An Ephemeral Contributor to a Toxic Legacy? Group-Specific Molecular Identification of Prey in Mercury-Contaminated Spiders by PCR

The biological toxicity of mercury has been well studied within in vitro and in vivo experimental systems. However, worldwide cases of industrial release into aquatic ecosystems have confirmed the potent effects of its entry into the food web. As environmental mercury becomes methylated, it is incorporated into biological tissues and undergoes biomagnification, reaching progressively higher concentrations in higher trophic positions. Through this effect, top predators such as bass have been found with mercury concentrations of up to 10,000,000 times the level in the surrounding environment. The consumption of such contaminated fish and other wildlife by humans has resulted in severe neurotoxicity, affecting nearly every neurosensory system and irreversibly damaging the brain. Because mercury is transferred and concentrated through the consumption of prey, its movement through contaminated food webs is of great interest. This thesis is an effort to understand the movement of dietary mercury from a contaminated aquatic ecosystem into the terrestrial ecosystem surrounding it. In the South River, a tributary of the Shenandoah River in Virginia, two decades of industrial mercury release have contaminated both the nearby aquatic and terrestrial ecosystems, but the dietary connection between them is unknown. Terrestrial spiders near the South River have been found with mercury levels higher even than fish from the river itself. These spiders are contributing the majority of dietary mercury to bird populations living near the river which are experiencing a multitude of negative effects due to this toxin. This thesis attempts to link the predation of terrestrial spiders to the aquatic emergence of Ephemopterans (mayflies), a known contributor of biomass to terrestrial environments. Mayflies from the river have exhibited significant levels of contamination, and are a plausible prey item. Terrestrial spiders and mayflies were collected from the South River during Summer 2008, and their mitochondrial DNA was amplified using PCR with universal invertebrate primers. This DNA was then sequenced to compare mayfly and spider mitochondrial genotypes and design mayfly-specific primers. During Summer 2009, terrestrial spiders were collected from the same sites along the river where emerging mayflies were collected. A small subset of these spiders were fed mayflies in feeding trials to establish how long mayfly DNA can be detected in spider gut contents. A large-scale gut content analysis was then carried out on 111 spiders. The mayfly-specific primers were tested on all collected mayflies, and successfully amplified the DNA of 76% of these individuals overall. Feeding trials were carried out with varying lengths of spider digestion time, and mayfly DNA was identified for up to 24 hours without any time-dependent decrease in detection ability. When the gut contents of 111 terrestrial spiders were analyzed, none of the samples showed any presence of mayfly DNA. Mayfly abundance may have been too low, and mayfly genetic diversity may have escaped the utilized primers. However, the efficacy of the methodology developed in this study makes it is unlikely that the high levels of contamination in these spiders are caused by preying on mayflies. These results provide important knowledge of the dietary connection between the aquatic and terrestrial food webs at the South River. If other aquatic insects can be linked to the diets of these spiders, the source of mercury can be localized to the river itself, rather than to historic deposits in the floodplain. Efforts of remediation to prevent further spreading of mercury depend on its environmental location, so further studies of the terrestrial spider prey are necessary.