Arrange the following estuarine zones in decreasing order of salinity.

polyhaline

mesohaline

oligohaline

euhaline

Euhaline ⇒ Polyhaline ⇒ Mesohaline ⇒ Oligohaline

Explanation:

Let's first define each of these levels:

- Polyhaline: a category of salinity that has medium-high levels of salt

- Mesohaline: a category of salinity that has medium levels of salt

- Oligohaline: a category of salinity that has low levels of salt

- Euthaline: a category of salinity that has high levels of salt

Now, we can order these in decreasing order of salinity:

Euhaline ⇒ Polyhaline ⇒ Mesohaline ⇒ Oligohaline