Phreatobius cisternarum
Encyclopedia
Phreatobius cisternarum is a species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of catfish
Catfish
Catfishes 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 heaviest and longest, the Mekong giant catfish from Southeast Asia and the second longest, the wels catfish of Eurasia, to detritivores...

 in the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...

 Phreatobius. This Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

ian fish is one of the few fish species that live underground in phreatic
Phreatic
The term phreatic is used in Earth sciences to refer to matters relating to ground water below the water table . The term 'phreatic surface' indicates the location where the pore water pressure is under atmospheric conditions...

 habitats. It has proved problematic in its classification, which remains uncertain.

Taxonomy

P. cisternarum was collected in 1903 and first described by E. A. Goeldi
Émil Goeldi
Émil August Goeldi , was a Swiss-Brazilian naturalist and zoologist....

 in 1905. This fish species and genus were redescribed in 2005.

Distribution and habitat

P. cisternarum is found in Brazil
Brazil
Brazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...

. They inhabit underground habitats both north and south of the mouth of the Amazon River
Amazon River
The Amazon of South America is the second longest river in the world and by far the largest by waterflow with an average discharge greater than the next seven largest rivers combined...

 as well as the Marajó
Marajó
Marajó is an island located at the mouth of the Amazon River in Brazil. It is part of the state of Pará.- Geography :With a land area of 40,100 km² , which compares to the size of Switzerland, it is the largest island to be completely surrounded by freshwater in the world...

 Island. This fish inhabits superficial, phreatic
Phreatic
The term phreatic is used in Earth sciences to refer to matters relating to ground water below the water table . The term 'phreatic surface' indicates the location where the pore water pressure is under atmospheric conditions...

 habitats, but does not live in the deeper artesian aquifer
Artesian aquifer
An artesian aquifer is a confined aquifer containing groundwater under positive pressure. This causes the water level in a well to rise to a point where hydrostatic equilibrium has been reached. This type of well is called an artesian well...

s.

These fish are found commonly in hand-dug well
Water well
A water well is an excavation or structure created in the ground by digging, driving, boring or drilling to access groundwater in underground aquifers. The well water is drawn by an electric submersible pump, a trash pump, a vertical turbine pump, a handpump or a mechanical pump...

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
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

 5-6).

Physical characteristics

This small fish reaches a maximum length of 5.5 centimetres (2.2 in). 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
Lateral line
The lateral line is a sense organ in aquatic organisms , used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...

 is reduced. The pelvic fin
Fish anatomy
Fish anatomy is primarily governed by the physical characteristics of water, which is much denser than air, holds a relatively small amount of dissolved oxygen, and absorbs more light than air does.- Body :...

 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
Superficial may refer to:*Superficial , an album by Heidi Montag*"Superficial" *The Superficial, a website devoted to celebrity gossip...

 blood; this red coloration does not extend to the fins. The fish is darker on the dorsal surface
Dorsum
Dorsum is a Latin word. In science, it could mean:* Dorsum , the posterior side of an animal* Dorsum , a term used in astrogeology for a ridge* Theta Capricorni, a star on the back of the Goat...

 due to some pigmentation in the skin. The barbel
Barbel (anatomy)
A barbel on a fish is a slender, whiskerlike tactile organ near the mouth. Fish that have barbels include the catfish, the carp, the goatfish, sturgeon, the zebrafish and some species of shark...

s are whitish and fade to transparent tips.

Ecology

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
Aquatic respiration
Aquatic respiration is the process whereby an aquatic animal obtains oxygen from water.-Fish:In most fish respiration takes place through gills. Lungfish, however, possess one or two lungs...

 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).
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