Fish are aquatic, cold blooded vertebrates that breathe by means of gills and use fins for locomotion.
 
Fish are divided into two main groups BONY FISH, e.g. the hakes, tuna, sole and trout, which have a skeleton made of true bone, and the CARTILAGINOUS FISH, e.g. the sharks and rays, whose skeleton is composed of cartilage. Fossil records show that fish have been around for over 360 million years. During this incredibly long period they have achieved a great diversity and have become adapted to live in nearly all aquatic habitats. There are over 20 000 fish species worldwide and this makes them the vertebrate group with the largest number of different species.
 
Bony fish are found in a wide variety of different shapes and sizes. Some are tiny, like the gobies, which may only grow up to one centimetre in length, while the huge game fish, such as the marlin, may grow to over three metres in length.
 
The oceans cover three quarters of the planet's surface and fish are found in just about all habitats in the sea. Open ocean fish can be associated with either the depths of the oceans (the abyssal zone (down to 6 000m deep)), or the surface water. Fish common in the coastal zone closer to land can live on coral reefs, offshore rocky reefs or sandy bottoms and inshore sandy or rocky shores or estuaries. Estuaries and intertidal rock pools are often very important nursery areas for the juveniles of certain fish species. Some fish are adapted to live near the sea bottom (demersal), while others remain higher up in the water column (pelagic) or right near the water surface (epipelagic).