If you live on a river delta or coastline, you may not be able to eat the local shellfish because of high levels of toxins present in the water. Why?

Answer: Shell fish is used as a biological indicator of pollution.

Explanation:

The clams, mussels and shellfishes are some of the organism which can be used as a biological indicator of the pollution.

This is so because they have the ability to filter and concentrate the pollutants such as pesticides, and other pollutants in water.

These organisms can be toxic to people as it includes pollutants. This can be a early warning that the nearby water is getting highly polluted.