Defense Date

12-4-2020

Document Type

Dissertation

Degree Type

Doctor of Philosophy

Degree Name

Oceanography/Marine Biology

First Advisor

Richard Spieler, Ph.D.

Second Advisor

Tamara Frank, Ph.D.

Third Advisor

Tracey Sutton, Ph.D.

Fourth Advisor

Alois Lametschwandtner, Ph.D.

Keywords

Climate Change, Ocean Acidification, Fish Respiration, Vascular Corrosion Casting, Mercox II, Coronary Circulation, Blood Oxygen Content

Abstract

Abstract

Fish stocks across the globe are increasingly being relied upon to supply the ever-growing human population with high quality protein and other nutrients. In addition to fishing pressure, stressors brought on by climate changes, including ocean acidification and hypoxic conditions, have the potential for providing substantial negative impacts on the efficiency of fish respiration, internal acid-base balance, and energy budgets. To meaningfully integrate the potential impacts of ocean acidification on fishes into management practices it is important to study the anatomical, physiological, and behavioral adaptations that enable fish to cope with decreasing seawater pH and how those mechanisms might be coupled with the functioning of the circulatory system.

The current average seawater pH is between 8.4 and 8.1. Climate modeling statistics, based on past and projected anthropogenic CO² output, have estimated that at current CO2 output trajectories we will see average seawater pH values around 7.8 by the year 2100, 7.6 by the year 2200 and potentially as low as 7.45 by the year 2300. Most marine fishes maintain internal pH homeostasis through the direct transfer of acid-base equivalents between the animal and its external environment. Compensation is achieved by adjusting plasma HCO₃⁻ levels in the blood through the differential regulation of H⁺ and HCO₃⁻ effluxes which are coupled to the influx of Na⁺ and Cl⁻. As a result, these compensatory changes can negatively impact respiration and osmoregulation. When there is decreased availability of oxygen in water, due for example to increase temperature, fish maximize the efficiency of O₂ uptake by regulating heart rhythm, blood pressure, and blood flow through the gills. Fish also rely on pH alterations of their blood to maximize hemoglobin uptake and delivery of oxygen and utilize the combined Bohr and Root effects to realize blood oxygen concentrations 25-30 times higher than what is available in the environment. The combined effects of ocean acidification and internal cellular metabolism could result in an overall acidification of blood and internal fluids impacting respiration and oxygen delivery to tissues and requiring fishes to expend substantial amounts of energy to compensate and acclimate to the changes. Fish responses and ability to compensate for impacts of ocean acidification is expected to be species specific but has not been extensively investigated.

To test the hypothesis that ocean acidification will result in a decrease in blood pH which will decrease the effectiveness of the Root and Bohr effect and in turn, cause an increased respiration rate to compensate, four species: snapper, crevalle jack, yellow stingray, and nurse shark, were subjected to experimental pH trials representing projected seawater pH levels for the years 2060 (pH 8.0) 2100 (pH 7.8) and 2200 (pH 7.6). Blood pH values and respiration rate were recorded for each individual at each experimental trial. Data analyses indicated that decreasing the pH of ambient seawater did cause a statistically significant decrease in the mean blood pH along with a corresponding significant increase in the mean respiration rate of each species at each level tested.

To fully understand how the environment can affect fish respiration, it is also important to understand the basic anatomy of the circulatory system and how it can benefit oxygen delivery to specific tissues. The heart of most modern fish species is often referred to as “venous” because it only has two chambers (atrium and ventricle) and is comprised mostly of spongy trabeculated myocardial tissue. With over 30,000 species cataloged to date, fish hearts exhibit an immense diversity in terms of shape, thickness, and composition. Fishes also exhibit a wide diversity of strategies for supplying oxygen to cardiac tissue ranging from passive diffusion through the cells and tissue layers from the luminal blood, complex networks of coronary vessels supplying oxygenated blood directly from the gills, or both with varying degrees of dependence on either strategy. The current literature indicates that, with some notable exceptions, i.e., seven species of anadromous salmon, five species of tuna, and seven species of shark, the hearts of most fishes gets oxygen and nutrients via diffusion from the luminal blood flowing through it. However, general anatomical information on the presence and extent of coronary circulation exists for less than a dozen species of fish and detailed routing from gills to heart and back has apparently never been described in any fish species.

