How important is water s buoyancy as a limiting factor on sea creature s size

A.� Water is a relatively dense environment, able to support the weight of the world's largest living animals, blue whales. And so, theoretically, there should be no gravitational limit on the size of sea creatures.

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Even a small increase in an animals length results in a much greater proportional increase in their mass. For land animals this has important consequences for mobility and their supporting structures, like skeletons that a sea creature doesn't have to deal with.

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Q.� So, why are whales so much bigger than the largest fish

A.� Well of course the supporting element of your environment isn't the only factor to consider when it comes to size and there are several additional aspects that affect the size of fish in comparison to whales.

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Q.� Like what

A.� One very important factor is the difference between how whales and fish absorb and transport oxygen.

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Oxygen provides an important fuel for plants and animals. Whales absorb oxygen from air via their lungs, which are basically sponges, optimised for soaking up oxygen. The air contains a much greater saturation of oxygen than water does, so when a whale breaks the surface to take in a lungful of air it has access to a much greater amount than a fish does, as much as fifty times more.

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Q.� And fish absorb oxygen differently

A.� Yes, gills are very efficient at absorbing oxygen from water, but they cant do it as well as lungs can with air. So oxygen absorption is one limiting factor when it comes to fish size.

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Q.� Are there others

A.� Yes, the difference between a fish and a whale's heart may also be a limiting factor when it comes to size. A whale has two sides to its heart: each side is able to deal with different blood pressures and so sends blood with different pressures around to different parts of the body very efficiently.

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However fish do not have this advantage: blood flows straight from the gills to the body, as a result, blood pressure has to be low enough not to burst the blood vessels, but high enough to actually make it to all the required body parts. A larger body for the fish means an increase in blood pressure.

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Obviously fish can only achieve a certain size before the required blood pressure would be too high for the vessels to withstand.

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Interested in how different groups of animals compare Click here to ask The AnswerBank.

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by Lisa Cardy