Analysis of the ontogeny of regional endothermy in Pacific bluefin tuna (Thunnus orientalis) and the effects of feed restriction in yellowtail kingfish (Seriola lalandi)

Author: Arif Malik

Malik, Arif, 2019 Analysis of the ontogeny of regional endothermy in Pacific bluefin tuna (Thunnus orientalis) and the effects of feed restriction in yellowtail kingfish (Seriola lalandi), Flinders University, College of Science and Engineering

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Early in their post-larval development, tunas undergo an ontogenetic transition from ectothermy to regional endothermy but it is largely unknown when this occurs or how it is regulated. In bluefin tunas, the tissues that are endothermic are the red (‘slow-twitch’, predominantly aerobic) skeletal muscle, the viscera and the cranium (eye/brain). We observed a linear increase in the red muscle temperature elevation above the ambient temperature in juvenile Pacific bluefin tuna (PBT, Thunnus orientalis). More specifically, we estimated that significant temperature elevation in the red muscle of juvenile PBT occurs at a minimum body size of approximately 29 cm fork length (FL), corresponding to an age of approximately 5.5 months. The red muscle mass of the PBT specimens scaled slightly less than isometrically with increasing body mass (BM) indicating that increasing red muscle mass as a proportion of total BM was not the explanation for the increasing red muscle temperature elevation with increasing body size. Similarly, the activity per g tissue of mitochondrial marker enzymes either decreased or remained constant with increasing body

size. This suggested that the development of red muscle endothermy in PBT is not a result of an increase in mitochondrial abundance and therefore heat generated by an increase in the aerobic metabolic activity in the red muscle. Instead, increasing temperature elevation in the red muscle above the ambient water temperature correlated well with the proliferation of red muscle countercurrent heat-exchanging blood vessels known as retia mirabilia (singular = rete). Similarly, increasing temperature elevation in the viscera of PBT correlated well with increases in the proliferation of the visceral rete. Together these findings indicate that it is likely that the development of the retia is the most important factor supporting the development of regional endothermy in PBT. The temperature elevation we observed in the red muscle of our PBT specimens with increasing body size was not paralleled with an increase in the transcript abundance of peroxisome proliferator-activated receptor-gamma (PPAR gamma) coactivator-1α (PGC-1α), a master regulator of mitochondrial biogenesis and function in mammals, with increasing body size. This suggests that the function of PGC-1α may be different in fishes than it is in mammals.

Yellowtail kingfish (YTK, Seriola lalandi) is an important aquaculture species in South Australia but in winter it suffers from impaired feed digestion due to suboptimal water temperatures. It was found that CS and COX enzyme, per g tissue, were 57% and 20% greater, respectively, in the liver of feed restricted YTK compared to satiated YTK. This suggested that the aerobic metabolic capacity of the liver increased in response to feed restriction. This did not correspond with an increase in PGC-1α transcript abundance. Therefore, it is unlikely that the feed restriction-induced increase in mitochondrial abundance was a result of regulation by PGC-1α.

Keywords: Pacific bluefin tuna, Thunnus, regional endothermy, retia mirabilia, temperature elevation, citrate synthase, cytochrome c oxidase, pyruvate kinase, PGC-1α, PPARα, MCAD, Yellowtail kingfish, Seriola

Subject: Biological Sciences thesis

Thesis type: Doctor of Philosophy
Completed: 2019
School: College of Science and Engineering
Supervisor: Kathy Schuller