Freshwater Mussels of Florida

Chapter 5. Mussel Distribution

Chapter 5 Mussel Distribution The North American freshwater mussel fauna is represented by the families Margaritiferidae , Mycetopodidae, and Unionidae, with approximately 375 mussel species (Williams et al. in review). Unionidae is the largest family with about 365 species occurring from southern Mexico to northern Canada and Alaska. The two remaining families are much smaller with Margaritiferidae represented by five species and Mycetopodidae by three species, and they have far more restricted distributions in North America. Margaritiferidae is limited to southern Canada and northern United States and a few small streams in Alabama and Louisiana, with the exception of one species that is widespread in the Mississippi basin. Mycetopodidae is limited in NorthAmerica to southern Mexico but is more widespread southward into Central and South America. The southeastern United States is the epicenter of aquatic biodiversity in North America and has been recognized as far exceeding that of other regions of the continent as well as most other temperate regions of the world. This is true for some invertebrates (e.g., mussels, snails, and crayfishes) and vertebrates (e.g., fishes and turtles) (Lydeard and Mayden 1995). Mussel diversity in this region is highest in Alabama and the Mobile basin with 182 mussel species in 44 genera (Williams et al. 2008, in review; Campbell and Lydeard 2012a). Other southeastern states have noteworthy mussel diversity: Tennessee with 129 species (Parmalee and Bogan 1998), Georgia with 120 species (J.D. Williams unpublished data), and Kentucky with 104 species (Cicerello and Schuster 2003). The exceptionally high aquatic species diversity in the Southeast owes its origin and existence to the geologically old and stable land mass, physiographic diversity, absence of glaciers, Pleistocene sea level fluctuations, high rainfall, and abundant water resources feeding numerous rivers. Of the 60 mussel species known from Florida, 58 are native and 2 (Anodonta suborbiculata and Toxolasma parvum) were introduced from other parts of the United States. Five of the native species—Amblema plicata, Elliptio crassidens, Megalonaias nervosa, Pyganodon grandis, and Utterbackia imbecillis—are widespread in the Mississippi basin and eastern Gulf Coast rivers to north-central Florida. The Greater Floridan Region is defined herein as the river basins of Florida from the Perdido River east to St. Marys River along with their upstream reaches in Alabama and Georgia (Figure 5.1). There is a total of 65 species (62 native and 3 introduced) of Margaritiferidae and Unionidae (Williams et al. 2011) in the Greater Floridan Region, which represents approximately 20 percent of the mussel fauna in the United States. Of the 65 species, 5 taxa are found only in Alabama and Georgia portions of the region. The geographic position of Florida is unique in that it encompasses several major Gulf and Atlantic Coast basins of the southeastern United States. This setting is important zoogeographically in that Florida has likely been connected to southern Atlantic Coast basins to the north (e.g., Satilla and Altamaha in Georgia) and Gulf Coast basins to the west (e.g., Mobile and Pascagoula in Alabama and Mississippi) during its geologic past. However, having access to the distinct and more diverse freshwater faunas of these two disjunct coastal areas has not had a positive effect on increasing diversity. In fact, Florida has a somewhat depauperate fauna in comparison to adjacent states. This is due in part to being farther away from those centers of diversity and having been partially inundated Mussel Distribution 60 during interglacials. This has resulted in a corresponding reduction in number of species and genera (Table 5.1). A similar pattern is evident in the diversity of Florida fishes (Swift et al. 1986). Figure 5.1. The Greater Floridan Region. This region is inclusive of all river basins, from their headwaters to the ocean, between Perdido River in Alabama and Florida and St. Marys River in Florida and Georgia. Mussels are primarily limited in their dispersal capabilities by the fishes that serve as hosts for their glochidia. Depending on the dispersal habits of the host fish, movements may be limited to less than one hundred meters per generation (e.g., darters of the family Percidae) or several kilometers (e.g., suckers of the family Catostomidae) (Matthews 1998). Juvenile mussels move only a short distance once they drop off of the host fish Mussel Distribution 61 and become embedded in the substrate; likewise, adults of most species have very limited movement. Most mussels are intolerant of saline environments, which further limits potential movements...