When we think about fish, we always imagine water too, their natural world. Fish have inhabited water nearly since life began on Earth, when the evolutionary process was initiated that has led to an enormous family: 20,000 to 30,000 different species of fish have been recorded.

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Any journey through the Internet's world of fish found quickly reveals the meaning of ''ichthyology'': the science dedicated to the study of these cold-blooded aquatic vertebrates. The cyber-excursion also allows the traveler to explore the worlds of scientific classifications, glossaries, collections, digital aquariums, and even mythology.

Fish constitute an essential part of life on this planet, and they are key to the aquatic ecosystems that cover more than three-quarters of the Earth's surface. As far as their connection to humans, fish provide an important source of protein, though over-exploitation of this natural resource has caused profound distortions in the environment.

Following is a guide for beginning your navigation through the watery world of fish: