Bryconops colaroja

Bryconops colaroja is one of the smaller members of the genus Bryconops, measuring around 7.8 cm (3.1 in) in standard length.[2] In general, the genus Bryconops has a similar body shape to sister genus Piabucus, described as resembling a minnow or smelt. [3] Its scales are mostly greenish-silver, slightly darker on the back than on the rest of the body. The uniform gradient of the scales, lacking prominent markings, is a feature that can be used to differentiate it from other members of the genus; for example, B. colanegra and B. melanurus have distinct blotches of pigmentation in the cheek and eye region.[4]

Bryconops colaroja is the only member of the genus Bryconops to have a caudal fin that is completely red, aside from congener Bryconops imitator (which gets its name from its similarity to B. colaroja).[5][6] The margins of the fin rays are the only places where melanophores are present, making dark lines radiate inwards from the fin margin.[4] This distinct color is the origin of the fish's scientific name; "cola" means "tail", and "roja" means "red".[7] It is sometimes called the redtailed bryconops because of this,[8] but this is not in wide use, and B. colaroja has no accepted common name.[9]

Though Bryconops colaroja has gill rakers that are notably denticulated - that is, adorned with toothlike protrusions - its congener B. colanegra is known to have gill rakers with higher denticulation.[4] The teeth are multicuspid, which is a feature that the Bryconops clade shares with sister clade Iguanodectinae.[10]

Bryconops colaroja was described in 1999 as a part of the subgenus Creatochanes, making its full name Bryconops (Creatochanes) colaroja.[4] It has no synonyms, nor is it often mistaken for its congeners, aside from B. imitator.[11] The red tail makes it fairly easy to identify.

Bryconops colaroja, as with all members of the genus Bryconops, was once considered a part of the family Characidae, and is still listed there by some sources.[12][13] However, research in 2011 by Oliveira et al. moved the genera Bryconops, Iguanodectes, and Piabucus to the family Iguanodectidae,[14] which is where B. colaroja currently stands.[2][11]

Bryconops colaroja is known only from the Cuyuní river basin, leaning on the border between Venezuela and Guyana;[2][15] however, it has not been collected from the Cuyuni river main, only smaller creeks and tributaries.[4] It demonstrates a preference for slightly acidic waters, as well as a rocky or sandy substrate.[1]

The blackwater nature of B. colaroja's habitat indicates that it does not need well-oxygenated waters to survive.[4] Part of what characterizes a blackwater river is a high concentration of humic substances, which are the result of microbial activity. This microbial activity consumes much of the oxygen in the water, and as such fish with high-oxygen needs are not suited for blackwater environments.[16]

Though little is known of B. colaroja's population trends, it is considered a species of least concern by the IUCN.[1] A potential threat is that the Cuyuni river main is regularly threatened by mining activity; the waters have been disturbed and riverbanks destroyed in the past, as it is a source of gold.[20] The Cuyuni river is generally in poor health, which places B. colaroja (as well as the other native species) at an increased risk of endangerment.[21] However, its presence has been noted by mining operations in the past, which in turn has increased their willingness to take precautions to protect it.[8]