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First published online October 30, 2009
Journal of Experimental Biology 212, 3708-3718 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.031427

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Transectional heat transfer in thermoregulating bigeye tuna (Thunnus obesus) – a 2D heat flux model

Jess Boye¹, Michael Musyl², Richard Brill^3,* and Hans Malte^1,

¹ Department of Zoophysiology, Institute of Biological Sciences, University of Aarhus, Denmark
² Joint Institute for Marine and Atmospheric Research, Pelagic Fisheries Research Program, University of Hawaii at Manoa, Honolulu, HI 96822, USA
³ Cooperative Marine Education and Research Program, Northeast Fisheries Science Center, National Marine Fisheries Service, NOAA, Woods Hole, MA 02543, USA

Author for correspondence (hans.malte{at}biology.au.dk)

Accepted 6 August 2009

We developed a 2D heat flux model to elucidate routes and rates of heat transfer within bigeye tuna Thunnus obesus Lowe 1839 in both steady-state and time-dependent settings. In modeling the former situation, we adjusted the efficiencies of heat conservation in the red and the white muscle so as to make the output of the model agree as closely as possible with observed cross-sectional isotherms. In modeling the latter situation, we applied the heat exchanger efficiencies from the steady-state model to predict the distribution of temperature and heat fluxes in bigeye tuna during their extensive daily vertical excursions. The simulations yielded a close match to the data recorded in free-swimming fish and strongly point to the importance of the heat-producing and heat-conserving properties of the white muscle. The best correspondence between model output and observed data was obtained when the countercurrent heat exchangers in the blood flow pathways to the red and white muscle retained 99% and 96% (respectively) of the heat produced in these tissues. Our model confirms that the ability of bigeye tuna to maintain elevated muscle temperatures during their extensive daily vertical movements depends on their ability to rapidly modulate heating and cooling rates. This study shows that the differential cooling and heating rates could be fully accounted for by a mechanism where blood flow to the swimming muscles is either exclusively through the heat exchangers or completely shunted around them, depending on the ambient temperature relative to the body temperature. Our results therefore strongly suggest that such a mechanism is involved in the extensive physiological thermoregulatory abilities of endothermic bigeye tuna.

Key words: 2D model, archival tag, endothermy, isotherm, mathematical modeling, partial differential equation, temperature gradient, vascular countercurrent heat exchanger, white muscle, tuna

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