson, and A. Fusaro

Revision Date: 11/26/2019

Citation Information:
Kipp, R.M., J. Larson, and A. Fusaro, 2024, Pisidium henslowanum Shepard, 1825: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, https://nas.er.usgs.gov/queries/FactSheet.aspx?speciesID=130, Revision Date: 11/26/2019, Access Date: 6/16/2024

This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.


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Citation information: U.S. Geological Survey. [2024]. Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [6/16/2024].

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