Anodonta suborbiculata

Anodonta suborbiculata (I. Lea, 1834)
Flat Floater

Anodonta suborbiculata, 14.2 cm shell length
Weiss Reservoir, Cherokee County, Alabama

Diagnosis

Anodonta suborbiculata is a large and distinctive species.  Shells of this species are nearly circular, laterally compressed, and extremely fragile.  Hinge teeth are absent and the periostracum color ranges from light tan to dark green.   A. suborbiculata is not likely to be mistaken for other species in Alabama.  Pyganodon grandis, a similar species, is also common in lakes and reservoirs throughout North America.  P. grandis from can sometimes also be very round, especially in reservoirs.  However, Pyganodon grandis are typically thicker shelled, more elongate, and inflated.

Ecology

A. suborbiculata is known to be a rapid colonizer of impounded waters, and also can be abundant in large river backwaters.  It prefers fine sand or silt substrate where its light shell allows it to remain near the surface.  A. suborbiculata uses a wide range of fish hosts including many species of sport and bait fish.  Our examination of soft tissues revealed large numbers (> 200) of parasitic mites (Acari: Unionicolidae) present in individuals from Weiss Reservoir (M. Gangloff, unpublished data). 

Distribution

Our recent surveys found Anodonta suborbiculata primarily in reservoir habitats (Weiss, H. Neely Henry Lakes) but also at several riverine Coosa River sites including the Coosa River downstream of the Terrapin Creek confluence, and in Hurricane and Line creeks (Gangloff 2003).  Hurd (1974) and all other prior surveys did not detect this species, indicating that it is probably a recent invader of the Mobile Basin.  Paul Hartfield (USFWS - personal communication) first noticed the species in the early 1990s in Weiss Reservoir.  

Status

This species is native to the Interior (Mississippi River) Drainage and appears to have expanded its natural range to include the Mobile, Apalachicola, and a number of other coastal drainages in Eastern North America.

References

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.

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.