The skin of many fishes contains large epidermal club cells (ECCs) that are known to release chemicals (alarm cues) that warn other fishes of danger. Initial research on ECCs focussed on their role in predator avoidance behaviour, however later research revealed that these cells might also have immune functions. Anthropogenic activities have dramatically increased over the past decades, with the consequence that many organisms simultaneously get exposed to multiple environmental stressors. We have seen considerable reductions in stratospheric ozone with a concomitant increase in global ultraviolet radiation (UVR). Metal pollution associated with industrial activity is also increasing on a global scale. Cadmium (Cd) is one such ubiquitous pollutant which is known to be toxic to organisms at extremely low concentrations. The main goal of my PhD research was to understand how multiple environmental stressors play a role in altering ECC investment and chemically-mediated predator-prey interactions by indirectly elucidating the evolutionary role of ECCs. The first experiment investigated the effects of in vivo ultraviolet radiation (UVR) exposure on ECC investment, physiological stress responses and potency of alarm cues in fathead minnows (Pimephales promelas). Subsequently, I investigated the interactive effects of UVR and/or waterborne cadmium (Cd) exposure using the same end points. I found that minnows exposed to UVR, either in the presence or absence of Cd, showed consistent decrease in ECC investment compared to non-exposed controls. There was a significant increase in cortisol levels of UVR exposed minnows compared to unexposed minnows. However, the combined exposure of UVR and Cd reduced cortisol levels relative to that in UVR only exposure. Surprisingly, there was no difference in the potency of the cues prepared from the skin of UVR and/or Cd exposed or non-exposed fish indicating that UVR and/or Cd exposure combined may have little influence on chemically-mediated predator-prey interactions. In aquatic systems, much of the negative effects of UVR are minimized by dissolved organic carbon (DOC) which is known to attenuate rates of UVR across the water column. In my third study, I investigated if DOC played a role in ameliorating the effects of in vivo UVR exposure on physiological stress and ECC investment in fathead minnows. I used two sources of DOC, a commercial soil based DOC (Sigma Aldrich Humic Acid) and a terrigenous source of DOC (Luther Marsh Natural Organic Matter). I found that fish exposed to UVR, in the presence of either source of DOC, in the presence and absence of UV blocking filter, maintained high ECC investment and reduced cortisol levels compared to fish exposed to UVR only. Studies that have examined factors that influence ECC investment have often been hampered by large variation in baseline levels of ECC. The larger the baseline variation in ECC number, the more difficult it is to elucidate factors responsible for changes in ECC investment. While I did not find this problematic in my work with UVR and Cd, others have failed to find effects in manipulative experiments. Consequently, my fourth study examined between and within variation in ECC investment across multiple sites in Saskatchewan and tried to investigate if holding fish under controlled laboratory conditions for up to 28 days would help reduce variation in ECC investment between and within populations. I found some evidence that I could reduce within population variation in ECC investment through time, but could not reduce among-population variation in mean ECC investment.
| Identifer | oai:union.ndltd.org:USASK/oai:ecommons.usask.ca:10388/ETD-2015-04-1510 |
| Date | 2015 April 1900 |
| Contributors | Chivers, Douglas P., Niyogi, Som |
| Source Sets | University of Saskatchewan Library |
| Language | English |
| Detected Language | English |
| Type | text, thesis |
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