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Species Description

Triangular Kidneyshell

Ptychobranchus greenii


Phylum: Mollusca
Class: Bivalvia
Order: Unionoida
Family: Unionidae


The Triangular Kidneyshell is a medium-sized freshwater mussel that usually measures less than 4.0 inches (100 mm) in length. Its shell is oval to elliptical in shape with a broadly rounded posterior ridge. The outer shell appearance is quite variable. The periostracum (outer shell surface) is often straw yellow in younger specimens and yellow-brown in older specimens. The outer shell may also be marked with wide and broken or fine and wavy green rays.

Life Cycle

Some of the specific details about the complex life cycle of Ptychobranchus greenii are not currently known, but the life history of this endangered mussel is presumed to be similar to related species. Male Triangular Kidneyshell mussels release sperm into high quality rivers with moderate to swift currents. Sperm enters females through siphon-like regions and fertilization of eggs occurs within female shells. These fertilized eggs develop into special larva called glochidia. Glochidia continue to develop and are released into the water column when fully matured, usually during the spring. The parasitic glochidia must find and attach to the gills or fins of the appropriate host fish to complete development. Recent studies have determined that females release glochidia in the form of conglutinates. A conglutinate is a mass of glochidia that resembles a fish or another organism that is used to attract fish. Fish will be attracted to this lure and this increases the probability that fish will come in contact or consume the mass of larvae, thus allowing the glochidia to attach to the gills of the proper host fish. The conglutinate released by the female Triangular Kidneyshell is round and pearl colored, with 2 black eyespots that make it resemble fertilized fish eggs. Recent research suggests that suitable fish hosts for the Triangular Kidneyshell are the Blackbanded Darter (Percina nigrofasciata), Logperch (Percina caprodes) and Warrior Darter (Etheostoma bellator). The glochidia parasitize the fish host for a variable length of time, likely depending upon water temperature, fish species and other factors. Larvae metamorphose into juvenile mussels on the fish and then release from the host to find a suitable substrate, usually the gravel bottom of clean rivers with moderate to swift currents.

Natural History

Many of the details about the natural history of the Triangular Kidneyshell are not currently known, but they are believed to be similar to better known, related species. Larvae (glochidia) are parasitic upon tissue of fish hosts while completing the metamorphosis into juvenile mussels. Adult mussels are typically sessile and are found attached to the gravel bottom of a high quality river. Adult mussels are filter feeders and usually feed upon plankton and detritus from their aquatic environment. Triangular Kidneyshell mussels bring water from their habitat into their shells through specialized regions that are similar to the true siphons of clams. The water is then filtered over the gills and food particles are trapped and eventually digested.


Historically, the Triangular Kidneyshell was found in many rivers throughout Georgia , Alabama and Tennessee . Its historic range included the Black Warrior River and tributaries, the Cahaba River and tributaries and the Coosa River tributaries. However, this mussel is now limited to portions of the Black Warrior River drainage in Alabama and the Conasauga River in Georgia .

Conservation Status

age and Conasauga River .

Similar Species

This highly variable species of freshwater mussel has similar species that can make identification difficult. Several species of Pleurobema, including the Southern Pigtoe (Pleurobema georgianum) and Southern Clubshell (Pleurobema decisum), are quite similar and often must be differentiated through the process of elimination by professionals.