Purely diffusive O2 transport typically is insufficient to sustain aerobic metabolism in most multicellular organisms. In small animals, however, a high surface-to-volume ratio may allow passive diffusion alone to supply sufficient O2 transfer. The purpose of this thesis was to explore the impacts of internal convection on the exchange of respiratory gases in a small complex organism, the larval zebrafish (Danio rerio). Thus, I tested the hypothesis that internal convection is required for the normal transfer of the respiratory gases O2 and CO2 and maintenance of resting aerobic metabolic rate. Use of morpholino knockdown of the VEGF-A and TNNT2 proteins allowed examination of two independent models lacking internal convection. Using micro-respirometry, I demonstrated that loss of internal convection reduces resting rates of O2 consumption and CO2 excretion in larvae at 4 days post fertilization. I also used the scanning micro-optrode technique to demonstrate that acute loss of internal convection resulted in reduced rates of cutaneous O2 flux, a trait that was reversed upon the restoration of internal convection. Finally, I demonstrated that in larval zebrafish, loss of internal convection resulted in decreased hypoxic performance and loss or severe reduction of the hypoxic cardiorespiratory responses. The results from these experiments showed that internal convection is i) required to maintain resting rates of respiratory gas transfer in the larval zebrafish, ii) important in facilitating the hypoxic cardiorespiratory responses in larval zebrafish and iii) augments O2 extraction capacity in the face of progressive hypoxia.

<p>The osmorespiratory compromise is the trade-off between high gill permeability for oxygen uptake and low gill permeability for conservation of ions in fish. The fundamental purpose of this study was to examine facets of the osmorespiratory compromise in freshwater fish under conditions of extended exercise and acute hypoxia, in light of previous research identifying very different gill morphometric and ionoregulatory modifications in the hypoxia-tolerant Amazonian oscar (<em>Astronotus ocellatus</em>) and the hypoxia-intolerant rainbow trout (<em>Oncorhynchus mykiss</em>). A technique using [³H]polyethylene-4000 ([³H]PEG-4000) for branchial paracellular permeability measurement was developed, and then applied to investigate the osmorespiratory compromise during extended swimming. Methods were developed to overcome the challenges of renal [³H]PEG-4000 loss, respirometer surface adsorption, and freshwater drinking of the chemical. In both trout and oscar, corrections were employed for these sources of error – leading to findings that in both species, branchial [³H]PEG-4000 permeability was not rectified and freshwater drinking was quite high. In both species, during an 8-h swim (1.2BL/s), oxygen consumption rate increased by 75-90%; drinking rate remained high but did not increase. Branchial paracellular permeability increased by 61% during exercise in trout but remained constant in oscar. The methods developed here can be widely applied to future studies of branchial paracellular permeability.</p> <p>Unidirectional fluxes (by ²²Na) of sodium, and net fluxes of potassium, ammonia, and urea were observed during a 2-h nomoxia:2-h hypoxia (30% O₂ saturation):2-h normoxic recovery protocol – to identify adaptive trends across phylogenies and/or environments in North and South American teleosts. Strategies for coping with hypoxia appeared to be environmentally, rather than phylogenetically linked, since both the oscar (perciform) and the tambaqui (<em>Colossoma macropomum</em> – characiform) displayed characteristic permeability reduction (of apparent transcellular origin); both frequently encounter severe hypoxia in their natural habitat. Two North American perciforms, pumpkinseed (<em>Lepomis gibbosus</em>) and bluegill sunfish (<em>Lepomis macrochirus</em>) which live in less hypoxic environments, increased branchial ion leakage as in the hypoxia-intolerant trout. Four Amazonian tetra species (all characiformes: <em>Paracheirodon axelrodi, Hemigrammus rhodostomus, Moenkausia diktyota,</em> <em>Hyphessobrycon bentosi rosaceus</em>) which experience intermediate hypoxia in their native Rio Negro presented variable responses. Finally, during a 4-h swim at1.2BL/s, branchial ion fluxes were not reduced but elevated in oscar, indicating that ionoregulation in this species occurs primarily transcellularly, and that adaptive strategies to one manifestation of the osmorespiratory compromise (hypoxia) may not apply to another (exercise).</p> / Master of Science (MSc)

