Identification of a fish host of the inflated heelsplitter Potamilus inflatus (Bivalvia: Unionidae) with a description of its glochidium.

INTRODUCTION

Potamilus inflatus is a federally threatened mussel that inhabits large rivers in the southeastern United States (U.S. Fish and Wildlife Service, 1992). The historical range of P. inflatus has decreased markedly in the last decade prompting concern over the conservation status of this organism. Historically, the inflated heelsplitter was known from the Amite and Tangipahoa rivers in Louisiana, the Pearl and Tombigbee rivers in Mississippi, and the Black Warrior, Coosa and Tombigbee rivers in Alabama (Hurd, 1974; Stern, 1976; Hartfield, 1988). Presently it is limited to the lower and middle reaches of the Amite and Pearl rivers in Louisiana and in the Black Warrior River between the Demopolis Lock and Dam upstream to the Oliver Lock and Dam in Alabama (U.S. Fish and Wildlife Service, 1992). Little is known about the natural history of P. inflatus, however, such information is critical to effective conservation and species management.

Reproduction of mussels in the family Unionidae differs from other bivalves. Before fertilization the eggs pass into the suprabranchial chamber and then into the water tubes of the gills where they are fertilized (Pennak, 1989). The developing embryos are retained in the marsupium, a modified portion of the gill (Thorpe and Covich, 1991). Members of the genus Potamilus are long-term breeders; the eggs are fertilized in the summer and the embryos are not released for almost a year (Heard and Guckert, 1970). A critical stage in the development of all unionid mussels is the attachment of the glochidium larvae on a suitable fish host. After attachment to a host, the glochidium is encysted as the tissue of the fish grows to cover it (Pennak, 1989). During this stage the juvenile mussels of some species develop their adult shell and anatomy (Surber, 1912, 1913, 1915; Cummings et al., 1990) that will enable them to begin life as a filter-feeding member of the benthic community. While all species of unionids do not appear to be host-specific, the genus Potamilus parasitizes the freshwater drum (Aplodinotus grunniens) almost exclusively (Surber, 1913; Wilson, 1916; Cummings et al., 1990). A single exception was reported by Surber (1913) who found glochidia of P. ohiensis on white crappie (Pomoxis annularis).

The objective of this study was to identify the fish host(s) of Potamilus inflatus. Such information may prove useful in management and recovery of the species, as unionids are dependent upon their fish hosts during a critical period of their natural history. Two species of Potamilus (P. inflatus and P. purpuratus) are present in the Black Warrior River. All described glochidia of Potamilus can be distinguished from those of other unionids by their axe-head …