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What helps fish swim?

Source: licencja: CC 0.

Link to the lesson

Before you start you should know
  • the importance of different structural features in the lives of animals;

  • how fish are structured;

  • how fish have adapted to life in the water.

You will learn
  • to compare air and water resistance;

  • to explain the importance of fishes' streamlined shape;

  • explain the importance of the swim bladder.

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nagranie dźwiękowe abstraktu

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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Fishes' fins
Source: Anita Mowczan, licencja: CC BY 3.0.

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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Comparing air and water resistance
Source: Tomorrow Sp. z o.o., licencja: CC BY 3.0.

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.

Task 1

Before you do the experiment „The relationship between shape and resistance”, write down the research question and the hypothesis. Then follow the attached instruction. Make observations during the task, and finally - conclusions.

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Analysis of the experiment. Pytanie badawcze (Uzupełnij). Hipoteza (Uzupełnij). Obserwacje (Uzupełnij). Wnioski (Uzupełnij).
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Pobierz załącznik

Plik PDF o rozmiarze 107.00 KB w języku polskim

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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How the swim bladder works
Source: Tomorrow Sp. z o.o., licencja: CC BY-SA 3.0.
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Exercise 1
Identify the traits which make fish well-adapted to swimming. Możliwe odpowiedzi: 1. Glands which produce mucus., 2. The presence of a swim bladder., 3. Excellent hearing., 4. Gills which take in oxygen.
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Exercise 2
Decide if the statements related to experiment 3 are true or false. In the experiment carried out, the jar is the equivalent of a fish, and the air contained in it is the equivalent of a bladder. Możliwe odpowiedzi: True, False. The air filling the jar allows it to float in water. Możliwe odpowiedzi: True, False. A screw-top jar filled with air will float on the surface of the water. Możliwe odpowiedzi: True, False. A screw-top jar filled with water will sink. Możliwe odpowiedzi: True, False. Glass is heavier than water. Możliwe odpowiedzi: True, False
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Exercise 3
Why is the swim bladder important for fish? Możliwe odpowiedzi: 1. It is fishes' main respiratory organ., 2. It increases the speed at which a fish can swim., 3. It accumulates the oxygen required to breathe., 4. It makes it possible for fish to swim to the surface.

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.

Homework
Task 2.1

Explain how the shape of motorboats allows them to reach high speeds.

Keywords

swim bladder, streamlined shape, resistance

Glossary

glands
glands
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nagranie dźwiękowe słówka

gruczoły – struktury zbudowane z komórek, które wydzielają różne substancje; wytworami gruczołów są na przykład śluz u ryb i pot u ludzi

fish scales
fish scales
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nagranie dźwiękowe słówka

łuski ryb – cienkie kostne płytki w skórze, które pełnią funkcję ochronną i ułatwiają poruszanie się w wodzie

tail
tail
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nagranie dźwiękowe słówka

ogon – część ciała występująca o większości kręgowców, której szkieletem jest końcowy odcinek kręgosłupa; u ryb stanowi główny narząd ruchu

swim bladder
swim bladder
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nagranie dźwiękowe słówka

pęcherz pławny – narząd we wnętrzu ciała wielu ryb mający postać worka napełnianego w razie potrzeby gazem; umożliwia zmianę głębokości zanurzenia

fin
fin
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nagranie dźwiękowe słówka

płetwa – narząd występujący u zwierząt wodnych służący do poruszania się, zmiany kierunku ruchu lub zachowania wybranej pozycji