''Phreatobius cisternarum'' is a

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

catfish Catfish (or catfishes; order (biology), order Siluriformes or Nematognathi) are a diverse group of ray-finned fish. Catfish are common name, named for their prominent barbel (anatomy), barbels, which resemble a cat's whiskers, though not ...

in the

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

'' Phreatobius''. This

Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

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 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 M ...

Island. This fish inhabits superficial,

habitats, but does not live in the deeper

artesian aquifer An artesian well is a well that brings groundwater to the surface without pumping because it is under pressure within a body of rock or sediment known as an aquifer. When trapped water in an aquifer is surrounded by layers of Permeability (ea ...

s. These fish are found commonly in hand-dug

well A well is an excavation or structure created on the earth by digging, driving, or drilling to access liquid resources, usually water. The oldest and most common kind of well is a water well, to access groundwater in underground aquifers. The ...

s 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 In biology, an integument is the tissue surrounding an organism's body or an organ within, such as skin, a husk, Exoskeleton, shell, germ or Peel (fruit), rind. Etymology The term is derived from ''integumentum'', which is Latin for "a coverin ...

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 epithelia ...

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 (belly) surface of fish, and are the lower of the only two sets of paired fins (the other being the laterally positioned pectoral fins). The pelvic fins are homologous to the hi ...

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 Superficial may refer to: *Superficial anatomy, is the study of the external features of the body *Superficiality, the discourses in philosophy regarding social relation * Superficial charm, the tendency to be smooth, engaging, charming, slick and ...

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 describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...

due to some pigmentation in the skin. The

barbel Barbel may refer to: *Barbel (anatomy), a whisker-like organ near the mouth found in some fish (notably catfish, loaches and cyprinids) and turtles *Barbel (fish), a common name for certain species of fish **''Barbus barbus'', a species of cyprini ...

s 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 Respiration may refer to: Biology * Cellular respiration, the process in which nutrients are converted into useful energy in a cell ** Anaerobic respiration, cellular respiration without oxygen ** Maintenance respiration, the amount of cellul ...

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 Cutaneous respiration, or cutaneous gas exchange (sometimes called skin breathing), is a form of respiration in which gas exchange occurs across the skin or outer integument of an organism rather than gills or lungs. Cutaneous respiration may be ...

, as they are small, live in conditions with low

dissolved oxygen Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It can ...

, 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 Brazil Fish described in 1905 Heptapteridae