• SharkieSophie

Sharks Packing Heat

Updated: Apr 26

Fish are cold-blooded... right? Wrong! It might surprise you to learn that "endothermy" (the ability to elevate the body temperature above that of the surrounding environment) has evolved independently among several fish lineages, including tuna (Thunnus spp.) and billfish (Order Istiophoriformes). Endothermy is even found in sharks; in several species of the Lamnidae family (Dickson & Graham, 2004).


The porbeagle shark

The porbeagle shark (Lamna nasus), salmon shark (Lamna ditropis), the mako sharks, both shortfin and longfin (Isurus oxyrinchus and I. paucus respectively) and the great white shark (Carcharodon carcharias) are all capable of keeping their body temperature between 6.5 - 10.0 °C above that of the surrounding water. However, unlike other endotherms that warm their entire body, these Lamniform sharks just heat their locomotor muscles, stomach and gut, brain and eyes. Therefore, we refer to them as "mesotherms".


They are able to do this due to a system of "retia mirabella", which act as counter-current heat exchangers in the circulatory system, warming the vital organs. In comparison to many endotherms, elevated body temperatures in these sharks do not appear to occur as a result of "thermogenesis" (aka producing heat), but rather a remarkable ability to retain metabolic heat which is generated by locomotion and digestion (Bernal et al, 2012).



And the results are absolutely spectacular!

Mesothermy allows these sharks to maintain their metabolic rate even in cold water, which means they have much more efficient muscle contraction and digestion. This gives them more energy for swimming, which is why this group are amongst the most active and dynamic of all shark species!


The shortfin mako shark is the fastest shark alive; clocking in at 60 miles per hour! They can also jump higher than any other shark; breaching up to 9 metres out of the water!


The maintenance of elevated body temperatures has allowed the white shark to expand their range globally, as they are able to tolerate a wide thermal range. Whatsmore, white sharks, especially larger individuals (above 3.5 m total length) are able to tolerate comparatively long periods of time in cold environments. Their metabolism makes them impressive hunters of swift, large prey, even in cold waters of extreme latitudes (Dickson & Graham, 2004, Bernal et al, 2012).


The bigeye thresher shark (Image source: http://www.edgeofexistence.org)

It is also thought that the thresher sharks might be mesothermic. Rete mirebella are found in both the common (Alopias vulpinus) and bigeye (A. superciliosus) threshers, but not in the pelagic thresher (A. pelagicus). However, the system is not as highly developed as found in the other five mesothermic sharks, and only acts to warm the brain and eyes. It is thought that endothermy evolved in these sharks to buffer these sensitive organs against drastic changes in temperature that they experience during their daily vertical migrations, which can be as much as 16 °C (Dickson & Graham, 2004, Bernal et al, 2012).



References

Bernal D, Carlson JK, Goldman KJ, & Lowe CG (2012). Energetics, Metabolism, and Endothermy in Sharks and Rays. In: Carrier JC, Musick JA & Heithaus MR (Eds.) Biology of Sharks and Their Relatives 2nd Edition. Access online.


Dickson K & Graham JB (2004). Evolution and Consequences of Endothermy in Fishes. Physiological and Biochemical Zoology, 77:6, 998-1018. Access online.


By Sophie A. Maycock for SharkSpeak.

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