They inhabit underground habitats both north and south of the mouth of the Amazon River as well as the Marajó Island.

[8] The pelvic fin is highly variable in length between specimens, and can be reduced to finger-like projections.

[8] Phreatobius cisternarum hide and fit tightly within crevices of rocks, where they stay most of the time, leaving only for food.

Because food is rare in their environment, these fish are able to survive long periods of starvation, which is partially because of their inactivity.

[9] These fish indicate they have some form of respiration that allows them to breathe air, however this behavior is only exhibited under stressful conditions.

[6] It is supposed that these fish also have a form of cutaneous respiration, as they are small, live in conditions with low dissolved oxygen, and have high blood circulation to their skin (which provides its red coloration).