Phreatobius cisternarum is a species of catfish in the genus Phreatobius. This Brazilian fish is one of the few fish species that live underground in phreatic habitats. It has proved problematic in its classification, which remains uncertain.
Distribution and habitat 
P. cisternarum is found in Brazil. They inhabit underground habitats both north and south of the mouth of the Amazon River as well as the Marajó Island. This fish inhabits superficial, phreatic habitats, but does not live in the deeper artesian aquifers.
These fish are found commonly in hand-dug wells on Marajó. These wells are 4-13 metres (13-43 ft) deep. In these wells, these fish are more common during the dry season when the water depth recedes to about 30 centimetres (12 in), and are rarer during the rainy season when the depth can increase by several metres. The water is warm and acidic (pH 5-6).
Physical characteristics 
The head of this species is distinctly wider than the body. The integument is thick and opaque. The lateral line is reduced. The pelvic fin is highly variable in length between specimens, and can be reduced to finger-like projections. The red coloration is generally uniform over the body and head and is due to superficial blood; this red coloration does not extend to the fins. The fish is darker on the dorsal surface due to some pigmentation in the skin. The barbels are whitish and fade to transparent tips.
P. cisternarum hide and fit tightly within crevices of rocks, where they stay most of the time, leaving only for food. These fish are not a burrowing species, nor do they hide under vegetative matter, restricting themselves to these crevices. They do not interact much with each other and do not seem to be gregarious. Nothing is known about the reproductive habits of this species.
P. cisternarum feed on macro-invertebrates such as worms. They either swallow their prey whole, or take a bite out of it, rolling their body, and twisting a piece off. This firm grip is due to extremely large jaw muscles, which are well adapted to feeding on relatively large prey; worms eaten in captivity can be as long as the fish itself. It is unknown what invertebrates P. cisternarum actually feeds on in its natural habitat. Becauses food is rare in their environment, these fish are able to last up to a year without feeding, which is partially because of their inactivity.
These fish indicate they have some form of respiration that allows them to breathe air, however this behavior is only exhibited under stressful conditions. 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).
- World Conservation Monitoring Centre (1996). Phreatobius cisternarum. 2006. IUCN Red List of Threatened Species. IUCN 2006. www.iucnredlist.org. Retrieved on 08 Apr 2007.
- Muriel-Cunha, Janice; de Pinna, Mário (2005). "New data on Cistern Catfish, Phreatobius cisternarum, from subterranean waters at the mouth of the Amazon River (Siluriformes, Incertae Sedis)" (PDF). Papéis Avulsos de Zoologia (Museu de Zoologia da Universidade de São Paulo) 35: 327. ISSN 1807-0205.
- Fernandez, Luis (2005-05). "Risk of Extinction of a Rare Catfish of Andean Groundwater and its Priority for Conservation" (PDF). AMBIO: A Journal of the Human Environment 34 (3): 269–270. doi:10.1639/0044-7447(2005)034[0269:ROEOAR]2.0.CO;2. ISSN 0044-7447. Retrieved 2007-04-09.
- Muriel-Cunha et al. (2005): page 328
- Muriel-Cunha et al. (2005): page 329
- Muriel-Cunha et al. (2005): page 334-336
- Froese, Rainer and Pauly, Daniel, eds. (2011). "Phreatobius cisternarum" in FishBase. December 2011 version.
- Muriel-Cunha et al. (2005): page 330-333
- Muriel-Cunha et al. (2005): page 337-338