An Assessment of Potential Contaminant Effects on Freshwater Mussels in the South Florida Ecosystem
Ruessler, D.S.,1* Kernaghan, N.J.2, Wieser, C.M.1, Wiebe, J.J.2, Gross, T.S.1,2
1USGS - Florida Integrated Science Center, Gainesville, Florida
2College of Veterinary Medicine, Department of Physiological Sciences
University of Florida, Gainesville, Florida
Approximately 2 million acres of the Everglades drainage system contain sediments and biota with elevated mercury and other contaminants related to human activities. The current study examined species diversity, health, and reproductive status of freshwater mussels in South Florida in relation to habitat alteration and contaminants. Mussels were present in only 23 of the 47 suitable sites examined. Although species diversity and abundance widely varied between sites, two species Elliptio buckleyi and Utterbackia imbecillis were each present at multiple sites. Approximately 25 adult mussels of each of the two species were collected per site and health assessed using: soft tissue, mantle, total gill and shell wet weights; shell length, shell width, and mantle glycogen concentrations. A body weight to length ratio was calculated as a body condition index (BCI). Histological analyses were utilized to determine sex and reproductive stage. Contaminant analyses indicated significant bioaccumulations of mercury, organochlorine pesticide residues and PCB's, which varied widely between sites. Foot biopsy or mantle tissues were analyzed for sex steroid concentrations (Estrogen and Testosterone) to assess reproductive status and potential contaminant related endocrine disruption. Health assessments indicated decreased body condition indices and mantle glycogen concentrations for mussels from sites with elevated mercury and organochlorine pesticide exposures. Likewise, endocrine analyses indicated decreased estrogen for females and decreased testosterone concentrations for males from sites with increased mercury and pesticide exposures. Exposure differences were also correlated to differing reproductive status. These data are among the first to indicate potential endocrine disrupting effects of contaminants in invertebrates. Overall, these data indicate that both habitat degradation and elevated contaminant levels from anthropogenic sources are probable causes of mussel declines.
Freshwater mussels are one of the most endangered groups of animals in North America. About 70 percent of the nearly 300 species in North America are classified as either endangered, threatened, extinct, or in serious decline (Williams et al, 1992). In the United States, 42 mussel species are federally protected with another 70 species proposed and/or candidates for listing (USFWS, 1994). Many threats have been documented for this drastic decline, such as siltation of suitable habitats, poor land management practices, channelization, host-fish declines, non-indigenous species, and decreased water quality from pollutants such as nutrients, herbicides, pesticides, and a wide variety of urban and industrial chemicals. However, little is known about the effects of environmental contaminants on freshwater mussels.
Much of the data indicating effects of environmental contaminants in wildlife, has focused on effects in fish, reptiles and/or other macro-faunal vertebrates. Indeed, as with many ecosystems, the macro fauna have been studied extensively, while invertebrates have largely been ignored. Aquatic invertebrates, such as freshwater mussels would be expected to be exposed to both sediment and water partitioning of contaminants. Indeed, exposures to environmental contaminants and potential effects may be related to the widespread population decreases frequently reported for freshwater mussels.
Our laboratory has recently developed freshwater mussels as models for general toxicity and as important environmental sentinels, early signals, of environmental effects for contaminants such mercury, pesticides and papermill effluents (Kernaghan et al., 1998a&b; Ruessler et al., 1998; Gross et al. 1998). These preliminary studies have utilized freshwater mussels (Elliptio buckleyi and Utterbackia imbecillis) as models for detecting the effects of environmental contaminants within controlled and natural environments. These efforts include the initial validation and development of several indices and biochemical assessments of health status as well as important indicators of reproductive health/function. Indeed, these methods have included the first reported validation of sex steroid monitoring procedures for freshwater mussels.
To assess whether chemical stressors/contaminants are significantly involved in the widespread declines in freshwater mussel populations, it is important to study populations that are potentially exposed and may be sensitive to contaminants. The current study focused on the south Florida/Everglades ecosystem and the endemic populations of freshwater mussels as a field application of mussels as environmental sentinels of contaminant effects and ecosystem health. These efforts include an ecosystem (Everglades) wide survey of endemic populations for initial assessments of biomarkers and bioindicators of effects such as health indices: body condition and organ somatic indices, tissue histopathology, and tissue glycogen, as well as indices of reproductive function: tissue sex steroids and reproductive success.