2006 NORTHEASTERN NATURALIST 13(1):39–42
A New Widespread Morphological Deformity in Freshwater
Mussels from New York
David L. Strayer*
Abstract - I describe a new, characteristic shell deformity in unionid mussels from several sites
in southern New York. The posterior end of the shell is strikingly shortened and distorted in
these deformed mussels. Because these misshapen mussels have been seen only in streams with
heavily agricultural or residential watersheds and only after 1990, they may be caused by an
agricultural or household chemical that has come into recent use.
The widespread pollution of fresh waters has increased the incidence of morphological
and developmental deformities in freshwater animals. Indeed, the occurrence
and frequency of such abnormalities is often used as an indicator of water pollution
(e.g., Johnson et al. 1993, Lenat 1993, Meregalli et al. 2000, Vuori and Kukkonen
2002). Morphological abnormalities have sometimes been described in freshwater
mussels (Unionidae), but typically have been attributed to mechanical damage or
parasites rather than pollution (e.g., Baker 1928, Parmalee and Bogan 1998, Vermeij
and Dudley 1985). Here, I describe a new, widespread deformity in freshwater
mussels from southern New York that seems to be associated with agricultural and
Deformed mussels are readily recognized by their severely shortened posterior ends
(Fig. 1). The posterior margin of the shell is irregular or ragged, the periostracal layers
often are thick and distorted, and the 2 valves often do not fit together well at the posterior
end of the shell, resulting in a marked gape. The deformation of the shell may be so
severe that it is difficult to identify the species. I have not observed the soft tissues of
animals affected by this deformity, but they presumably are distorted as well.
I saw this deformity at 5 widely separated sites in New York in 1992–2006. In
the Ramapo River (Orange County), which drains a suburban watershed, I found
deformed individuals of all 3 unionid species—Elliptio complanata (Lightfoot)
(eastern elliptio), Alasmidonta undulata (Say) (triangle floater) and Pyganodon
cataracta (Say) (eastern floater)—that live at this site. Similarly misshapen mussels
also turned up in 4 streams whose watersheds are dominated by row-crop
agriculture: Schoharie Creek (Schoharie County, Lasmigona costata (Rafinesque)
[flutedshell]), 2 sites along Rutgers Creek (Orange County, E. complanata), 2
sites along Five Mile Creek (Steuben County, Lasmigona compressa (Lea) [creek
heelsplitter]), and Webatuck Creek (Dutchess County, E. complanata). Unlike
the situation in the Ramapo River, I did not see deformed individuals of all of the
species present at these latter 4 sites. This deformity is thus geographically and
phylogenetically widespread. Although I did not record the frequency of misshapen
animals in affected populations, these shell anomalies were not rare, and involved
>10% of the animals that I saw at some sites.
I have examined thousands of unionid specimens in museum collections that
were collected from New York and other parts of the Northeast from 1820–1990, and
have seen a few misshapen specimens, but none with the characteristic deformity
shown in Figure 1. Further, although several authors (e.g., Baker 1928, Coker et al.
1921, Howells et al. 1996, Parmalee and Bogan 1998, Vermeij and Dudley 1985)
have discussed or illustrated shell anomalies in unionids, only Figure 9D of Howells
et al. (1996) even remotely resembles the specimens in Figure 1. All of this evidence
suggests that this type of deformity may be of recent origin.
Notes of the Northeastern Nat u ral ist, Issue 15/1, 2008
150 Northeastern Naturalist Notes Vol. 15, No. X
The cause of this deformity and its ultimate significance to unionid populations
are unknown. However, the geographical and temporal distribution of the abnormality
suggests that it might be caused by a chemical that has recently come into use in
agricultural or residential areas. A wide variety of chemicals, including insecticides,
herbicides, metals, organic contaminants, chlorine, and industrial, agricultural, and
Figure 1. Deformed unionid mussels from New York, along with normal shells from nearby sites
for comparison; actual shell lengths are given in parentheses. All specimens are oriented with
the anterior to the left. A. Alasmidonta undulata (deformed, 34 mm), Ramapo River, Tuxedo
Park, Orange County, NY; B. A. undulata (normal, 60 mm), Hackensack River, 1 km below
Lake Louise, Rockland County, NY; C. Elliptio complanata (deformed, 48 mm), Rutgers Creek
at Johnson, Orange County, NY; D. E. complanata (normal, 75 mm), Indian Kill 100 m above
mouth, Staatsburg, Dutchess County, NY; E. Lasmigona compressa (deformed, 50 mm), Five
Mile Creek above Kanona, Steuben County, NY; F. L. compressa (normal, 57 mm), Mud Creek
west of Sonora, Steuben County, NY; G. Lasmigona costata (deformed, 81 mm), Schoharie
Creek, Esperance, Schoharie County, NY; H. L. costata (normal, 76 mm), Mohawk River, Cohoes,
Albany County, NY. All specimens are in the collection of the New York State Museum.
2008 Northeastern Naturalist Notes 151
domestic effluents in general, are known to cause abnormalities in other freshwater
animals (Johnson et al. 1993).
Alternatively, this deformity may be caused by a parasite. Coker et al. (1921)
and Baker (1928) noted that parasitic infestations of unionids sometimes cause their
shells to grow irregularly (although they suggest that the anterior part of the shell
is usually distorted). If the shells shown in Figure 1 were distorted as a result of a
parasitic infestation, however, it is difficult to understand why such shells were not
found in the past in the Northeast.
I do not know whether this deformity affects functions such as feeding, reproduction,
or respiration, nor if it might affect population size or viability. The extent,
cause, and functional consequences of this deformity deserve further investigation.
Acknowledgments. Surveys during which these observations were made were
sponsored by the New York Natural Heritage Program, The Nature Conservancy,
and the New York State Department of Environmental Conservation. I thank the New
York State Museum and Bob Daniels for access to specimens. This is a contribution
to the program of the Institute of Ecosystem Studies.
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*Institute of Ecosystem Studies, PO Box AB, Millbrook, NY 12545; firstname.lastname@example.org.