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Size and shape underwater
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Size and shape underwater

Short video on size and shape
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In this section, you will learn about why different sized swimming organisms have the shape that we see. Hi, I’m Dr. Christopher Clemente, and I’m a specialist in animal physiology at the University of the Sunshine Coast. Have you ever noticed that animals like sharks or dolphins are really streamline? This kind of makes sense? Because after all, we want to be streamlined in the water. But what about other organisms, tiny animals like plankton? They don’t seem to be streamlined at all, what’s going on there? And what does it even mean to be streamlined anyway? Today I’m going to tell you why these animals adopt these different shapes, and how this is linked with speed and size.
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But first, I have to tell you something about the water that they are moving through. gases and liquids have viscosity and lack the stiffness and elasticity of solids. And this viscosity determines how fast a fluid will flow like honey or water. Now back in 1883, a guy called Osborne Reynolds famously studied the conditions which affect the flow of fluid in pipes, he found that the flow of fluid was determined by two effects. First, the viscosity of the fluid viscosity, he reasoned, is a measure of the group Enos of a fluid, the tendency for the bits to flow together. And he showed that this was proportional to the area of an object in contact with that fluid.
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But a second force was also important. The inertia of fluid inertia is the tendency for bits of matter to keep going, like what happens in space. It resists acceleration, and it reflects the individuality of the bits of fluid. Now, this property is proportional to mass. Now, he found that the wave fluid moved was dependent on the ratio of these two forces, and he called this ratio the Reynolds number. Why is this important for animals? Well, it turns out that the shape of animals changes in a predictable way as animals get larger.
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To illustrate this, let’s pretend that animals are a simple cube, a small cube, say with a length of one centimetre long, will have a surface area of six centimetres squared, but a volume and a mass equivalent for a one centimetre cubed, the ratio of these surface area to mass will then be six to one. However, if we double the length of the cube, the surface area increases, but not as fast as the mass or the volume. And this trend continues as we continue to increase the size of the cube. This means that the volume and the mass are going to increase faster than the surface area as animals get bigger.
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Since viscous effects are proportional to surface area, while inertial effects are proportional to mass, the movement of the small organisms is dominated by viscous effects, while those of large organisms tends to be dominated by the inertial effects. That means small animals or small Reynolds numbers, and to them, the water feels more viscous. But why does this matter? Well, because Reynolds numbers determine the way fluid flows. at really low Reynolds numbers, the flow is orderly with a gentle gradient, and drag force doesn’t depend on shape. Therefore streamlining is without value to reduce this drag, and animals can be whatever shape they want.
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But as Reynolds numbers get higher, the flow becomes disordered and velocity gradients, steep shape becomes important to reduce drag. But what on earth is drag anyway, it’s a little complicated, but think of it is the difference in pressure between the front and back of an organism relative to the flow, there’s going to be high pressure at the front of the organism. But as fluid speeds up to get around the object, the pressure drops, especially as the flow become separated. So the difference in this pressure between the front and back is going to push our object backwards. You can actually map the pressure of fluid both in front and behind an organism.
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For example, here, in front of both objects, the pressure is high, but decreases quickly for the streamline shaped because there is a small area of that disordered flow, which would have a lower pressure. Now we have all the information we need to understand the shape of animals like fish, they adopt this streamline shape To reduce the area of low pressure behind them, and therefore reduce this drag force for we can actually see a lot more. First, notice where the mouth is, it’s situated at the front where the pressure is high, and this will force water in and over the gills. But look where it comes out of the gills.
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The pressure at this point is that a minimum and so this is going to help to allow water to flow quickly over the gills. Next, look at the position of the eye. Evolution could have put the eye wherever it wanted on the tail, or even above the mouth so it could see what it was eating. Yeah, eyeballs a soft and are easily deformed by the pressure of water. So the best spot for the eye would be exactly where it crosses this axis, and the pressure changes are the lowest. So the shape of fish and all of the other organisms underneath the sea is driven by these forces.
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And the amazing thing is that this general shape has followed on in the forms of creatures that have evolved from fish, both below the water and on the land.

Different animals have different shapes underwater

some animals, like sharks and dolphins have a sleek torpedo shape, but other animals like plankton can be round or odd shaped. What drives the shape of animals underwater?

In this video we will explore the the forces acting on animals moving through an aquatic environment.

The movement of water is the result of both inertial and viscous forces. At very small body sizes viscous forces dominate, so the water feels like honey, and streamlining the body would be without value. At larger body sizes inertial forces are more important, the flow of water becomes more disordered, creating more drag. At these larger body sizes, becoming streamlined is important to reduce this drag.

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