Here's a snippet to pique your interest:
"At a conservative estimate at least 40% of the world's vertebrates are fish. On the one hand they are united by their adaptations to an aquatic environment and on the other they show a variety of adaptations to differing environmental conditions - often to extremes of temperature, salinity, oxygen level and water chemistry." One chapter begins with this truism: "All fish are united by the requirement to obtain food, while (at least during some phase of their life) avoiding being food." It's a challenging life, beneath the water's surface. Take the plunge and learn more.