7. Fish

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7.1 Fascinating fish
7.2 The internal structure of a fish

7.3 Fish reproduction
7.4 Fish and their varied diets

7.5 Gallery – Fish species

7.1 Fascinating fish

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Fish have a number of adaptations that make them suitable for aquatic life. Adaptations are characteristics that make the animal able to live in its environment, for example under water. Gills are an important adaptation for fish. They are used by fish to "breathe" oxygen under water: fish use gills to gather oxygen directly from water and to remove carbon dioxide from their systems.

Fish have adapted to aquatic life.

7.2 The internal structure of a fish

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The internal structure of a fish. The fish uses its gill to breathe under water. Its heart has one chamber and one hallway. Its digestive system consists of a stomach and an intestine. Female fish have ovaries. Fish use their swim bladder to regulate their depth movement.

7.3 Fish reproduction

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The process of fish reproduction is called spawning. It requires male fish sperm, or milt, and female fish eggs, or roe. The process of spawning involves females releasing ova (unfertilized eggs) into the water, while males simultaneously release sperm cells (milt) to fertilize the eggs. This process is known as external fertilization, as it does not take place inside the female fish.

Depending on the species, the development of fertilised eggs into fish takes from a few days to several months. The eggs develop and hatch into larvae (baby fish) without any help from the parents.

The larvae are not able to feed themselves, and carry a yolk-sac which provides their nutrition. Before the yolk-sac completely disappears, the tiny fish must become capable of feeding themselves.

Left: Fertilized eggs. Right: Hatched salmon larva. It gets its energy from a yolk-sac.

7.4 Fish and their varied diets

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When a young fish has used up all of the nutrition from its yolk-sac, it has to start consuming zooplankton.

Aquatic invertebrates slightly larger than zooplankton are also an important source of food for small fish. These invertebrates comprise the diet for many species of fish even as adults. Such invertebrate-consuming fish species include all small-sized species, such as small cyprinids like the common roach. Some larger species of fish also eat invertebrates. For example, the common bream likes to feed on insect larvae it finds near the bottom of the water.

The common roach.

Many species fish eat other fish. They are considered predators. The pike is an excellent example of a predator fish. It has a large mouth, sharp teeth, and it is lightning-quick. Perch become predators when they grow over 15 cm long.

Salmon and trout are likewise predators. They become predators once they move away from their birth locations to dwell in the sea or in lakes. Once there, they begin hunting for smaller fishes, such as herring.

The pike is a predator.

7.5 Gallery - Fish species

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