Pyganodon grandis

Pyganodon grandis (Say, 1829)
Giant Floater

Pyganodon grandis, 9.89 cm shell length
Opintlocco Creek, Macon County, Alabama

Diagnosis

The giant floater is easily recognized by its large size, thin shell, and lack of hinge teeth.  Shell thickness and shape are highly variable; P. grandis from large, fast-flowing streams and rivers are typically more ovate with a dark periostracum whereas reservoir forms are typically more elongate with a yellow-green periostracum.  Headwater forms may have relatively thick shells but tend to be more elongate and laterally inflated.

Some authors (e.g., Jenkinson 1973) have recognized a second species of Pyganodon in parts of the Mobile, P. cataracta.  Pyganodon cataracta resembles headwater Pyganodon collected from the South Fork of Terrapin and Opintlocco creeks.  Since there is no reliable way to discern these 2 species without molecular data (even glochidia are similar) I considered all Pyganodon encountered in recent surveys of the Mobile Basin to be P. grandis (Gangloff 2003).

Ecology

Pyganodon grandis is a species that is apparently tolerant of a wide range of habitat conditions.  It is known to inhabit moderate-to-slow flowing streams and rivers, reservoirs, farm ponds, and shipping channels.  Pyganodon grandis is abundant in low gradient streams in Coastal Plain Mobile Drainage streams including Catoma, Line, and Opintlocco creeks (Gangloff 2003).   Pyganodon grandis (and 7 other mussel species) were abundant (>100 mussels/50 m stream) in the middle reaches of Opintlocco Creek prior to its complete drying from August - December 2000.  Pyganodon grandis was found at this site the following May, but in greatly reduced numbers.
  
Because of it is tolerant of a wide range of environmental conditions, P. grandis is an excellent model organism and more is known about the reproductive and physiological biology of P. grandis than any other unionid.  A wide range of fish species have been reported as hosts for P. grandis (Watters 1994).  Glochidia are typically released along with large quantities of mucous that presumably entangles fish indiscriminately (Watters 1994, M. Gangloff, pers. obs.).  Pyganodon grandis in Macon County are gravid from October through May and a related study found its glochidia on 6 species of fish in Opintlocco Creek (Buntin, Feminella and Gangloff, unpublished data).     

Distribution

Pyganodon grandis is found throughout North America (Johnson 1970, Parmalee and Bogan 1998).  The range of P. grandis in the Upper Alabama River Drainage appears to be greater now than in the past (Gangloff 2003).  Herbert H. Smith made a single collection from the Coosa River in St. Clair County, Alabama (Gangloff 2003).  However, P. grandis was abundant in Hurd’s (1974) collections. 

Status

The range of P. grandis is expanding in North America (e.g., Gangloff and Gustafson 2000) and this species is regarded as stable throughout its range (Williams et al. 1993).

References

Gangloff, M. M. and D. L. Gustafson. 2000. The freshwater mussels (Bivalvia: Unionoida) of Montana.  Central Plains Archaeology 8(1):121-130.

Gangloff, M. M.  2003.   The status, physical habitat associations, and parasites of freshwater mussels in the upper Alabama River Drainage, Alabama.  Ph.D. Dissertation, Auburn University, 237 p.

Hurd, J. C.  1974.  Systematics and zoogeography of the unionacean mollusks of the Coosa RIver Drainage of Alabama, Georgia and Tennessee.  Ph.D. Dissertation, Auburn University, 240 p.

Jenkinson, J.  J.  1973.  Distribution and zoogeography of the Unionidae (Mollusca: Bivalvia) in four creek systems in East-Central Alabama.  Master’s Thesis, Auburn University 96 p.

Johnson, R. D.  1970.  The systematics and zoogeography of the Unionidae (Mollusca: Bivalvia) of the Southern Atlantic Slope Region.  Bulletin of the Museum of Comparative Zoology 140:263-450.

Parmalee, P. W. and A. E. Bogan.  1998.  The freshwater mussels of Tennessee, University of Tennessee Press, Knoxville. 328 p.

Watters, G. T.  1994.  An annotated bibliography of the reproduction and propagation of the Unionoidea (Primarily of North America).  Ohio Biological Survey Miscellaneous Contributions 1, 158 p.

Williams, J. D.,  M. L. Warren, K. S. Cummings, J. L Harris, and R. J. Neves.  1993.  Conservation status of freshwater mussels of the United States and Canada.  Fisheries 18: 6-22.

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Page Compiled by Michael Gangloff, 25 August 2003.