Metabolic and Oxidative Stress Responses of Cassiopea sp. to Environmental Stress : Towards a Better Physiological Understanding of Jellyfish's Tolerance

Abstract

Jellyfish are commonly seen as robust, noxious and unwelcoming animals. In recent decades, reports concerning jellyfish outbreaks and invasions are increasing worldwide. Regardless of the debate about the underlying drivers of jellyfish blooms, climate change and anthropogenic activities are commonly perceived as the main drivers. There is, however, scarcity of strong evidences to support this perception, because only few manipulative experiments have addressed the physiological responses of jellies to varying environmental stressors. The overall aim of this thesis is to use manipulative experiments and field excursions to test hypotheses about tolerance of jellyfish to stressors associated with climate change and anthropogenic activities for better prediction of their fate in the future. Due to climate change, extreme weather conditions are becoming more frequent and severe. In chapters 2 and 3, I investigate the metabolic and oxidative responses of the upside-down jellyfish Cassiopea sp. (Cassiopea hereafter) medusae to sudden changes in seawater temperature (i.e., either rise or drop by 6 AdegreeC from the control temperature). Medusae responded in contrasting manners to drop and rise in seawater temperature. While medusae treated at low temperature (20 AdegreeC) looked unhealthy and showed signs of decreased physiological performance (i.e., in term of decreased body mass and size) after two weeks, medusae treated at high temperature (32 AdegreeC) gained in body mass and size, indicating an enhanced performance for the same period. At the cellular level, medusae treated at low temperature suffered from oxidative stress-induced cellular damage and elevated metabolic demand, while no oxidative stress or signs of increased energy demand were evident in medusae at higher temperature. The overall results of these two chapters suggest that Cassiopea medusae are more tolerant to temperature rise than drop. They might benefit from global warming to spread and expand their populations in the future as well. Coastal systems experience a variety of pollutants, nutrient loading and other burdens associated with anthropogenic activities. In chapter 4, I investigate the anaerobic potential and oxidative stress responses in Cassiopea medusae collected from anthropogenically impacted and protected marine coastal habitats. While medusae from all investigated locations did not show signs of oxidative stress-induced damage (e.g., lipid peroxidation), the medusae from polluted locations had more anaerobic potential (e.g., high PK and LDH activities). While the results of Chlorophyll-a (Chla) contents measurements did not show clear trends in medusae from the studied locations, it seems that medusae Chla content is more sensitive to water clarity than to pollution status in the studied sites. Overall these results suggest that Cassiopea seems to be robust to the level of pollution at the studied sites and they might be anaerobically poised to live and thrive at such habitats. Noteworthy to mention here is that while the studies conducted in this thesis could work as a framework for future studies aimed for better understanding of jellyfish physiological responses, the thesis findings do not claim ultimate proofs for tolerance or sensitivity in all jellies. However, this thesis, and for the first time, highlights the feasibility and importance of understanding the underlying mechanisms of jellyfisha s physiological tolerance/sensitivity to changing environmental conditions. By using the epibenthic jellyfish Cassiopea as a representative model for studying tropical jellyfisha s ecological roles and responses to environmental stressors, this thesis encourages doing further researches on this jellyfish.