Basking like a fish in the warmth?
In the vast expanse of the ocean, a fascinating discovery has been made about the evolution of warm-blooded fish. Researchers have uncovered that the ability to maintain warmth in fish has developed independently multiple times, shedding light on the intricate web of life in the deep blue sea.
The ancestors of tunas and their close relatives, as well as the ancestors of the bigeye tuna, have independently evolved this remarkable trait. This discovery was made through a comparative genomics analysis of endothermic vertebrates, which revealed that the rapid evolution of 51 genes is shared among various marine animals, including bony fish, sharks, turtles, birds, and mammals.
One of the key findings is the presence of a counter-current heat exchanger in some tuna species. These heat exchangers are located in the muscle groups necessary for sustained swimming at high speeds. This mechanism allows these fish to produce heat energy, acting as a heater, particularly in their working muscles.
The same genes seem to play a role in the development of warm-bloodedness in different vertebrates. For instance, swordfish can warm their brain and eyes, while the opah can warm its entire body to about five degrees Celsius above the surrounding water temperature.
The evolution of endothermic fish occurred simultaneously with the evolution of whales, around 38 to 30 million years ago for swordfish relatives, 23 to 16 million years ago for mackerel relatives (including tuna), and 11 to 7 million years ago for the opah's relatives. However, it's important to note that no whale species played a role in the evolution of warm-blooded fish species, as whales are mammals and warm-blooded fish evolved independently.
Researchers have found that the evolution of endothermic fish occurred in four separate lineages: the opah belongs to the silverfish-like fish, swordfish, marlin, and butterflyfish to the swordfish-like fish, and endothermy has evolved twice within the family of mackerels and tunas.
The 51 genes mainly associated with the development of the nervous system, metabolism, and muscles are believed to have played a crucial role in this evolution. Specific genes were found to be subject to positive selection if endothermy was limited to the brain and eyes or encompassed the entire body, in vertebrates such as whales, penguins, and opah.
This discovery suggests that the ability to maintain warmth in fish is likely rare due to the high metabolic energy required. However, it is correlated with larger body size and a unique swimming style that allows for great endurance and rapid progress. The competition for prey with whales in the oceans may have favored the evolution of endothermic fish that could swim fast and endure.
This research not only deepens our understanding of the evolution of warm-blooded fish but also opens up new avenues for exploring the mysteries of life in the oceans. As we continue to unravel these secrets, we gain a deeper appreciation for the intricate tapestry of life that exists beneath the waves.
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