In the evolution of vertebrates, temperature has an important regulatory effect on the physiological activities of living organisms. The ability of terrestrial birds and mammals to successfully adapt to a variety of habitats is due to their ability to keep temperatures. Although most fish are thermotropic animals, the temperature of their bodies will change with the water environment, but the temperature is also equally important for them.
Compared with terrestrial environments, organisms in aquatic environments have more difficulty in obtaining constant temperature capabilities, and they must face the challenge of high specific heat and easy loss of heat in water. Since the first cases of thermostatic fish were discovered, at least 40 species of fish have overcome these difficulties and have a constant temperature capacity similar to that of mammals and birds. These thermostatic fish mainly include about 10 species of cartilaginous fish (from porcupines and rays, respectively) and about 30 species in bony fish, of which bony fish are mainly about 15 species in the order Swordfish, 15 species in tuna and moonfish in the order Tuna.
Porphyra (image from the Internet)
Fish temperature constancy is a relatively rare phenomenon, and less than one in 1,000 of the existing 40,000 species of fish have this ability. Through continuous swimming, they generate heat, maintain a constant temperature, and eventually evolve into thriving taxa among fish. Although these fish have different thermostatic capacity and thermostat mechanism, they all have one obvious common feature, which is to ensure that at least some important organs (such as the brain and eyes) have high temperatures, so that they can travel through different environments.
Fish have their own temperature limits and optimal adaptive temperatures, the ambient temperature beyond this limit may bring about the risk of death, therefore, many warm fish usually live in an environment where the temperature does not change much, then the resources it can obtain are also in this limited environment. The temperature of the thermostatic fish is relatively stable, so it has a wider ecological niche; in the marine environment, there is a large temperature difference between the upper and lower thermostatic layers, like this thermostatic fish can survive above the thermostatic jump, but also swim to the thermostatic layer to obtain prey, then it can obtain more resources.
On the other hand, temperature has an important role in the resolution of vision and the processing of information by the brain, and the sailfish, swordfish and tuna in thermostatic fish have a very good vision system, and in cold waters, maintaining a high temperature is very conducive to visual clarity and the brain's processing of visual information. However, this higher temperature also has a side effect, when the water environment is overheated, they have to swim to the lower temperature jump or more southern and northern waters, which can make them cooler.
Sailfish (image from the Internet)
Skeletal muscle of vertebrates occupies a large part of the body, whether it is flying in the sky, running on the ground or swimming in the water, skeletal muscle occupies a large proportion; in the process of animal evolution, like thermostatic animals, whether it is fish, birds or mammals, they rely heavily on skeletal muscle as a heat source, but in addition to skeletal muscle in mammals, some mammals will also use brown fat tissue as a heat source to heat heat. Like newborn babies, in the early stage, they also rely on brown adipose tissue thermogenicity to maintain the body's constant temperature, but with the slow development process, muscle thermogenation gradually replaces brown adipose tissue, and constant temperature fish use skeletal muscle as a heat source to produce heat from beginning to end. All thermogenesis processes rely first on the synthesis of ATP and then on the decomposition of ATP, yet this is a process of huge energy consumption, especially for animals in aquatic environments, maintaining the same temperature usually costs several times more calories than mammals.
In addition to this, the constant temperature of fish usually meets at least three conditions: a large body, a heat source, and a countercurrent exchanger for temperature preservation and exchange. Recent studies have shown that from the evolution of dinosaurs to birds, the size of the body is inversely proportional to the ability to keep thermostat, but it seems that fish with constant temperature ability usually have a larger body, which is the opposite of the evolution of the constant temperature of birds. Our recent study of the thermogenesis mechanisms of sailfish, swordfish and tuna of independent origin has found that there are a number of thermogenesis-related convergence genes in swordfish, sailfish and tuna, mainly in the glycolytic pathway. These alterations in convergent genes do alter the efficiency of heat production in eukeletal fish. In the formation of countercurrent exchangers, we have also found some genetic innovations unique to sailfish and swordfish, which may be related to their unique vascular networks, enabling them to achieve better temperature preservation and heat exchange in cold waters.
Swordfish (Image from the Internet)
For different origins of constant temperature fish, they acquire this constant temperature ability behind the driving force is different, sailfish and swordfish constant temperature capacity acquisition is about 66 million years ago, this time is the time of the alternation of the Mesozoic and Cenozoic, the continental plate has undergone earth-shaking changes, the land mass rises and falls, constantly splitting, slow drifting, collision and joining, ocean circulation, temperature drop, the north and south Poles appeared ice sheets, the texture and temperature of the ocean has changed greatly, In order to adapt to the changes in the ambient temperature at that time, sailfish and swordfish evolved thermostatic ability to cope with the environmental changes at that time and became one of the most successful fish at the moment.
Source: Institute of Deep-Sea Science and Engineering, Chinese Academy of Sciences