Fish as a heating system?
In the vast expanse of the world's oceans, a fascinating story of evolution unfolds. A group of fish, known as endothermic fish, have developed the ability to maintain warmth within their bodies, a trait usually associated with mammals and birds. This remarkable adaptation, while consuming enormous amounts of metabolic energy, has proven to be beneficial under certain conditions.
The similarities between endothermic fish and marine mammals like whales are striking. It is believed that they may have evolved due to a common external factor or competition for prey. Both species occupy similar ecological niches and travel vast distances through the world's oceans, much like small toothed whales and dolphins.
The evolution of endothermy in fish has occurred multiple times, with four distinct evolutionary lineages leading to endothermic bony fish today. Three distinct periods of evolution led to endothermic fish: around 38 to 30 million years ago for swordfish relatives, around 23 to 16 million years ago for mackerel relatives including tuna, and around 11 to 7 million years ago for shiny fish relatives with golden lizards.
Interestingly, the same genes appear to play a role in diverse vertebrates such as whales, penguins, and golden lizards. Specific genes were found to be subject to positive selection if endothermy was limited to the brain and eyes or if it encompassed the entire body.
Some tuna species, like the opah, have unique mechanisms to limit heat loss. The opah can warm its entire body, up to about five degrees Celsius above the surrounding water temperature. Other fish, such as swordfish and some tuna species, have thick layers of fat or counter-current heat exchangers to regulate their body temperature.
The evolution of whales occurred simultaneously with the evolution of endothermic fish. Researchers suggest that the evolution of whales was a crucial step in the development of endothermic fish, as it may have been determined by certain gene variants that predisposed large, enduringly swimming fish for warm-bloodedness.
Intriguingly, the species of fish that independently evolved endothermy parallel to whales are the Lamnid sharks (such as the great white shark and mako shark) and the tunefish (family Scombridae), both capable of maintaining elevated body temperatures for enhanced predatory performance.
A comparative genomics analysis of endothermic vertebrates found that various marine animals, including these warm-blooded fish, have a rapid evolution of genes associated with the development of the nervous system, metabolism, and muscles. This suggests that the ability to maintain warmth is linked to these critical biological functions.
Endothermic fish thrive in a variety of marine environments, from tropically warm waters to relatively cool marine regions. Larger fish, with their relatively smaller surface area through which heat is lost, tend to be more likely to warm parts of their body.
This fascinating journey of evolution highlights the incredible adaptability of life in the oceans and the intricate web of connections that bind diverse species together. As research continues, we can expect to uncover even more insights into the evolution of warm-blooded fish and their remarkable abilities.