The idea that the heart would be the last site in the body to receive oxygen and nutrients from the blood, much of which has been deoxygenated in post-gill capillary beds, is difficult to believe especially when the transfer of oxygen from the blood to tissues is a diffusion dependent process. The structure, composition, and distance of heart tissue layers from luminal blood would seem to act as barriers to diffusion from within the system. I hypothesized that luminal diffusion alone is not capable of supplying enough oxygen to the cardiac tissues found in the four target species. To examine this hypothesis, direct oxygen measurements were taken from various sites in each species including the ventricle, dorsal aorta, efferent branchial artery, caudal vein, and caudal muscle tissue using a high speed, fiber optic oxygen microsensor. Data analyses indicated that there was a 67% average difference between the mean dissolved oxygen content in venous versus arterial blood in teleost species and a 93% average difference between the mean dissolved oxygen content in venous versus arterial blood in elasmobranch species used for this study. The results of the average percent difference in blood dissolved oxygen content support the hypothesis that venous blood retains significantly lower dissolved oxygen compared to arterial blood. Apparently, there is still enough dissolved oxygen remaining in venous blood for diffusion into oxygen-depleted muscle tissues where the cells are in close contact with the capillaries. However, cardiac tissue in highly aerobic and is not closely associated with capillaries so it is questionable that diffusion from oxygen depleted luminal blood would be adequate. Fishes with thick, muscular hearts would presumably require a dedicated constant supply of oxygenated blood from the gills such as has been reported for some large, active species.

To meet the oxygen demands of the heart, I hypothesized that, rather than as exceptions, large active fishes, sharks, and rays have a dedicated vascular element (hypobranchial artery) from the gills that supplies a coronary circulation delivering blood to the tissue of the heart ventricle and the bulbus arteriosus. Vascular corrosion casts were made from representatives of each fish species. Microscopic analysis of these casts allowed me to identify and describe the origination, routing, and extent of the coronary circulation in crevalle jacks, yellow stingrays, and nurse sharks. It is generally expected that elasmobranchs have a coronary circulation, but it has never been identified and described in detail, prior to this study. Contrary to what is currently understood about the heart structure and composition of teleost species, a coronary circulation supplying the tissue of the bulbus arteriosus and ventricle was found in crevalle jacks. The singular, supplying coronary artery, was identified, and traced from its origination out of the efferent branchial artery of the third gill holobranch to the cardiac cavity, bulbus arteriosus, and ventricle. The coronary circulation supports arterioles and associated coronary capillary networks within the cardiac tissues that drain back into the lumen of the heart at the atrioventricular canal. A coronary circulation in teleost species has apparently only been described in salmon, tuna, and crevalle jacks. This may simply be due to inadequate investigation. I suggest that other highly active teleost species with muscular hearts would likewise have a coronary circulation and further research is needed to investigate this.

This study highlights the notion that anatomical investigation continues to be a relevant research topic that can lead to discoveries that have remained hidden by broad generalizations. It is clear that anatomical studies like this one are needed to continue identifying, describing, and cataloging the presence, extent, and variation of coronary circulation among different fish species. The results of this study also show that ocean acidification that is associated with climate change would have a significant impact on the physiological processes associated with fish respiration. The anatomical, behavioral, and physiological adaptations that allow fishes to cope with decreasing seawater pH and hypoxic conditions are highly variable ranging from increased respiration, regulating internal pH management, and maximizing blood oxygen supply and delivery that can be species specific. Further research is required to determine the extent to which these adaptations would impact the general and species-specific ecology of fishes.

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