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112 Nature

GILLS

4 GREAT WHITE SHARK

Sharks include the biggest and

most fearsome fish of all. The

great white shark is the largest

predatory fish, growing up to

7 m (23 ft) long. The whale shark,

which is a filter feeder, is even

bigger. It can grow to 18 m

(59 ft) in length and weigh

up to 21 tonnes.

Fish

Water exits

body through

gill slits

Oxygen is absorbed from

the water, through the gill membranes

HOW GILLS WORK

Tail

is swept from

side to side to

propel the fish

forwards

Dorsal fin

provides stability,

keeping the fish

upright as it swims

through the water

FISH CLASSIFICATION

• There are over 29,000 species
  of fish – more than half of all
  the world’s vertebrates. Fish
  divide into three major groups.


• The first group, and by far
  the largest, contains the bony
  fish. There are more than
  25,000 species alive today.
  
  
  • The second group contains the
    600 species of cartilaginous
    fish – sharks and rays.
  
  
  • The smallest group, with
    about 60 species, is also the
    most primitive. Its members,
    the lampreys, have skeletons
    but no jaws.
  
  
  
  

Like all animals, fish need a constant

supply of oxygen to survive. They do not

breathe air but extract dissolved oxygen

from the water using their gills – feathery

organs located behind the eyes and

supplied with many tiny blood vessels.

WHERE DO FISH LIVE?

Superbly adapted to life in water, fish are found

throughout the world’s oceans, from warm tropical

seas to icy polar waters. Some fish dwell near the

surface. Others live in the depths, where some use

. BIOLUMINESCENCE. Fish are also found in
freshwater habitats such as rivers, lakes, and swamps.

HOW DO FISH BREATHE UNDER WATER?

Water containing dissolved oxygen is drawn in

through the fish’s mouth, to pass over four or five sets

of gills on either side of the head. The gill arches hold

delicate, flap-like membranes with very thin walls.

Oxygen passes through these membranes into the

fish’s bloodstream, to be distributed around the body.

DO ANY FISH LIVE ON LAND?

No fish live on land but some can survive out of water

for years. For example, when the pool or lake where

they live dries up, African lungfish can survive while

buried in the mud, extracting oxygen from the air.

Fish are aquatic animals with an inner skeleton, including skull,

ribs, and backbone. Most fish have bony skeletons, but shark

and ray skeletons are made of rubbery cartilage. Fish extract

oxygen from the water using. GILLS , and swim using their

tail and fins. A fish’s skin is covered with tough scales.

1 MUDSKIPPER

Mudskippers come out on to

muddy shores to graze algae.

They keep their gills puffed out

with water, returning every so

often for a refill. Mudskippers

wriggle over the mud using their

pectoral fins.

Water enters

through the

mouth

Pectoral fin

controls direction – it

is held flat when

swimming in a

straight line