In wastewater the most common testate amoeba is from the Euglypha sp. (pictured below). This amoeba usually increases in number as soluble BOD falls below 40 mg/L and nitrificaiton is occurring. The testate amoebae population tends to trend upwards with longer sludge ages.
While usually seen with good water quality, the shell can remain visible after the amoebae is dead or inactive. Therefore, we should not rely on testate amoeba as a ideal indicator protozoa. For example, if you see testate amoeba as abundant but do not see activity of ciliated protozoa, there is a need to look further to see if any toxicity or other stress is occurring as the ciliates are much more sensitive to environmental changes than testate amoebae. Below is a photomicrograph of a Euglypha sp. testate amoeba at 400x phase contrast magnification.