Fish

Evolving more than 500 million years ago, fish were the earliest vertebrates to appear on earth. Although the label “fish” is commonly applied to a wide range of species, from the formidable great-white shark to the elegant pygmy seahorse, it is merely an informal term describing any aquatic vertebrate that can’t be classified as a mammal, reptile, amphibian or bird. This typically means any cold-blooded animal, covered with scales, that breathes using gills, and has fins. Indeed, the five living classes of fish, which includes hagfish, lampreys, cartilaginous fish (sharks and rays), lobe-finned fish (coelacanths and lungfish) and bony fish, are only distantly related to each other. Over-fishing, pollution, climate change, habitat destruction and invasive species present the greatest threats to the world’s dwindling fish stocks.

Explore ARKive for videos and images of endangered fish, and learn about fish conservation, biology and more in our species fact-files.