After decades of pollution regulation in the United States, many pollutants exist in the environment below levels that cause mortality; even so, sublethal levels of environmental pollutants can still result in indirect mortality and reduced abundance of organisms. Understanding the consequences of pollution at chronic sublethal levels is one of the main goals in toxicology and in environmental risk assessment. This dissertation focuses on understanding the impacts on multiple fish species from environmentally relevant sublethal levels of two developmental neurotoxicants that commonly occur in aquatic environments, methylmercury (MeHg) and 3,3',4,4',5-pentachlorobiphenyl (PCB126). The first chapter provides a background of fish behavior assays used in toxicology; gives background information on a commonly used fish species in toxicology, the Atlantic killifish (KF; Fundulus heteroclitus); and summarizes a relatively new technique to compare biological responses to chemical exposure, the Adverse Outcome Pathway (AOP). The second chapter summarizes the impact of MeHg on fish larvae behaviors by conducting an analysis that combines recent research results into feeding, swimming and startle response behavior affects after exposure. Mercury has long been known to cause neurological deficiencies especially during neurological development which can cause temporary and/or permanent impairments to brain function. One way to assess these impacts is by examination of behavior after exposure. This chapter analyzes fish larval behavior effects and constructs predictive models that could be used in future mercury risk assessments. The third chapter focuses on assessing changes in larval fish behavior after exposure to neurotoxicants using a new analytical approach. Typically in toxicological behavior assays, fish behavior is summarized as an average response over the length of the assay. However, average responses could miss more sensitive behavior changes brought on by chemical exposure. New behavior models such as hidden Markov chain models (HMMs) could potentially detect more behavior alterations and allow for increased accuracy in chemical exposure assessments. This chapter analyzes impacts detected from both traditional and HMM behavior endpoints in yellow perch larvae (YP; Perca flavescens) and how they are altered by MeHg and PCB126 embryonic exposure. The fourth chapter of this dissertation applies the new and traditional analytical techniques described in chapter 3 to the specific case of KF after embryonic exposure to MeHg and PCB126. This species of killifish is unique because some populations have naturally evolved tolerance to industrial pollutants which could highlight the biological mechanisms behind this evolution through comparisons between non-adapted and adapted populations. In this chapter, these comparisons are made using the AOP framework, allowing for visualization of molecular, organismal and population level effects from chemical exposure. The fifth chapter in this dissertation investigates different aspects of the AOP framework, specifically whether different biological responses can be shared over three different fish species after exposure to MeHg and PCB126. The AOP framework has been used to assess impacts across different species and chemicals by assuming common biological responses in organisms that share similar molecular, cellular and organ functions. This chapter investigates the use of AOPs in assessing the similarity of biological responses in gene expression, larval behavior and predicted cohort survival and growth. This work advances our knowledge of the sub-lethal impacts of two common neurotoxicants in aquatic ecosystems and their impacts on multiple levels of biological organization in fish larvae.
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