''Phreatobius'', also known as cistern catfishes, is a

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 ...

of very small

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 ...

es (

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

Siluriformes) from tropical South America.

Taxonomy

For quite some time ''Phreatobius cisternarum'' was the only

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), ...

in the genus. However, two new species were described relatively recently in 2007, ''Phreatobius dracunculus'' and ''Phreatobius sanguijuela''. ''Phreatobius'' has been classified with a number of different families: Clariidae, Plotosidae, Trichomycteridae, Cetopsidae, and Pimelodidae. Most recently, it has been classified in Heptapteridae. Its

phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...

position remains uncertain. The

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

placement of the genus has remained problematic and thus it is not classified in any of the existing families, though

Fishbase FishBase is a global species database of fish species (specifically finfish). It is the largest and most extensively accessed online database on adult finfish on the web.

treats it as a member of Heptapteridae. An

undescribed species In taxonomy, an undescribed taxon is a taxon (for example, a species) that has been discovered, but not yet formally described and named. The various Nomenclature Codes specify the requirements for a new taxon to be validly described and named. U ...

of ''Phreatobius'' lives in waterlogged

leaf litter Plant litter (also leaf litter, tree litter, soil litter, litterfall, or duff) is dead plant material (such as leaves, bark, needles, twigs, and cladodes) that has fallen to the ground. This detritus or dead organic material and its constituen ...

near (not in) streams. ''Phreatobius walkeri'', a ''

nomen nudum In Taxonomy (biology), taxonomy, a ''nomen nudum'' ('naked name'; plural ''nomina nuda'') is a designation which looks exactly like a scientific name of an organism, and may have originally been intended to be one, but it has not been published ...

'', is provided by the

Guinness Book of World Records ''Guinness World Records'', known from its inception in 1955 until 1999 as ''The Guinness Book of Records'' and in previous United States editions as ''The Guinness Book of World Records'', is a British reference book published annually, listi ...

, to describe this fish that stays on land for extended periods of time. Two additional quite distinctive species of ''Phreatobius'', as yet undescribed, are known from the Río Negro basin of Brazil.

Species

There are currently three recognized species in this genus: * '' Phreatobius cisternarum'' Goeldi, 1905 * '' Phreatobius dracunculus'' Shibatta, Muriel-Cunha & de Pinna, 2007 * '' Phreatobius sanguijuela'' L. A. Fernández, Saucedo, Carvajal-Vallejos & S. A. Schaefer, 2007

Distribution and habitat

The three scientifically described ''Phreatobius'' species all live underground and are known only from artificial wells penetrating near-surface aquifers of the

Amazon basin The Amazon basin is the part of South America drained by the Amazon River and its tributary, tributaries. The Amazon drainage basin covers an area of about , or about 35.5 percent of the South American continent. It is located in the countries ...

. The genus has one of the widest distribution of any hypogean fish genus, with ''P. sanguijuela'' from the upper Amazon, some 2000 km from reported locations of ''P. cisternarum'' near the Amazon River mouth, and ''P. dracunculus'' also approximately 1900 km from the Rio Branco drainage area.

Description

This genus can be recognized by the combination of characteristics, such as their dorsal and anal fins being continuous with a rounded

caudal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported only ...

, unbranched

anal fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish swim. Apart from the tail or caudal fin, fish fins have no direct connection with the back bone and are supported o ...

rays, a projecting lower jaw, bright red coloration, a lack of the

dorsal fin A dorsal fin is a fin on the back of most marine and freshwater vertebrates. Dorsal fins have evolved independently several times through convergent evolution adapting to marine environments, so the fins are not all homologous. They are found ...

spine locking mechanism, the first

pectoral fin Fins are moving appendages protruding from the body of fish that interact with water to generate thrust and help the fish aquatic locomotion, swim. Apart from the tail or caudal fin, fish fins have no direct connection with the vertebral column ...

ray being soft instead of spinous. Most of these traits are rare or unusual among vast groups of catfish. All three species look extremely similar. However, in ''P. cisternarum'' the eyes are tiny, while ''P. dracunculus'' and ''P. sanguijuela'' lack eyes altogether. The two eyeless species can be differentiated by fin-ray and vertebrae counts. It is also described that ''P. dracunculus'' is lighter in color, while ''P. cisternarum'' and ''P. sanguijuela'' are described as bright red.

References

{{Taxonbar, from=Q2103561 Catfish of South America Catfish genera Taxa named by Émil Goeldi Heptapteridae