Le réchauffement climatique cause plusieurs modifications abiotiques et biotiques dans les milieux naturels. La hausse de la température de l’eau cause une diminution de l’oxygène dissous dans les lacs et augmente la quantité de zone hypoxique observée. Une autre conséquence de la hausse de la température est l’augmentation du métabolisme et de la consommation d’oxygène des espèces ectothermes dont les poissons et les parasites. Le parasitisme est omniprésent dans les réseaux trophiques et a un effet néfaste sur l’hôte affecté. Les parasites et l’hypoxie peuvent limiter la portée aérobie (AS) des poissons pour la réalisation d’activités journalières. Ainsi, cette étude analyse l’effet dans le temps d’une infection de trématodes causant la maladie du point sur le métabolisme et sur la tolérance hypoxique de l’hôte puisque le développement de ces parasites suggère un effet sur le poisson qui varie selon le temps de résidence des parasites. Nous avons utilisé des crapets-soleil (Lepomis gibbosus) infectés par ces trématodes comme système modèle. Nous avons émis l'hypothèse que l'infection parasitaire réduirait la portée aérobie et la tolérance à l'hypoxie des poissons en fonction du temps du développement de l’infection. Afin d’étudier cette relation hôte-parasite, des tests de respirométrie et d’hypoxie critique ont été performés à cinq moments lors des deux premiers mois suivant l’infection. Les traits métaboliques aérobies (taux métabolique standard et maximal, AS), des indices de la tolérance hypoxique et du métabolisme anaérobiques (tension critique d’oxygène, pression partielle d’oxygène entraînant la perte d’équilibre, la concentration de lactate) et le taux d’hématocrite sont les variables analysées à l’aide de la respirométrie et de prélèvements sanguins. Nous démontrons ici que l’infection expérimental de ces trématodes n’affecte ni la portée aérobie ni la tolérance hypoxique et ce indépendamment du temps de développement du parasite. Un faible effet temporel, mais significatif, est observé entre les premiers jours d’expérimentations et les derniers, des différences principalement dues aux faibles différences non significatives des taux métaboliques standards et maximaux. Le stress induit par captivité et l’effet des changements saisonniers sur les taux métaboliques sont possiblement en cause. Pour l’instant, selon les conditions environnementales actuelles, le crapet-soleil démontre une résilience à l’infection parasitaire ainsi qu’à l’hypoxie. / Global warming is causing several abiotic and biotic changes in natural environments. The rise in water temperature causes a reduction in dissolved oxygen in lakes and increases the amount of hypoxic zone observed. Another consequence of rising temperatures is the increased metabolism and oxygen consumption of ectothermic species, including fish and parasites. Parasitism is ubiquitous in food webs and has a detrimental effect on the affected host. Parasites and hypoxia can limit the aerobic range (AS) of fish for daily activities. Thus, this study analyzes the effect over time of a trematode infection causing the blackspot disease on the metabolism and hypoxic tolerance of the host since the development of these parasites suggests an effect on the fish that varies according to the residence time of the parasites. We used sunfish (Lepomis gibbosus) infected with these trematodes as a model system. We hypothesized that parasite infection would reduce the aerobic range and hypoxia tolerance of fish as a function of the time of infection development. Respirometry and critical hypoxia tests were performed at five time points during the first two months post-infection to investigate this host-parasite relationship and its impact over time. Aerobic metabolic traits (standard and maximum metabolic rate, aerobic range), indices of hypoxic tolerance and anaerobic metabolism (critical oxygen tension, partial pressure of oxygen leading to loss of equilibrium, lactate concentration) and hematocrit levels were analyzed using respirometry and blood sampling. We demonstrate here that experimental infection with trematodes affects neither aerobic range nor hypoxic tolerance independently of parasite development time. A small but significant temporal effect is observed between the first and last days of experimentation, differences mainly due to small non-significant differences in standard and maximum metabolic rates. This may be due to stress induced by captivity and seasonal changes affecting metabolic rates. For now, considering actual environmental conditions, sunfish show high resiliency to parasitic infection and to hypoxia.