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Endothermic Sharks

Thanks to FIN tastic Allstar Jake for sharing this informative blog. Jake is 12 and loves sharing shark facts with his peers and teachers.

There are around 500 extant species of sharks and the majority of them are ectothermic or cold-blooded. However, the family Lamnidae has achieved endothermy and has gained numerous benefits because of this ability. They developed a rete mirabile which allows them to warm certain organs, therefore enabling them to preform functions better in a cold environment. Furthermore, this makes them more efficient predators. Other organisms that have this adaptation have increased their habitat range and resilience to the environment. Overall, the adaptation of endothermy is extremely beneficial.

Shortfin Mako shark (Isurus oxyrinchus) member of family Lamnidae Image: Andy Murch

Lamnidae gained the ability to raise their body temperature by developing a rete mirable (Retia mirabilia plural) or wonderful net in Latin. This refers to the complex arrangement of arteries and veins in their bodies. The arrangement targets several areas of importance in the shark’s body including the front of the viscera, the swimming muscle, and the brain as well as maintains temperatures needed to function correctly in a certain habitat. The Rete mirabile works by taking advantage of the heat generated by the red muscle, which is one of the two types of skeletal muscle, the other being white muscle. The red muscle breaks down fats to operate and is used to cruise at slow speeds while the white muscle needs glycogen (a chemical that is often used for short-term energy storage) to operate and is used for short bursts of speed. By using the red muscle, the blood flowing away from the muscle will be warmer than the original temperature of the blood as it was flowing towards the muscle. This heat would have been lost to the environment if this was an ectothermic shark. This is because the veins would then carry the warm blood back to the heart which will then be pumped to the gills to get oxygenated. In doing so, and if the shark is in a cold environment, the water will drain thermal energy from the blood because of its high specific heat (“The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius.”

However, in an endothermic shark the warm blood in the veins will travel through the rete mirabile and transfer the thermal energy it has to colder arteries by utilizing countercurrent blood flow. Countercurrent blood flow exchanges, in this case, thermal energy efficiently because in each stage thermal energy tries to reach an equilibrium with higher and higher amounts of heat from the vein, therefore gradually increasing temperature until the artery is no longer close enough to transfer heat or the artery reached nearly 100% of the original vein temperature. This ensures that the blood traveling to any organ will be warmer than the surrounding environment, furthermore increasing the shark’s metabolism.

Image Copyright Pearson Education, Inc. Published as Benjamin Cummings

By using the Rete mirabile, the family Lamnidae can achieve actions their ectothermic cousins can’t. By raising their body temperature, they are able to live in colder environments, therefore occupying a larger range of habitats as well as ecological niches. Additionally, these Lamnids escaped competition from other sharks and found new food sources as well. The increased metabolism also allows them to better hunt and digest newfound prey items. As part of the rete mirabile targets the cranial area they are able to remain alert and receive higher resolution images from their eyes. This could also have certain reproductive benefits such as the young will develop faster and have a higher chance of surviving in a cold environment due to the reduced competition mentioned earlier. However, to sustain a higher metabolism they need much more energy than a similarly sized ectothermic shark.

Sharks are extremely successful animals and 450 million years of sharks have proved this and the evolution of endothermy has made the family Lamnidae even more so. They developed a Rete mirabile which takes advantage of the heat generated by red muscles and redistributes it to increase their body temperature and metabolism. This allows the shark to gain numerous benefits including increasing their habitat range as well as their resilience to multiple environments, hunting and digesting efficiently, and allowing their young a greater chance of survival. This far outweighs the cost of having a higher metabolism. Furthermore, the advantage of developing endothermy is an extremely rewarding trait in terms of being successful.

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