Fish are characteristic aquatic animals that have a skull connected to a backbone.
They do not have limbs, but have fins and a tactile tail which helps them swim. They have gills which allow them to extract oxygen from the water and directly exchange it with the carbon dioxide in their blood.
Pet Fish Species
Though the term fish denotes all such aquatic life, it isn’t possible to group them in a manner in which other mammals are grouped. Fish are a collection of various different species and may be broadly classified on the basis of their habitats.
The different fish species include the tropical fish species, the saltwater fish species, the freshwater fish species, aquarium fish species and many others like these. There are other, more complicated scientific classifications of these fishes too.
Typically, fishes are ectothermic animals that have a streamlined body. This allows them to swim rapidly and maneuver their body in different depths and underwater pressures.
They have scales on their body and lay eggs that are sometimes fertilized within the body but mostly outside.
Each fish species is designed differently from the other, depending on the choice of habitat and the survival issues in the habitat. Tuna, salmon and jacks are marine fishes that are carnivorous and can swim very fast. However, in the same habitat, live the eel and the ray fish which have more maneuverability due to their cartilaginous bone structure, but are much slower than the tuna and salmon.
A lot of fresh water fish species are designed in a manner that their gills are able to extract oxygen from the water but they have another mechanism through which they are able to extract oxygen from the air too. Other fishes like the lungfish have lungs which are similar to mammals. There are some species of catfish that extract oxygen from their stomach. The body shape, number and placement of fins and the type of skin is also highly variable. Some species of fish are as small as 8 mm and some are as large as 16 meters. There are also eels which do not have scales on their body and the sea horses, anglerfish, and the gulpers which do not have a conventional fish-like form.
There are some species of fish that defy the one single characteristic of fishes—that they die almost immediately after removal from water. The mudskippers are fishes that perform most of their functions on flat muddy surfaces and go underwater only to hide from predators. There are other species of underwater organisms which are known as fish but are not really fish. Shellfish, starfish and cuttlefish are all misnomers.