One of the first science-related things we learn as children at school is that some animals, like birds and mammals, are warm-blooded, while others, such as amphibians, reptiles, and fish, are cold-blooded, also known as ectothermic. Cold-blooded animals cannot regulate their internal temperature, so their body temperature varies with that of the environment.

Well, sort of.

With reptiles and amphibians, yes, the animal’s internal body temperature does vary related to the environment. With fish, though, this is just partially true, as a few species are partially warm-blooded. And among these fish are five species of sharks, all in the Lamnidae family.

Lamnidae: Warm-blooded sharks

Endothermy is the ability of an organism to metabolically produce heat to achieve a stable body temperature. This is a pretty cool evolutionary trait, as organisms that can regulate their temperature are a bit more independent from environmental thermal fluctuations¹.

The Lamnidae is a family of elasmobranchs belonging to the order Lamniformes. Members of this family are also known as isurids, or mackerel sharks². This unique elasmobranch family comprises three genera and five species, including the white shark (Carcharodon carcharias), one of the most recognized and infamous sharks responsible for the highest number of unprovoked and provoked attacks recorded worldwide³. The other members of this family are the longfin mako (Isurus paucus), the shortfin mako (Isurus oxirynchus), the porbeagle (Lamna nasus), and the salmon shark (Lamna ditropis).

Sharks in this family have the unique ability to elevate their internal body temperatures above that of their surrounding environment by re-using some of the heat produced by the muscles they use to swim. This allows these sharks to inhabit colder waters while maintaining higher-intensity and sustained swimming.

How does this work?

These five species of sharks have large masses of red muscle along their sides, in a more anterior position nearer their spines. This allows their bodies to remain rigid and then retain heat generated by the muscle contraction. And that, in turn, creates internal body temperatures that are higher than surrounding waters and can remain fairly stable, even as the sharks move from warm surface waters to colder, deep waters⁴.

The retention of heat is possible due to an arrangement of highly branched blood vessels known as the “rete mirabile,” a Latin phrase meaning “wonderful net.” As arterial, oxygenated blood flows toward the muscles near the core of the body, it passes through the rete mirabile, and the blood runs countercurrent to warm blood leaving the muscles. The oxygenated blood warms as it passes through the rete and travels toward the swimming muscles. So, the heat generated by the activity of the large, swimming, red muscles returns to the muscles themselves, rather than transports via the blood to the gills, preventing heat from being lost to the external environment.

Why is this important?

All species in this family are oceanic pelagic sharks, seating at the top of the oceanic food web. They are among the most active oceanic apex predators⁵, playing important ecological roles in maintaining healthy oceanic ecosystems⁶. Part of their predatory success could definitely be attributed to their capacity to partially produced heat.

References

1. Wang, X., Qu, M., Liu, Y., Schneider, R. F., Song, Y., Chen, Zhang, S., (2022). “Genomic basis of evolutionary adaptation in a warm-blooded fish. The Innovation,” 3(1), 100185.
2. Castro, JI (2011). “The sharks of North America.” Oxford University Press, Inc.
3. International Shark Attack File (2021). www.floridamuseum.ufl.edu/shark-attacks
4. Helfman, GS, Collette, BB, Facey, DE, Bowen, BW, (2009) .”The Diversity of Fishes: Biology, Evolution, and Ecology” (Second Edition). Wiley-Blackwell
5. Shadwick, RE, (2005). “How Tunas and Lamnid Sharks Swim: An Evolutionary Convergence.” American Scientist. 93:524 – 531
6. Myers, RA, Baum, JK, Shepherd, TD, Powers, SP, Peterson, CH, (2007). “Cascading Effects of the Loss of Apex Predatory Sharks from a Coastal Ocean.” Science. 315:1846-1850

3