What is the only distinguishing feature of fish?

Perhaps no other animal has such a distinctive feature as that of fish. After all, if you think about it, no one has scales and gills. Only such animals that live-live constantly in the water – 24 hours a day, 7 days a week, 365 days a year, can and should have gills, as well as scales. Other animals that do not live so much time in the water, but in a different way – constantly, do not need such attributes of life, because they simply do not need them. But, do not forget about the swim bladder, which the fish acquired in order to comfortably live in the water, namely, to stay at the depth at which it is necessary. Ordinary animals and people do not need it. And one more sign is the flat or streamlined shape of the body and fins, as well as the presence of the fins themselves, which is also dictated by constant living in the water.