The whale shark is the largest living fish, commonly reaching 9 m and sometimes as much as twice that length.
It is a sluggish fish that feeds on plankton and is found only in tropical seas. One of the largest freshwater fish is the arapaima of South America, which reaches a length of 2.4 m. The European catfish grows to a length of 4.5 m and weighs around 300 kg.
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Electric eels are well known, and they can stun their prey with a powerful shock. Other fish can also produce electricity to kill their prey or to defend themselves. They include electric rays, which are common in many warm seas, and also electric catfish living in African rivers. Fish called mormyrids, which live in very muddy waters in Africa, use electrical fields as a form of radar. This allows them to move about safely and to find food. They can also communicate by means of these electrical signals.
Sharks do not produce electricity, but they possess organs in their skin that can detect tiny electrical currents produced by the muscular movements of other fish. In this way they can hunt without being able to see their prey.
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Most fish have an organ called a swim bladder, which is filled with gas to make them buoyant. The amount of gas in the swim bladder can be altered so that when the fish swims in deep water, and the pressure squeezes the swim bladder, more gas is pumped into it to compensate. When fish are caught in very deep water and pulled quickly to the surface, their swim bladder expands rapidly and may burst. Some fish, such as sharks and tuna, do not have swim bladders. They have to keep swimming so that they do not sink.
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Flying fish are able to launch themselves above the surface of the water to escape from predators. They support themselves in the air with large, outstretched fins. It is this which gives the appearance of flight.
The lower lobe of a flying fish’s tail is extended so the fish can still propel itself along by beating it in the water while its body is above the surface. A single ‘flight’ above the water can cover a distance of 150 m, at a speed of 55 km/h.
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Fish have colonized almost every aquatic habitat and are found from the water’s surface down to the deepest depths. They even live in warm springs and spring water containing large amounts of dissolved minerals. A fish’s shape is modified to suit its lifestyle. Fish such as tuna and sharks, are streamlined because they are powerful swimmers. Fish living in mountain torrents are often flat and have suckers to help them cling to stones to avoid being swept away.
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Many fish that live in water containing little oxygen are able to breathe air. They usually have to do this in foul water, when bacterial decay reduces oxygen levels. Some fish have a special breathing organ called a labyrinth, and they rise to the surface every few minutes to breathe air into this organ. Many catfish breathe air in a similar way — they swallow it and it passes into their gut where oxygen is absorbed through the thin lining of the gut wall.
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Fish are able to breathe underwater because they have special organs called gills. Gills are bars of tissue at the side of the fish’s head. They carry masses of finger-like projections that contain tiny blood vessels. Water enters the fish’s mouth and flows over the gills. The gill filaments take in dissolved oxygen from the water and pass it into the fish’s blood. In this way the gills have the same function as the lungs of air-breathing animals.
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The giant salamander of Japan is a very strange-looking creature. It grows to a length of 1.5 m and lives in fast-flowing mountain streams. It breathes through its wrinkled and baggy skin. The giant salamander is a very sluggish animal that cannot move about on land.
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The skin of some frogs and toads contain poisons, which are among the most powerful known to humans. In South America, poison from the poison arrow frog is added to the arrow tips used by the Indians for hunting. Common toads contain poison that they exude through their skin if attacked. Dogs and cats commonly experience this poison; however they seldom suffer serious effects from it but quickly learn to avoid these amphibians. Cane toads are very large toads, which contain a drug capable of causing hallucinations if it is eaten.
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The bodies of some frogs and toads are adapted to survive in very dry conditions in deserts. During the brief rainy season, their bodies absorb huge amounts of water and become swollen like a bladder. These frogs and toads bury themselves deep in the ground to protect themselves from the heat and Sun. They can survive for several years if necessary, until the next rains arrive. Once the rain comes, the amphibians emerge from their underground holes and breed quickly, laying their eggs in puddles. The tadpoles develop very rapidly before the puddles dry up.
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Nearly all amphibians have aquatic larvae called tadpoles, which usually lead an independent existence while they grow. In most amphibians, the male clasps the female and sheds sperm over the eggs as they are laid. The eggs of salamanders and caecilians are fertilized inside their bodies. Amphibian eggs are delicate and must be kept moist. The developing tadpoles may have a large rounded head and body before their legs appear, as in frogs and toads. The tadpoles of salamanders and newts are shaped like the adult animals, but they have feathery gills.
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Frogs and toads have developed some of the most unusual and varied forms of reproduction known among vertebrate animals. The midwife toad drapes fertilized eggs around the hind legs of the male, who returns to the water every night to damp them down. He returns to the water when the eggs are hatched, so the tadpoles emerge safely.
The Surinam toad is even more odd. As the eggs are fertilized, the male presses them onto the female’s back. They sink into her skin and each egg forms a small cell in which the tadpole develops. After 80 days it emerges into the water as a small toad. The eggs and young of the Darwin’s frog, found in South America, are brooded inside the male’s vocal sac. Some frogs lay their eggs in water-filled holes high up in trees in the tropical forest, while others lay them on the damp ground and sit on them to keep the eggs moist.
Most amphibians live in moist places near rivers, streams or lakes. They have thin moist skin and cannot usually tolerate dry conditions. Amphibians are thought to be the descendants of the first animals to emerge from the sea onto land. Most amphibians are still partly aquatic, especially in their breeding habits.
Why are amphibians important to humans?
Frogs and toads are particularly important in tropical regions because they keep insect pests under control. They eat enormous numbers of insects, and their tadpoles feed on pests such as mosquito larvae. Unfortunately, frogs are increasingly caught as food, and so their numbers are greatly reduced. Their thin skin makes them very susceptible to the poisonous effects of crop sprays, contributing further to their reduced numbers.
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The olm is a blind newt-like amphibian that lives in flooded caves in Eastern Europe. Like other cave-dwelling animals, it does not have any pigment in its skin and it never leaves the water.
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From an evolutionary point of view, amphibians are halfway between fish and reptiles. There are 4,400 living species of amphibian. This group includes frogs and toads, newts and salamanders, and an obscure group of burrowing creatures which are called caecilians.
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