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.