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Characteristics of fish

Fish are vertebrates which have adapted to living in water. They swim in the water with the help of finsfinfins and appropriate body movements. The driving body of fish is their tailtailtail, which ends in a caudal fin. In turn, the remaining fins are used to maintain the correct direction of movement and a suitable position. Fishes' skin is covered in scalesfish scalesscales and numerous mucus‑producing glandsglandsglands, which reduce friction when moving in water. Many species of fish have a swim bladderswim bladderswim bladder inside their bodies. This is a membraneous sack filled with gas. Because of this, the fish is able to swim at any depth.

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Ilustracja przedstawia rybę widoczą z lewej strony. Głowa ryby jest skierowana w lewo. Za głową ryby widać płetwy piersiowe pectoral fin. Za nimi na spodzie ciała ryby są płetwy brzuszne pelvic fin. Za nimi na spodniej stronie jest płetwa odbytowa anal fin. Na grzbiecie znajduje się płetwa grzbietowa dorsal fin. Na końcu ciała jest płetwa ogonowa caudal (tail) fin.

Where is it easier to move?

Fish spend their whole lives in water and move around in it without any problems. Does it come easily to all organisms? Those of us who have been to the sea or a lake know that running is different in wet sand, different in knee‑deep water, and in turn, running in waist‑deep water is almost impossible. Now we can do some experiments to demonstrate the differences between air and water resistance.

Comparing air and water resistance

Experiment 1

Demonstrating the difference between air and water resistance.

You will need

• two identical coins, for example fifty groszy;

• a tall glass container.

Instruction

1. Fill the previously prepared containder with water.

2. Put both coins at the same height, one just above the surface of the water, and the other next to the container of water, then drop them at the same time.

3. Observe the speed at which both coins fall.

4. Repeat the experiment a few times, to be sure of the result.

Summary

If the coin falling in water moved more slowly and landed later, that means that water has a higher resistance than air.

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Zdjęcie przedstawiające dwie dłonie trzymające monety na tej samej wysokości. Jedna nad wysokim naczyniem z wodą, a druga obok, w powietrzu nad blatem, na którym stoi naczynie.

How can we overcome water resistance?

When swimming, fish must overcome water resistance. However, the same laws affect all mediums, for example air. It is worth noting that both birds and planes, which reach considerable speeds during flight, have a streamlined body shape (or hull) and a long forward‑facing beak. Moving in water is similar, however water puts up more resistance than air. This is why elongated, spindle‑shaped objects move more easily in water. This means a narrow, elongated front, which widens in the middle, and narrows towards the back. Think about what shape most fish are, as well as the shape of kayaks, boats, yachts, ships or submarines. To confirm this, carry out the following experiment.

Not all fish have a streamlined body shape, which is why they can't swim fast. For example, it is hard to call seahorses good swimmers. However, the shape of their bodies camouflages them perfectly when they hide among underwater plants. The ability to swim fast is therefore not needed.

How does the swim bladder work?

„A body immersed in water experiences an apparent loss in weight that is equal to the weight of the fluid displaced by it”. For this reason, fish immersed in water, when compared to land animals, are „lighter” by the weight of the displaced water. This is why the biggest and heaviest animals known to us can live in the seas and oceans. Fish, just like people or ships, are heavier than water, but don't sink in it. They can also swim on its surface or dive deeply.

The deeper a fish dives, the greater the pressure it experiences from the weight of the water surrounding it. To be able to balance out this pressure and swim at different depths, fish are equipped with a swim bladder. It is in the form of a chamber, which, when required, fills itself with gas. Then, the fish swims up, closer to the surface. We can understand this better by carrying out an experiment.

Swim bladder

Experiment 2

Investigating the principles of the swim bladder.

You will need

• a small screw‑top jar or a glass screw‑top bottle,

• a large, tall container (for example a tall vase, aquarium, bucket, etc.).

Instruction

1. Fill both the large container and the small jar with water.

2. Screw the top on the jar, and put it into the large container.

3. Take the jar out of the bottom of the container and pour out a little of the water. Screw the top on the jar, and put it back into the container.

4. If the jar sinks, pour out a little more of the water and try again.

5. If the jar floats to the surface, add a little water and try again.

6. By repeating these actions and changing the amount of water and air, you can achieve a state in which the screw‑top jar with the pocket of air will float between the bottom and the surface of the water.

Summary

The glass and the lid are heavier than water, therefore when the jar is filled with water it will sink. Air is lighter than water, therefore a jar containing a lot of air will float to the surface.

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Zdjęcie przedstawiające mały zakręcony słoiczek napełniony wodą i odrobiną powietrza, unoszący się w dużym pojemniku z wodą powyżej dna, ale poniżej powierzchni – ujęcie z boku.

Summary

• Water puts up greater resistance to moving objects than air.

• Objects with a streamlined shape move faster in air and in water.

• The swim bladder makes it possible for fish to balance out water pressure, which makes it easier for them to move in water.

Keywords

swim bladder, streamlined shape, resistance

Glossary