Forming a much larger group than the cartilaginous fish, bony fish have skeletons made of bone. Their bodies are usually covered with overlapping scales. They also have gas-filled swim bladders inside their bodies that allow them to stay afloat even when they are not moving.
Some bony fish live in fresh water (rivers and lakes), while others live in the seas and oceans. Many feed on plant material, but some are carnivorous. Among the largest predators are the barracudas, which strike at great speed with their powerful jaws. Even larger is the blue marlin, which can be over four metres in length. Probably the most extraordinary of the large bony fish is the ocean sunfish, or mola mola. Its round body can be as large as a small car, and it “rows” itself through the water using its fins.
Most bony fish have good eyesight, and can see in colour. Their eyes are on the sides of their heads, giving them a wide field of vision. Some kinds of bottom-dwelling fish have eyes that point upwards, to spot predators or prey above them. Fish such as salmon, which are preyed upon by many other animals, swim in large groups for protection in the same way as some animals, for example, cattle, herd together on land.
Seahorses rely on camouflage to keep themselves safe, while angelfish stay near clumps of seaweed or rocks. Some fish hide in the stinging tentacles of sea anemones.
Several kinds of fish have protective armour made of fused scales, while others have sharp spines. Bright colours are often a warning signal to predators. The markings of the lionfish warn that its long, sharp spines are poisonous. Other fish inflate their bodies suddenly to startle an attacker.
Most fish have a familiar streamlined body shape, but several kinds look very different. Eels have long bodies with narrow dorsal fins that do not stick out from their bodies like those of other fish. Seahorses live in shallow waters and swim upright. They grasp on to seaweed with their tails to avoid being swept away by the current.
Many ocean-living fish are found close to the surface, where there is warmth and light and tiny plants, called plankton, to eat. Small plankton-eating fish may be prey for larger fish.
Fewer fish live in the deeper, darker waters. The oarfish lives between 300 and 600 metres down. With its ribbon-like body and red “mane”, it may be the sea monster described in old sea legends. Some deep-living fish travel up through the water to hunt, while others feed on scraps of food that drift down from the surface. Some deep-living fish are able to make their own small lights to attract prey towards their gaping mouths.
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