The vast underwater world of fish is truly amazing. Fish are known to have colonized almost every aquatic environment on this planet Earth. From the freezing depths of the Arctic Ocean to the shallow pools of the Sahara desert, the finfish have permeated far reaches well beyond any man-made borders. Meanwhile, geographical separations have created an abundance of variation among these fish. Today, we can see the product of over 400 million years of population fragmentation and natural selection. Many new freshwater and saltwater fish species have been created as species of fish have become too different to interbreed after long periods of separation. In fact, this process has lead to over 25,000 different species of fish known today, and many more are being discovered each year. This makes fish the most diverse group of the major vertebrae groups.
— Fish Laboratory