Fish are a vertebrate class, like mammals, however they are cold-blooded and live in water. Fish are one of the oldest classes of animals, having been on the Earth for more than 500 million years. Additionally, while all fish share the common traits of having a backbone and living in water, they are one of the most diverse classes with about 32,000 different living species (which is more species than amphibians, reptiles, birds, and mammals combined). Due to the unique properties of water to transmit scent, taste, sound, and electricity, many fish have evolved to rely less on their sense of sight and more on their other senses.
The diet of fish is very diverse as the number of fish species is very large. Some fish are carnivores, while other species of fish are herbivores or omnivores. Fish generally eat small organisms, algae, and sometimes can even eat other fish. The food that fish eat depends on their species, size, and environment.
The lifespan of a fish usually depends on the species. A fish may live anywhere between a year to 200 years – it all depends on the species of fish. The lifespan of a fish typically depends on their size, species, and whether they live in the wild or captivity.
Fish do not sleep in the same form and manner that land mammals sleep. Fish sleep by reducing their activity and metabolism while remaining alert to any potential danger. While sleeping some fish float in place, in a secure spot within the mud, or find a nest. During sleep, fish perform restorative functions.