The lamprey, the first warm-blooded fish
Lampris guttatus, of the moonfish family, is an endothermic animal, that is, it maintains the temperature of its blood above that of its environment, and this throughout its body. An advantage for this predator who can thus travel long distances in cold waters.
Lampris guttatus or opah is a large silverfish of the moonfish family that lives in cold, low-light waters. He swims by quickly waving his red pectoral fins. Fish usually living at these depths tend to be slow and stagnant. They remain ambushed to catch prey instead of actively pursuing it, which would consume a lot of energy.
There are fish like tuna or sharks that warm up parts of their bodies like muscles, to promote their swimming performance. But they have to return regularly to the surface waters to warm up and are not totally endothermic, unlike mammals and birds, who manage to maintain a body temperature above the ambient temperature.
Researchers at the National Oceanic and Atmospheric Administration (NOAA) based in La Jolla (San Diego, USA) found that lamprey was unique in looking at gill samples. Indeed, they found that blood vessel that carries warm blood in the gills wind around those that carry the cold blood that has absorbed oxygen. This organization suggests a “counter-current heat exchange”: warm blood from the center of the body helps to warm the cold blood that comes from the respiratory surface of the gills, where the blood is loaded with oxygen. This heat exchange in the gills could allow almost the entire body of fish to maintain high temperatures, even at cold depths.
The opah, a fully endothermic moonfish
In an article in Science, researchers describe the results of their work on this moonfish. They attached temperature-recording devices to live fish off the west coast of the United States and found that the body temperatures of lamprey remained warmer than the surrounding water: the fish had an average muscle temperature of about 5 ° C above the surrounding water. The fish were also tracked by satellite, which showed that they spent most of their time between 45 and 300 meters, without regularly returning to the surface.
Lamprey produces energy through the constant agitation of its fins and minimizes heat loss through a series of countercurrent exchanges in its gills. In addition, fat tissue surrounds the gills, the heart and the muscles where the fish generates a lot of its internal heat. Unlike other fish, lamprey spread warm blood throughout the body, which enhances physical performance in cold, nutrient-rich waters below the oceanic thermocline (the boundary between shallow and deep waters).
According to Nicholas Wegner, the main author of the article, the higher body temperature of the fish could increase its muscular capacity, favor its visual and cerebral functions and help it resist the effects of cold on its organs. “Before this discovery, I had the impression that it was a slow-moving fish, like most other fish in cold environments. But, as he can heat his body, he is a very active predator that pursues agile prey like squids and can migrate great distances.