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). Believe it or not, endothermy is even found in sharks; in several species of the Lamnidae family. So how do these sharks warm their bodies? And why?
Sharks Are Hot!
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 (Dickson & Graham, 2004; Bernal et al, 2012).
It is also thought that basking sharks (Cetorhinus maximus) and 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 (Dickson & Graham, 2004; Bernal et al, 2012; Dolton et al, 2023).
However, unlike other endotherms (including us!) that warm their entire body, these Lamniform sharks just heat their locomotor muscles, stomach and gut, brain and eyes. The threshers only heat their eyes and brain. Therefore, we refer to them as "mesotherms" - meaning intermediate heat in Greek (Bernal et al, 2001; Dickson & Graham, 2004; Bernal et al, 2012).
How do They Do It!?
Mesothermic sharks can elevate their body temperatures above that of the surrounding water thanks a system of "retia mirabila" (aka "rete mirabile"), which act as counter-current heat exchangers in the circulatory system.
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. The retia mirabila simply stop the shark from losing so much of it's body heat and returns this energy back to the vital organs (Bernal et al, 2001; Bernal et al, 2012).
Why Be Warm?
It is thought that endothermy evolved in thresher sharks to buffer their sensitive organs against drastic changes in temperature. These sharks undergo daily vertical migrations through the water column, following their prey to great depths. During these excursions they experiences changes in water temperature as vast as 16 °C. Their mesothermy allows them to stay active and hunt under such extreme conditions (Dickson & Graham, 2004; Bernal et al, 2012).
In the species with more advanced mesothermy, the ability to maintain their metabolic rate even in cold water, means these sharks have extremely efficient digestion and explosive muscle contractions. So they have A LOT more energy for swimming. That is why this group includes some of the most active and dynamic of all shark species in the world! They have the strength and speed to take down very large, agile prey and they can put on remarkable displays if you are every lucky enough to be watching... (Bernal et al, 2001; Dickson & Graham, 2004; Bernal et al, 2012).
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! This is known as "breaching". Even large great whites that can weigh in at a couple of tonnes are able to launch their entire bodies out of the water when they are hunting!
Controlling their body temperature also means these sharks can tolerate colder waters very well. This is what has allowed these species to expand their ranges out of the tropics and into the deep without having to compromise how active they are (Dickson & Graham, 2004; Bernal et al, 2012).
For example, white shark have a global range, where their cold-blooded cousins tend to have much more localised distributions. They are also able to thrive in the shallows, but also spend time at great depths, rather than being confined to areas within certain temperature ranges (Dickson & Graham, 2004; Bernal et al, 2012).
There's no denying it... these sharks are hot!
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.
Bernal D, Sepulveda C & Graham JB (2001). Water tunnel studies of heat balance in mako sharks. Journal of Experimental Biology, 204:23. Access online.
Dickson K & Graham JB (2004). Evolution and Consequences of Endothermy in Fishes. Physiological and Biochemical Zoology, 77:6, 998-1018. Access online
Dolton HR, Jackson AL, Deaville R, Hall J, Hall G, McManus G, Perkins MW, Rolfe RA, Snelling EP, Houghton JD & Sims DW (2023). Regionally endothermic traits in planktivorous basking sharks Cetorhinus maximus. Endangered Species Research, 51, 227-232. Access online.
By Sophie A. Maycock for SharkSpeak.
awesome footage. what a fantastic sight.