Scales

Scales are clear plates which are usually small and round but can appear in many different shapes and sizes. Some fish have no scales at all but they have extra tough skin. Fish often feel slimy to touch. This is not from the scales but from the skin underneath which produces a special mucus and helps the fish to glide easily through the water.
Breathing

Oxygen is found in water in a dissolved form. Fish breathe in water using their gills. Water flows past the rich blood supply in the gills and oxygen passes into the fish's blood through special, thin skins called membranes. The fish's heart then pumps blood around the body distributing the oxygen.
Bony fish
Bony fish have bony skeletons and an air bag inside them called the swim bladder which keeps them afloat in the water.
Cartilaginous fish
Cartilaginous fish have skeletons made of bendy cartilage (not bone). They include sharks, skates and rays. Most of the cartilaginous fish are hunters!
Predators
Big fish eat little fish and little fish eat even smaller fish. Fish have several features and tactics to escape from their predators. Lionfish have sharp spines, pufferfish can swell up to twice the size, plaice change their colour or shape through camouflage and others use a burst of speed to trick the hunters. Fish eat many different things including microscopic plants and animals, shellfish and corals.
Sharks
Sharks are powerful and efficient predators. They detect their prey with specialised senses. They have large eyes to see their prey, a good sense of smell to hunt for the scent, and electricity detecting organs around the nose to detect their prey. They charge at speed and bite their prey with bone crunching power. A shark continuously grows new teeth at the back of its jaws. These gradually move forward until they reach the front of the mouth, so they can replace teeth that have worn away. 'Jaws' the film star was a great white shark. These sharks can grow up to 9 metres long.