Updated: Jan 23, 2021
There are about 32,500 species of fishes (Nelson, 2006) and according to FishBase (global fish species database) 33,100 species have been described by April 2015.
To give a single definition to fish is difficult considering the kind of variations seen in them. We have the jawless fishes such as lampreys, hagfish which lack fins, jaws, stomach. Eel is another example. We also have lungfish which is an elongated freshwater fish with one or two sacs that function as lungs, enabling it to breathe air. Many species of fishes such as Clingfishes lack scales. We have jawed fishes to which the bony and cartilaginous fishes belong. With this, to get a simple, generalised definition of fish as a cold blooded aquatic animals having gills, fins, scales may not really take into account the kind of variations being observed.
According to Berra (2001) fishes could be defined as a poikilothermic, aquatic chordate with appendages (when present) developed as fins, whose chief respiratory organs are gills and whose body is usually covered with scales. Poikilothermic are animals whose internal temperature varies considerably.
Ornamental fishes simply put are attractive, colourful fishes which are kept as pets. Color talks a lot about the fish, be it its health condition, readiness to spawn or their feelings. Various hues seen in fishes are produced by different types of chromatophores. Several distinct kinds of chromatophores could be seen in fishes, as follows: iridophores, leucophores, melanophores, xanthophores, erythrophores, cyanophores. The genetics, diet have a huge role in determining the colour of the fishes.