Fish Fish are the ultimate aquatic organism. Fish can be defined as cold-blooded aquatic vertebrates that have gills throughout life and limbs in the form of fins.
Fish are a heterogeneous group.
There are approximately 19,000 living fish species, nearly half of all know vertebrate species. More new species of fish are discovered every year than all other species of vertebrate combined. Fish shapes vary as greatly as their size and the range from brilliant to drab. Fish survive in nearly every conceivable aquatic habitat on earth and are found in every marine environment. Southeast Asia claims the largest number of freshwater species in the world.