Why are shellfish population often a good indicator of water quality

"Water quality" may include the amounts of dissolved oxygen and the levels of algal growth, pollutants which may be present and the pH level. Water quality effects also which types of organisms can survive in a body of water.
 Shellfish require a high level of dissolved oxygen and their abundance is an indication of good water quality. Shellfish are sensitive to changes in the ecosystem and they cannot easily escape changes in the water quality. These are one of the reasons why shellfish are good indicators of water quality.

Because the earth is not round ,but it may seem round when viewed from ,our planet but true some points earth is not round was an oblate spheroid -a square that it squashed as globe however is not even a perfect oblate spheroid.in fact isaac newton's first proposed that earth was not perfectly round but it was an oblate spheroid instead.that mean a square that is squashed as its poles and swollen at the