ORCID ID

https://orcid.org/0000-0001-6356-0337

Date Awarded

2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Virginia Institute of Marine Science

Advisor

Richard W. Brill

Committee Member

Mary C. Fabrizio

Committee Member

Emily B. Rivest

Committee Member

Kevin C. Weng

Committee Member

Holly A. Shiels

Abstract

This dissertation examines physiological stress responses of coastal elasmobranch fishes and investigates mechanisms that maintain oxygen delivery under stress. Elasmobranch fishes are, in general, understudied despite their susceptibility (due to K-selected life histories) to unsustainably high fishing mortality and the effects of climate change. Knowledge of physiological stress responses is, therefore, necessary to understand species-specific resilience and overall susceptibility to stressors. In Chapter II, I describe the hypoxia tolerances of clearnose skate (Rostaraja eglanteria), thorny skate (Amblyraja radiata), and summer flounder (Paralichthys dentatus, a representative teleost species) under conditions of temperatures and acidification projected to occur by the end of the century due to climate change. At the least-stressful temperature, all three species exhibited significant increases in standard metabolic rate and decreases in hypoxia tolerance with a 0.4 unit drop in pH. Clearnose skate were, however, found to be among the most hypoxia-tolerant elasmobranch species known. To investigate the physiological adaptations underpinning this extreme hypoxia tolerance, I measured blood-oxygen affinity (measured as P50 - the oxygen partial pressure (pO2) at which hemoglobin is 50% saturated with O2) of clearnose skate (Chapter III). Clearnose skate exhibited a relatively high P50 (4.9 ± 0.6 kPa) at 20 °C that was not different from that of the equally hypoxia-tolerant epaulette shark (4.3 ± 0.6 kPa at 28 °C). Yet, these values are significantly higher than those of many other elasmobranch species, suggesting blood-oxygen affinity may not be correlated with hypoxia tolerance in elasmobranch fishes as it is in teleost fishes. At the cellular level, the erythrocyte intracellular pH influences hemoglobin-oxygen (Hb-O2) affinity. In teleost fishes, decreases in Hb-O2 affinity (Bohr effect) and maximum blood oxygen carrying capacity (Root effect) result from acidosis. These responses can be mitigated through red blood cell (RBC) swelling. The aim of Chapter IV was, therefore, to investigate the prevalence of RBC swelling in elasmobranchs fishes. Blood samples were taken from captive individuals of five elasmobranch species: Clearnose skate, sandbar shark (Carcharhinus plumbeus), blacktip reef shark (C. melanopterus), sicklefin lemon shark (Negaprion acutidens), and epaulette shark (Hemisyllium ocellatum) following exhaustive exercise and acute air exposure. None of the measured hematological parameters (hematocrit, blood hemoglobin concentration [Hb], RBC counts, RBC volume, and mean corpuscular hemoglobin content; MCHC) were indicative of RBC swelling, although published results show swelling occurs in sandbar and epaulette shark RBCs. Other impairments to the cardiorespiratory may result from external stressors, such as those associated with interactions with fishing gear. I hypothesized the cause of post-release mortality in elasmobranch fishes is diminished myocardial function. To test this hypothesis, I measured changes in the functional properties of isolated ventricular myocardial muscle from clearnose skate, smooth dogfish (Mustelus canis), and sandbar shark during hyperkalemia (7.4 mM K+), acidosis (a pH decline of 0.8 units), and reduced oxygen (to 31% saturation) at two temperatures (Chapter V). Stressors had relatively small and species-specific detrimental impacts on myocardial function that were only partially ameliorated adrenergic stimulation (i.e. by application of isoproterenol, an adrenaline analog). Overall, this dissertation implies that coastal elasmobranch fishes maybe highly resilient to the effects of directional climate change and the physiological stress associated with interactions with fishing gear.

DOI

http://dx.doi.org/10.25773/v5-6hq9-zy38

Rights

© The Author

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