Aquatic animals, in general, face more of a challenge when extracting oxygen from their environment than do terrestrial animals. Why?

water has less oxygen than air

Explanation:

All animals require oxygen in order to breath. However aquatic animals face more challenge in obtaining oxygen compared to terrestrials this is because of water having less oxygen.

The inadequacy of oxygen is due to air being less dense than water hence less flow of oxygen.

Therefore, in order for aquatic animals to obtain oxygen, they will have to process 30 times more water to get the same amount of oxygen as terrestrial animals.

Due to this problem, aquatic animals poses gills that have an extremely large surface area to absorb oxygen.

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In order to extract oxygen aquatic organisms face laborious tasks,This is because oxygen concentration of aquatic  medium is very low, (compare to the atmosphere) because of   the need to  mix and   dissolve in water. There it takes more processing, mechanisms for these organisms to extract  dissolved oxygen, compare to terrestrial animals

In addition, the relatively high density of water compare to air reduces  the  flow and diffusion  of dissolved oxygen. Therefore to extract oxygen which must flow on their respiratory surfaces , aquatic organism  must    expend  large amount of energy to expand these respiratory surfaces  for a large surface area  needed for extracting Oxygen.  

Explanation: