Ectrepopterus uruguayensis

Ectrepopterus uruguayensis reaches a maximum of 5.1 cm (2.0 in) SL (standard length, without the tail fin included). The body is elongated and horizontally compressed, and is deepest within the range of the dorsal-fin origin. The caudal peduncle is relatively long and usually straight, occasionally with slight concavity above and below. The eye is wider than the snout is long, and the lower jaw protrudes slightly past the upper. The lateral line is incomplete, with 5–9 pored scales (most often 6). There are 10–12 dorsal-fin rays (ii, 8–10 - meaning two unbranched rays and 8 to 10 branched rays), 11–13 pectoral-fin rays (i, 10–12), 7 or 8 pelvic-fin rays (i, 6 or 7), and 22–31 anal-fin rays (iii–v, 19–26). There are hooks on the pectoral-fin rays; while hooks on various fin-rays are common in various characid species, their presence on the pectoral fin in particular is unusual, and can be used as diagnostic criteria for E. uruguayensis when compared to similar species. The lower lobe of the caudal fin is sometimes longer than the upper, which Fowler cited amongst further diagnostic criteria; however, this is not uncommon in small characins, and is not present in all specimens of E. uruguayensis. As such, it should not be regarded as a distinguishing feature, though it is of occasional note during research. The base scale color for E. …

Ectrepopterus uruguayensis was originally described from Uruguay, though Fowler failed to provide specific details. For a period of roughly 20 years from 1990 to 2010, there were no records of the species anywhere else; before then, there were instances of occasional specimens from the portion of the Río Uruguay in Brazil. A 2013 report of specimens found in Argentina broadened this range somewhat, and specimens were rediscovered in the Pampas region of Brazil in May 2023, again expanding the known range. Still, the area of occurrence remains relatively restricted. Generally, E. uruguayensis displays a preference for streams, and occurs in shallow areas (less than 1 m depth) with a notable current, though it has been cited from semi-still waters and areas of up to 2 m depth. It is found in regions with aquatic and riparian vegetation both. While it has its preferences, it also demonstrates some adaptability to weather conditions that influence its habitat; for instance, heavy rainfall may result in turbid water with a muddy substrate, while a drought might result in transparent water and a depth as little as 30 cm (12 in). Ectrepopterus uruguayensis is a generally amicable fish that occurs in low-density populations, with an extensive list of syntopic species from various fish families. Syntopic species also in the family Characidae, to which E. …

The genus name originates in Greek. "Ectrepos" means "reversed", and "pterus" means "fin" or "wing", in reference to the caudal fin; the upper lobe is shorter than the lower. The suffix "-ensis" denotes a location, and E. uruguayensis was originally described from Uruguay. Ectrepopterus uruguayensis lacks a common name, but is referred to under the general "tetra" label in English and the "mojarra" label in Spanish.