Pygidianops is a 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...

 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...

es (order

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 Siluriformes) of the family

Family (biology)

In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...

 Trichomycteridae

Trichomycteridae

Trichomycteridae is a family of catfishes commonly known as the pencil or parasitic catfishes. This family includes the infamous candiru fish, feared by some people for its alleged habit of entering into the urethra of humans....

. It includes three 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...

, P. cuao, P. eigenmanni, and P. magoi.

Distribution

P. eigenmanni is from the Rio Negro basin 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...

. P. cuao is known only from Cuao River drainage basin. P. magoi is known only from lower Orinoco

Orinoco

The Orinoco is one of the longest rivers in South America at . Its drainage basin, sometimes called the Orinoquia, covers , with 76.3% of it in Venezuela and the remainder in Colombia...

 mainstem between Ciudad Bolívar

Ciudad Bolívar

Ciudad Bolívar is the capital of Venezuela's southeastern Bolivar State. It was founded with the name Angostura in 1764, renamed in 1846, and, as of 2010, had an estimated population of 350,691....

 and Barrancas in Venezuela

Venezuela

Venezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south...

.

Description

Species of Pygidianops share the extreme reduction of pigmentation, loss of the dorsal fin

Dorsal fin

A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

, the loss or extreme reduction of pectoral fins, a reduced 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...

, and the reduction or complete loss of eyes in some species. These fish are markedly miniaturized, yet retain a relatively well-ossified skeleton comparable in both bone differentiation and degree of calcification to that observed in larger trichomycterids.

P. cuao is distinguished from congeners by the presence of diminutive eyes (vs. eyes absent in both other species), posterior naris absent (vs. nares bilaterally paired), and the presence of a triangular skin flap at mouth corner (vs. skin flap absent). P. magoi is distinguished from all congeners by the absence of pectoral and anal fins (vs. fins present), posterior naris absent (vs. present, nares bilaterally paired), four laterosensory pores on the head (vs. six), and by the presence of 9–10 caudal fin rays (vs. 12–13). With all of its fins lost except the caudal fin, P. magoi represents the most extreme fin loss among ostariophysans

Ostariophysi

Ostariophysi is the second-largest superorder of fish. Members of this superorder are called ostariophysians. This diverse group contains almost 8,000 species, about 28% of known fish species in the world and 68% of freshwater species, and are present on all major continents except Antarctica...

. P. eigenmanni lacks eyes but has its anal and pectoral fins.