Dehydration Stress in the Wolf Spider Schizocosa Ocreata (Araneae: Lycosidae): Tolerance, Resistance, and Coping Mechanisms
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Dehydration Stress in the Wolf Spider Schizocosa ocreata (Araneae: Lycosidae): Tolerance, Resistance, and Coping Mechanisms Dissertation Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University. By: Samantha Kelly Herrmann, BA Graduate Program in Evolution, Ecology, and Organismal Biology The Ohio State University 2015 Dissertation Committee: Dr. J. Andrew Roberts, Advisor Dr. Richard Bradley Dr. Roman Lanno Copyright by Samantha Kelly Herrmann 2015 Abstract Dehydration stress is a potential challenge for any terrestrial organism that must seek out free water and limit the amount of water lost to the environment. Water is required for maintaining homeostasis and so dehydration can affect a number of functions in the body, including nutrient transport, structure and mobility, and thermoregulation. Animals that experience dehydration can respond by tolerating it or by making behavioral and physiological adjustments to evade it or mitigate its effects. We examined the effects of dehydration stress on the brush-legged wolf spider, Schizocosa ocreata (Hentz 1844), by investigating three potential modes of responding to dehydration stress: 1) tolerance 2) behavioral adjustment and 3) stress hormone (octopamine) production. We investigated dehydration tolerance by examining survivorship under varying humidity regimes, and measuring total body water content, critical water loss mass, and water loss rates. Using controlled humidity chambers, we specifically compared male and female S. ocreata, which we hypothesized to have different dehydration tolerances due to differences in morphology, metabolism, reproductive strategy, and life history traits. Males and females survived significantly longer at higher relative humidity (>55% RH), but females survived significantly longer than males at each RH level. Females had significantly lower critical water loss mass and lower water loss rates than the males, while the males had higher body water content relative to dry mass. We concluded that ii females have a greater dehydration tolerance than the males and would likely be more successful long term in dry environments. Behavioral responses to dehydration stress can help the individual minimize the effects stress or locate depleted water resources. We investigated the behavioral responses to dehydration stress by examining three aspects of wolf spider behavior: exploratory behavior, response to perceived threat, and microhabitat selection. We used dehydration chambers to stress the spiders and compared their behavioral responses to those of low-stress spiders. While there was no difference between the groups in propensity to explore a novel environment, we found that dehydration-stressed spiders displayed reduced anti-predator behaviors and chose cooler microhabitats than the low- stress spiders. The results from this study provide an understanding of how spiders behaviorally mitigate stress and the tradeoffs they may make in responding to dehydration stress. Finally, we studied the effects of dehydration stress on production of the stress hormone octopamine. In arthropods and other invertebrates, octopamine modulates many behaviors and mobilizes lipids to help the animal prepare for a “fight or flight” situation. We used dehydration chambers with and without desiccant to compare dehydration-stress spiders to container-stress spiders, both of which we compared to low-stress, minimally handled control spiders. We hypothesized that wolf spiders demonstrate a graded hormonal response to stress. Both the dehydration-stress spiders and the container-stress spiders had elevated levels of octopamine compared to the low-stress spiders and there was no difference between the dehydration-stress spiders and container-stress spiders, iii suggesting that octopamine production is an all-or-nothing response to stress. This study confirmed that the neurohormone octopamine is produced in response to stress in a wolf spider, and has provided information critical to understanding how behavioral responses to stress are likely moderated. Altogether, these studies greatly expand our understanding of dehydration stress response, and stress in general, in an important behavioral model system. This work has significant implications for the design of a wide array of behavioral experiments from predator avoidance, to courtship and mate seeking in an environmentally variable and structurally complex deciduous forest habitat. iv Dedication To my incredibly supportive and patient family. v Acknowledgments I’d like to thank all of my undergraduate researchers, without whom I’d not have been successful on this project, including