Fish are animals that live only in water. They breathe with gills and usually have scaly skin. They are found all over the world, in rivers, lakes, and the sea. Fish are vertebrates because they have a spine, like mammals, birds, reptiles, and amphibians.
There are many different types that can look very different. They are distinguished primarily by whether their skeleton consists of cartilage or bones, which are also called bones. Sharks and rays belong to the cartilaginous fish, most other species are bony fish. Some species live only in the salt water of the seas, others only in the freshwater of rivers and lakes. Still, others migrate back and forth between the sea and rivers over the course of their lives, such as eels and salmon.
Most fish feed on algae and other aquatic plants. Some fish also eat other fish and smaller water animals, then they are called predatory fish. Fish also serve as food for other animals, such as birds and mammals. Humans have been catching fish to eat since time immemorial. Today, fishing is an important part of the economy. The most popular edible fish include herring, mackerel, cod, and pollock. However, some species are also overfished, so they are threatened with extinction and must be protected.
The expression “fish” is important in our everyday life. In biology, however, there is no uniform group with this name. There is a class of cartilaginous fish, which includes the shark, for example. But there are also bony fish like the eel, the carp, and many others. They don’t form a class, but a series. There is no group name for cartilaginous fish and bony fish together. They form a subphylum of vertebrates. Explaining this in more detail would be very complicated.
How do fish live?
Fish do not have a special temperature. Her body is always as warm as the water around her. For a special body temperature, it would take too much energy in the water.
Fish “float” in the water and usually only move slowly. Their muscles are therefore only supplied with a small amount of blood, which is why they are white. Only in between are there strong blood supply muscle strands. They are red. The fish need these muscle parts for a short effort, for example when attacking or when fleeing.
Most fish reproduce by eggs. These are called roe as long as they are still in the womb of the mother. The insemination by the male takes place outside of both bodies in the water. The ejection of the eggs is called “spawning”, the eggs are then the spawn. Some fish simply leave their eggs lying around, while others stick their eggs to rocks or plants and swim away. Still, others take great care of their offspring.
There are also few fish that give birth to live young. In addition to sharks and rays, this also includes some species that we are particularly familiar with from the aquarium. These fish need visual intercourse so that the eggs can be fertilized in the mother’s womb.
What special organs do fish have?
Digestion in fish is almost the same as in mammals. There are also the same organs for this. There are also two kidneys that separate urine from the blood. The joint body outlet for feces and urine is called the “cloaca”. The female also lays her eggs through this exit. There are only a few species with a special exit for the living young animals, for example with special carp.
Fish breathe through gills. They suck in water and filter out the oxygen. They return the water with the carbon dioxide to their surroundings.
Blood circulation in fish is simpler than in mammals.
Fish have a heart and a bloodstream. However, both are easier in mammals and birds: the heart first pumps the blood through the gills. From there it flows directly onto the muscles and other organs and back to the heart. So there is only one circuit, not a double one like in mammals. The heart itself is also simpler.
Most fish can see and taste like mammals. They just can’t smell because they don’t come into contact with air.
This is what a swim bladder looks like.
The swim bladder is particularly important in fish. They only exist in bony fish. The swim bladder may fill or empty more. This makes the fish appear lighter or heavier in the water. It can then “float” without power. It can also lie horizontally in the water and prevent it from accidentally tipping forwards or backward.
The lateral line organs are also special. They are special sense organs. They stretch over the head and all the way to the tail. This allows the fish to feel the flow of the water. But he also senses when another fish comes near.