a All fish are aquatic, living either in fresh or salt water. They are divided into three main classes — the jawless fish (Agnatha), bony fishes (Osteicthyes) and cartilaginous fishes (Chondrichthyes). There are more species of fish than all the species of amphibians, reptiles, birds and mammals combined.

a Cartilaginous fishes include sharks, rays, skates and chimaeras.

a Bony fish also have teeth that are fixed onto the upper jaw. They have a swim bladder (an air filled sac that helps them with buoyancy) that opens into the gullet. Examples include herring, eels, sturgeon, salmon, carp, catfish, anglers, cod, flying fish, seahorses, etc.

a There are very few jawless fish species today, examples include lampreys and hagfish. These primitive fish have no jaws, and have a skeleton made of cartilage.

a All fish are vertebrates (have a backbone) and make up almost half of all known vertebrate species. They have lived on earth for more than 500 million years.

a A group of fish is called ‘school’.

a A person who studies fish called an ichthyologist.

a The study of history of fishes is known as ichthyology.

a Fish are cold-blooded, so their internal body temperature changes with the change in the temperature of its surroundings.

a Fish have excellent senses of sight, touch, taste and many possess a good sense of smell and ‘hearing’. Fish feel pain and suffer stress just like mammals and birds.

a Fish eat other fish, fish eggs, molluscs, aquatic plants, algae, zooplankton, terrestrial insects, water birds, turtles, frogs, snakes and mice.

a Sharks and rays are flexible as their skeletons are made of bendable cartilage rather than bone, and they have rough scales.

a Most fish lay eggs, some lay up to 200,000 at a time, but many of these eggs get eaten by other sea creatures.

a Fish have a specialised sense organ called the lateral line which works much like radar and helps them to move in dark or murky water.

a Fish ‘breathe’ by pushing water over rows of gills at the back of their mouth. When the fish opens its mouth to take in water, the gills take oxygen from the water and the water then passes out through the gill slit.

a There are about 40 known species of the flying fish, which have thin wings that allow it to jump and glide in a way that looks like it is flying but it is actually not flying.

a Stingrays use electric sensors to find food. This enables it to find its prey even if it is buried in the sand.— Complied by Aamnah Arshad