They are found most often in freshwater, floating in the open water amongst reeds and filamentous algae. Actinophrys is notable for having more than one nucleus. In general, heliozoans are spherical and are frequently enveloped by a shell, or test, made of silica or organic material.
Organisms secrete the shell in the form of scales or pieces in a gelatinous covering.
The shells exhibit a wide variety of shapes, which can be used in species identification. Heliozoans reproduce asexually by binary fission or by budding, but several genera have flagellated forms that may possibly be sex gametes.