Fish are known to be the oldest vertebrates in the world.
It is believed that even five hundred million years ago, fish were swimming in the waters of the earth. Fish are cold blooded vertebrates that have scales all over the body. They have two sets of fins that are paired together, along with several unpaired fins.
The skeleton of a fish contains a lot of hard and fine bones.
However, there are also fish like the ray fish and sharks which have soft and cartilaginous skeletons that allows them greater underwater maneuverability.
Fish have gills that allow for underwater breathing. With the help of gills, fish are directly able to exchange gases from the blood. Most of the small fish feed on aquatic plants; however, there are other fish that feed on small fish.
There are many kinds of fish and they all differ in sizes, shape and color. The smallest fish may measure somewhere around 8 mm and largest one may measure about 16 meters. There are about 28,000 known species of fish which fall between these two sizes. About 27,000 of these fish have a bony structure and the rest include sharks, lampreys, rays, hagfish, and chimeras. Fish inhabit all the various kinds of water bodies and can also be kept in aquariums.
Types of Fish:
Since there is such a huge number of fish all over the world, they have been classified into different categories, depending upon their habitats. Aquarium fish, tropical fish, marine fish, saltwater fish, and freshwater fish are some of the classifications of fish.
- Freshwater Fish: As the name suggests, these fish are found in fresh water where the salinity of water is negligible. 41% of the total fish found in the world are present in fresh water.
- Marine Fish: These are typically sea water fish and may be present at varying depths and in different ecosystems. A lot of these fishes require a tropical climate to thrive. The tropical fishes are also largely a subset of the freshwater and the marine fish.
Apart from these basic types of fish, there are a large number of species that can be kept easily in an aquarium. Both freshwater and saltwater fish can be kept in aquariums. However, both cannot be kept together at the same time because they both require water of different salinity. There are also certain other species which live in saltwater but move to freshwater sources at the time of breeding.