CREATION OF A GLOCHIDIA KEY FOR ST. CROIX RIVER NATIVE MUSSELS THAT PRODUCE SMALL GLOCHIDIA

Jennifer Bury and Mark Hove, University of Minnesota

Minnesota is home to 48 native freshwater unionid mussel species. The St. Croix River alone contains 43 species including the last known reproducing population of the federally endangered winged mapleleaf, Quadrula fragosa (Conrad, 1835). Mussels have the highest ratio of species listed as threatened, endangered, or of special concern, making them the most endangered group in North America. Mussels play important roles in the aquatic ecosystem by filtering particulates and serving as a valuable food source for a variety of animals. Yet, many basic aspects of their life cycle are unknown. The mussel reproductive cycle includes a parasitic stage where the larva (glochidium) must attach to the gills or fins of a host fish or they will die. If the larva attaches to a suitable host species, the glochidium is encapsulated in a cyst where it undergoes transformation into a juvenile, which then drops off of the host fish. Some mussel species can use a variety of host fishes, but others specialize upon only a few. For many species, the host or hosts are unknown. If host information is limited, as is the case with Quadrula fragosa, then mussel conservation strategies may not be effectively implemented. The best way to confirm that a fish serves as a host for a particular mussel species is to observe glochidia metamorphosis from naturally infested fish. To facilitate identification of unknown glochidia valves from naturally infested fish, a key was created for small glochidia based on shell morphology and dimensions measured from scanning electron micrographs. Keys to mussel glochidia are needed to facilitate the identification of glochidia that fall from naturally infested fishes. Glochidia release periods can be pinpointed by using this key to identify glochidia collected by drift sampling.

SUGGESTED READING

Hoggarth, M. A. 1999. Descriptions of some of the glochidia of the Unionidae (Mollusca: Bivalvia). Malacologia 41: 1-118.

Williams, J. D., M. L. Warren, Jr., 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.