Tag Archives: Buoyancy

Swim Bladder vs. Oily Liver

Many animals naturally float at the surface and have to force their way down below the surface. Fish however, have a very convenient adaptation. Two ways to animals in the ocean control their buoyancy, or the upward force exerted by a fluid, are with a swim bladder and an oily liver. Osteichthyes (bony fish) use swim bladders that are filled with oxygen taken in by their gills. The more air in the swim bladder the more buoyant the fish and the less air in the swim bladder the less buoyant the fish. The swim bladder is similar to human lungs in the way that it expands and deflates. Chondrichthyes (cartilaginous fish) use an oil filled liver to control their buoyancy. The oil lightens the shark’s heavy body to keep it from sinking and saves the sharks energy when using its fins to keep itself moving. The oily liver is also used for other daily functions such as digestion. The name for this oil is squalene. Without these organs they would not be able to control their location in the water column. These organs can help them stay neutrally buoyant.

A fun way to test how oxygen and oil react in water is to fill a tank with water and submerge each at different times. Push an upside down cup into the water, air will be trapped inside the cup. Next take a second cup upside down and push it into the water but allow water to fill it. Then you should be able to move the cups close together and pour the air filled cup into the water filled cup. The air will try to escape to the surface but will get caught in the second cup. Do this a second time with a cup full of oil instead of air. A lid will be needed to cover the cup of oil in order to turn it upside down to submerge it. Once submerged in water remove the lid and pour the oil into the cup filled with water. This demo also gives you a chance to observe how the oxygen and oil are positively buoyant and why they need these organs to assist in the vertical movement in the water column.

Salinity and Water Density

When swimming in salt water you can make observations on more than just the salty taste. An observation you may have made is that you are more buoyant, or you float more, in salt water than in fresh water. This is a result of the oceans salinity, the amount of salt in the water. Density is the amount of matter in a given space or area. When a volume of water is replaced by an item of greater density than the water, the object will sink. Likewise, if a volume of water is replaced with an object that is less dense than the water, it will float towards the surface.

This physical happening can be observed when pouring fresh water into a pitcher of salt water. The fresh water, when poured slowly, will remain on the top of the salt water. A small layer forms where the two bodies of water meet, called a Halocline. You can also observe the difference in density by using an egg. Place an egg in fresh water and watch as it sinks to the bottom. When placed in salt water it will float. The egg was denser than the space it replaced in the fresh water causing it to sink and was less dense than the salt water causing it to float. When placing the egg into the pitcher that had the fresh and salt water the egg floated right at the Halocline.


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