Anatomy and Morphology of Fish and Cephalopods
Fish do not form a uniform group of animals such as birds or mammals. Fish are rather all vertebrates that can be characterised by tooth-bearing jaws, living in the water and having paired and unpaired fins. They form the most species-rich vertebrate group in aquatic ecosystems. Their shape, size and internal structures vary considerably. Currently, more than 34,000 different fish species are known, and new species are being described every year. The majority are bony fish, mainly Teleostei, followed by cartilaginous fish species and jawless fish. Half of the fish species known today can be found in freshwater; the other half inhabit marine ecosystems. Fifty percent of the fish species occurring in marine waters live in warm and nearshore water zones (e.g. coral reefs). However, fish can be found in all marine habitats, with some remarkable adaptations to prevailing biotic and abiotic conditions. These adaptations enable them to live in the deep sea, polar regions and close to the coast.