The Warmblood Water Stud

The Bluefin tuna is a remarkable and unique species that mainly inhabits the Atlantic Ocean, but is known for its ability to migrate large distances. In the documentary, Superfish: Bluefin Tuna, David Attenborough describes the school-like behavior of the fish, as well as the uncanny ability for the species to thrive in almost any condition. The Bluefin tuna is considered to be one of the most elite swimmers in the ocean, migrating over 10,000 km during the summer months to mate in the warmer waters of the Mediterranean Sea [1, 2]. Moreover, this capability allows the fish to enhance their hunting range and potential, increasing their chances of survival [3]. Their tolerance to traverse through a widely variable temperature range is an ode to the fishes’ abilities to regulate their own body temperature, an adaptation uncommon to the majority of fish [4]; this rare characteristic is known as endothermy.

Endothermy is the ability to raise body temperature through internal heat production. Bluefin tuna accomplish this by warming their muscles and internal organs with the metabolic heat conserved through a specialized breathing mechanism known as countercurrent heat exchange, wherein the warm blood from the fishes’ veins transfers heat to the new, colder blood entering the gills [4]. They bolster this warming system by simultaneously increasing their cardiac output at colder temperatures to adjust to the variable conditions [5]. Though endothermy is an exceptionally advantageous and unique adaptation to water-dwelling species, it is not exclusive to the Bluefin tuna [4]. Endothermy has evolved a number of times in various subspecies of fish, which achieve this through a number of relatively distinct techniques [6]. For example, the butterfly mackerel accomplishes endothermy through the use of an organ that develops from tissue beneath the brain [4]. Though endothermy is common to land residing organisms, this characteristic is much harder to maintain, and thus more admirable, in water inhabitants as the environment is not as conducive to a productive heat exchange. All hail the Bluefin tuna!

Superfish: Bluefin Tuna, starting at approximately 23:00

by Georges Daoud, Edward Bae & Jake Rosen

References

  1. Stephan L Katz. 2002. Design of heterothermic muscle in fish. The Journal of Experimental Biology 205: 2251-2266.
  2. Reglero P, Ortega A, Balbín R, Abascal FJ, Medina A, Blanco E, de la Gándara F, Alvarez-Berastegui D, Hidalgo M, Rasmuson L, Alemany F & ø Fiksen. 2018. Atlantic bluefin tuna spawn at suboptimal temperatures for their offspring. Proceedings. Biological Sciences 285: 1870.
  3. Madigan Daniel, Carlisle Aaron, Gardner Luke, Jayasundara Nishad, Micheli Fiorenza, Schaefer Kurt, Fuller Daniel & Barbara A. Block. 2015. Assessing niche width of endothermic fish from genes to ecosystem. Proceedings of the National Academy of Sciences of the United States of America. 112 (27): 8350-8355
  4. Barbara A. Block & John R. Finnerty. 1992. Endothermy in fishes: a phylogenetic analysis of constraints, predispositions, and selection pressures. Environmental Biology of Fishes 40: 283-302.
  5. Jayasundara N, Gardner LD & BA Block. 2013. Effects of temperature acclimation on Pacific Bluefin tuna (Thunnus orientalis) cardiac transcriptome. American Journal of Physiology. Regulatory, Integrative and Comparative Physiology 305: 9.
  6. Block BA, Finnerty JR, Stewart AF & J Kidd. 1993. Evolution of endothermy in fish: mapping physiological traits on a molecular phylogeny. Science 260 (5105): 210-4.

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