Gaseous Exchange in Aquatic Invertebrates.

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Gaseous Exchange in Aquatic Invertebrates

All aerobic organisms need a regular supply of oxygen from their

environment in order to respire. This is much easier for terrestrial animals, as

21% of the air is made up of oxygen, compared to less than 1% in water.

Aquatic animals have to overcome the problem of getting enough oxygen to

support metabolic reactions in an environment where there is very little of it.

Some simple organisms such as amoebae and flatworms are able to

carry out gaseous exchange over their whole surface area, as they have a

high surface area to volume ratio. As they are submerged in water, gasses

can easily diffuse in and out of the organism using osmosis.

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        As animals become bigger and more complex, the surface area to

volume ratio decreases, and it becomes impossible for the organism to get

enough oxygen for metabolic reactions through its surface. Instead, they need

to have specialised respiratory surfaces. Many aquatic insects have

‘respiratory siphons’ which allow the insect to breathe air from above the

surface of the water while their bodies remain submerged. The insect

breathes through ‘spiracles’, which are pores in the skin, which can be

opened and closed by valves. These lead ...

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