Bernard Paniccia, Matthew Harcha, Madison Nashu, Jessica O’Hara, Christina Lehn, Abigail Fresch, and Christa Eyster. Additionally, the advice I received from my committee members Dr. Richard Bradley and Dr. Roman Lanno was hugely valuable and I appreciate the time they were willing to spend with me. I also would like to thank The Department of Evolution, Ecology, and Organismal Biology, The Ohio State University at Newark, and the American Arachnological Society for support. Above all, I am immensely grateful for the support of both my advisor, Dr. J. Andrew Roberts, and my labmate, Dr. Ryan Bell. Without their patience and encouragement this would not have been possible. vi Vita June 2004……………….St. Charles North High School May 2008……………….B.A. Zoology and Environmental Sciences, Miami University August 2008-Present……Graduate Teaching Assistant, Department of Evolution, Ecology, and Organismal Biology, The Ohio State University Autumn 2012 & 2013….Instructor of Record, Human Biology, Department of Center for Life Science Education, The Ohio State University 2013…………………….Center for Life Science Education Graduate Teaching Award Field of Study Major Field: Evolution, Ecology and Organismal Biology vii Table of Contents Abstract………………………………………………………………………..…………..ii Dedication………..………………………………………..……………………..………..v Acknowledgements ……….…………………………………………………….………..vi Vita………………………………………………..………………….…………..……...vii List of Tables……………………………………...……………………………….……..ix List of Figures……………………………………..…………………………………..…..x Chapter 1: Introduction to dehydration stress: what are the challenges and how do animals respond?......................................................................................................1 Chapter 2: Dehydration Tolerance in the Wolf Spider Schizocosa ocreata: A comparison of survivorship, body water content and critical water loss, and water loss rates between sexes………………………………………………….………………..10 Chapter 3: Behavioral coping mechanisms to dehydration stress in the wolf spider Schizocosa ocreata…………………………………………………….…………30 Chapter 4: Physiological responses to water loss in the wolf spider Schizocosa ocreata: the role of octopamine in dehydration stress………………………...…………..48 References………………………………………………………….…………………….64 viii List of Tables Table 2.1: Post-hoc analysis of survivorship between males and females…………26 ix List of Figures Figure 2.1: Mean survival time for males and females in each humidity regime…….....27 Figure 2.2: Survivorship in varying humidity regimes in (A) females and (B) males......28 Figure 2.3: Body water content in males and females…………………………….…..…29 Figure 3.1: Aerial view of the exploratory arena…………………………………...……44 Figure 3.2: Time spent in each zone of the exploratory arena…………………..…….…45 Figure 3.3: Freeze time after receiving a puff of air…………………………..…………46 Figure 3.4: Preference index of dehydration-stress and low-stress spiders………...…....47 Figure 4.1 Sample chromatogram of wolf spider hemolymph…………………………..62 Figure 4.2: Normalized octopamine levels among three groups of spiders………..……63 x Chapter 1: An Introduction to Dehydration Stress: What Are The Challenges and How Do Animals Respond? Abstract Throughout its lifetime, an animal may encounter a variety of environmental stressors, including thermal stress, dehydration stress, wind stress, etc. All of these have the potential to disrupt homeostasis, and so organisms are expected to respond to stressors behaviorally and physiologically to reduce or mitigate the effects of that stressor. Dehydration stress receives little attention in the literature, yet water is essential to maintaining proper body functions in all animals. Animals can respond to dehydration stress a number of ways, such as migrating towards environments with more moisture to reducing their respiratory water loss via discontinuous gas exchange. In the following review, I discuss the concept of stress in biology and how animals can respond to stress. I then address dehydration stress and the potential problems it causes individuals. Finally, I cover the ways animals respond both behaviorally and physiologically to avoid or reduce the impact of dehydration. 1 The Concept of Stress in Biology Stress has traditionally been broadly defined as the “non-specific response of the body to any demand” (Selye 1978). Unfortunately, the term has come to be used in a myriad of different ways, and might be used to describe the event causing a behavioral or physiological response in an organism, or it might describe the response itself (see McEwan and Wingfield 2003). In the work presented here, I follow the McEwan and Wingfield (2010) definition of “stress” as real or implied disruption of homeostasis, or the maintenance of physiological conditions within an optimal range within an organism where the “stressor” is