Dazed and confused: pesticides alter physiology, behavior, and predator-prey interactions of juvenile and adult blue crabs (Callinectes sapidus)

Toxicants (i.e., pesticides) and predators may have large and interacting effects on natural communities by removing species (lethal effect) or by altering organismal physiology or behavior (sublethal effect). Studies evaluating the effects of sublethal concentrations of pesticide mixtures are limited, especially in coastal systems [1]. The purpose of this dissertation research was to investigate both lethal and sublethal effects of realistic pesticide exposure scenarios on two life-stages (juvenile and adult) of an important invertebrate estuarine predator, prey, and fishery species, the blue crab (Callinectes sapidus). Importantly, blue crab populations are declining throughout the U.S., but the potential role of pesticides in declines remains largely unexplored. In a series of laboratory experiments, I investigated: (1) lethal and sublethal effects of a single exposure to carbaryl (carbamate), malathion (organophosphate) and resmethrin (pyrethroid) + PBO (synergist), individually and in mixtures, on juvenile and adult blue crab survival and neuromuscular functioning by measuring changes in mortality, righting time (RT), and eyestalk reflexes. These responses serve as a proxy for the direct effects of exposure on survival and indirect effects on coordinated behaviors critical to blue crab survival (e.g., predator escape or foraging). Pesticides were selected because they are three of the mostly commonly used throughout the U.S. and have different modes of action. Effects observed at the organismal level were subsequently evaluated and linked with changes in (2) predator-prey interactions (mesocosms), and (3) physiological responses (enzyme assays). Lastly, (4) differences in susceptibility between juvenile (post-planktonic) and adult life-stages were also evaluated in behavioral and predator-prey experiments. Sublethal, legally allowable concentrations of individual pesticides and pesticide mixtures negatively affected juvenile and adult blue crabs by (1) reducing survivorship and locomotor functioning, (2) altering predator-prey interactions via changes on foraging rates and increased vulnerability to predators, and (3) increasing metabolic costs (e.g., AChE synthesis). These findings underscore the importance of studying pesticide effects in an ecological context, as juvenile life-stages were not always the most vulnerable, some effects varied non-linearly with concentration, and interactions between individual pesticides in mixtures were not necessarily predicable based on individual exposures. Notably, blue crabs were most sensitive to exposures including pyrethroid (resmethrin) + PBO, which are representative of common co-components of vector control products. Pyrethroid use for mosquito abatement and disease control is expected to increase, and the application of such products near aquatic systems should be carefully evaluated. In blue crabs, behavioral changes (e.g., RT) provided a reliable and sensitive endpoint, indicating altered physiological (i.e., increased AChE activity) and predator-prey interactions (i.e., reduced foraging, increased vulnerability to predators) in the pesticide exposures studied. Results also highlight the importance of studying individual responses with increasing levels of biological organization, e.g., changes in species interactions, as increases in RT unexpectedly corresponded with increased consumption rates in juvenile crabs (e.g., hyperactivity, Chapter 2). In the context of fisheries management and environmental regulations, RT may be a useful endpoint when measured in combination with other responses to indicate chances of survival or altered trophic relationships [2, 3].