''Phreatobius cisternarum'' is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

catfish Catfish (or catfishes; order Siluriformes or Nematognathi) are a diverse group of ray-finned fish. Named for their prominent barbels, which resemble a cat's whiskers, catfish range in size and behavior from the three largest species alive, ...

in the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

Phreatobius ''Phreatobius'' is a genus of very small catfishes ( order Siluriformes) from tropical South America. Taxonomy For quite some time ''Phreatobius cisternarum'' was the only species in the genus. However, two new species were described relatively ...

''. This

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

ian fish is one of the few fish species that live underground in

phreatic ''Phreatic'' is a term used in hydrology to refer to aquifers, in speleology to refer to cave passages, and in volcanology to refer to a type of volcanic eruption. Hydrology The term phreatic (the word originates from the Greek , meaning "well" ...

habitats. It has proved problematic in its classification.Muriel-Cunha et al. (2005): page 328

Taxonomy

''Phreatobius cisternarum'' was collected in 1903 and first described by E. A. Goeldi in 1905.Muriel-Cunha et al. (2005): page 329 This fish species and genus were redescribed in 2005.

Distribution and habitat

''Phreatobius cisternarum'' is found in

. They inhabit underground habitats both north and south of the mouth of the

Amazon River The Amazon River (, ; es, Río Amazonas, pt, Rio Amazonas) in South America is the largest river by discharge volume of water in the world, and the disputed longest river system in the world in comparison to the Nile. The headwaters of t ...

as well as the

Marajó Marajó () is a large coastal island in the state of Pará, Brazil. It is the main and largest of the islands in the Marajó Archipelago. Marajó Island is separated from the mainland by Marajó Bay, Pará River, smaller rivers (especially ...

Island. This fish inhabits superficial,

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

This small fish reaches a maximum length of SL. There is great variation between members of this species. The head of this species is distinctly wider than the body. The integument is thick and opaque. The

lateral line The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...

is reduced.Muriel-Cunha et al. (2005): page 330-333 The

pelvic fin Pelvic fins or ventral fins are paired fins located on the ventral surface of fish. The paired pelvic fins are homologous to the hindlimbs of tetrapods. Structure and function Structure In actinopterygians, the pelvic fin consists of two en ...

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 Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

due to some pigmentation in the skin. The barbels are whitish and fade to transparent tips.

Ecology

''Phreatobius cisternarum'' hide and fit tightly within crevices of rocks, where they stay most of the time, leaving only for food.Muriel-Cunha et al. (2005): page 334-336 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. ''Phreatobius 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. Because food is rare in their environment, these fish are able to survive long periods of starvation, which is partially because of their inactivity. A captive individual did not feed for a year, but remained in good condition. 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).Muriel-Cunha et al. (2005): page 337-338

References

{{Taxonbar, from=Q2670148 Cave fish Catfish of South America Fish of Brazil Fish described in 1905 Heptapteridae