How Do Fish Breathe and Protect Themselves Underwater?

Fish are vertebrates (animals with backbones) that live in the sea and in fresh water. Like all animals they have to breathe in oxygen. Lungs would simply fill up with water, so instead fish have feathery gills at the back of the head that extract oxygen from water as it flows over them.


  • A goldfish can live up to 40 years.
  • A shark’s skin is covered in tiny, backward-pointing tooth-like structures which feel like rough sandpaper.
  • When flatfish are young they swim upright, but when they settle on the sea bed, one side becomes their ventral (lower) surface, and the other becomes the dorsal (upper) surface. Their left eye moves up to the top of their head to join the right one.
  • With fins almost as long as its body, the Atlantic flying fish, can skim above the waves at up to 60 kph (37 mph) to escape predators,

You won’t believe it!

The eel-like pearlfish has a strange relationship with sea cucumbers, sausage-like relatives of starfish. The fish hunts for food at night, and lives inside the sea by day, going in and out of creature’s bottom!

Sea cucumber is a great hiding place for pearlfish
Sea cucumber is a great hiding place for pearlfish

Tell me more: fish features

Powder Blue surgeonfish
Powder Blue Surgeonfish

powder-blue surgeonfish

Streamlined body: The ideal-shape to move through water.

Overlapping scales: These form a smooth surface for water to flow over.

Dorsal fin: This controls how much the fish rolls to one side or the other.

Tail fin: The tail gives forward thrust to push fish through the water.

Slimy mucus: This makes fish slippery and protects against parasites.

Lateral line: Sensors here detect sound vibration and water movement.

Eyes: Vision is important for navigation and to find mates and food.

Operculum: A flap that covers the gills and opens to let water flow out.

Pelvic fin: Used to steer the fish.

Pectoral fin: Used to control the direction of movement.

Fish families

“FISH” is a general name given to a range of vertebrates with streamlined bodies that live in water. There are three types of fish.

These long thin fish have a sucking mouth to grasp food. The group includes lampreys (pictured) and hagfish.

Jawless fish
Jawless fish

Rays (pictured), skates, and sharks all have a skeleton made of flexible
cartilage instead of bone.

Cartilaginous fish
Cartilaginous fish

This is the largest group of fish. They have a skeleton made of bone and come in all shapes and sizes.

Bony fish
Bony fish

In numbers

13 m
The length of the whale shark, the biggest fish.

7.9 mm
(0.3 in) The length of Paedocypris progenetica, the world’s smallest fish. It lives in peat swamps on the Indonesian island of Sumatra.

110 kph
(68 mph) The speed of the sailfin.

20,000 km
(12,430 miles) The distance the great white shark migrates on its return trip between South Africa and Australia.

The age in years of the longest-living fish – the rougheye rockfish.



Seahorses are slow movers and never exceed 0.0016 kph (0.001 mph).

To anchor themselves in strong currents, they can wind their tail around a piece of weed.

The male has a pouch in his body into which the female lays up to 600 eggs. He incubates the eggs in his pouch until they hatch.

Although seahorses are bony fish, their skin is not covered with scales.

Slithery slime

hagfish - Slithery slime
Slithery slime. Photo: PHY.ORG

Hagfishes are jawless fish that are also known as slime eels. Here’s why:

  • An average sized hagfish can produce enough gooey mucus to fill an 8-litre (14-pint) bucket.
  • They use slime as a defence against predators while they feed on dead and dying fish.
  • Hagfish get rid of their own mucus by tying a knot in their bodies and then sliding it forwards.

Reef life

Reef life
Reef life

Many fish that live on coral reefs have bright colours and patterns to break up their outlines, making it harder for predators to catch them or for prey to see them coming.

Monsters of the deep

Anglers fish
Anglers fish. Photo: PETER DAVID

Down in the cold, dark gloom of the ocean depths lurk some very strange fish. This deep-sea anglerfish can emit light in order to attract prey. It also has flexible bones, allowing the jaws and stomach to expand and create room for prey twice the size of its entire body.

Electric shockers!

Electric eels live in the murky waters of South American rivers and produce 500 volt electric pulses to stun prey and deter predators.

Electric catfish from tropical Africa are nocturnal hunters that use their electric organs to generate 300 volts to stun prey.

Electric rays can release 220 volts to stun other fish.

Elephantnose fish produce weak electrical currents to help with navigation.

Sharks and rays detect the weak electric currents produced by some prey and go in for the kill.

The stonefish, found in the Indian and Pacific Oceans, has spiny fins filled with venom that can be fatal to people who step on them.

Four sharks to avoid

Great white shark
This scary predator does attack humans, though it doesn’t really like the taste.

Great white shark
Great white shark. Photo: WWL

Hammerhead shark
With widely spaced eyes, this shark is prone to making unpredictable attacks.

Hammerhead shark
Hammerhead shark

Tiger shark
A stripy scavenger found in coastal waters, and known to attack people.

Tiger shark
Tiger shark. Photo: Oceana

Blue shark
The most widespread of all sharks, the blue commonly circles prey before attacking.

Blue shark
Blue shark. Photo: Wikipedia

Sharks are more at risk from humans than we are from them. Shark fishing, especially for shark fins, has seriously depleted numbers of some species.