Complex sentence: In terms of numbers, actinopterygians are the dominant class of vertebrates, comprised of nearly 95% of the 25,000 species of fish, and they are ubiquitous throughout fresh water and marine environments from the deep sea to the highest mountain streams.
Split into simpler sentences: In terms of numbers, actinopterygians are the dominant class of vertebrates, comprised of nearly 95% of the 25,000 species of fish. They are ubiquitous throughout fresh water and marine environments from the deep sea to the highest mountain streams.