Astyanax aurocaudatus

Carlastyanax aurocaudatus usually reaches a size of 6.0 cm (2.3 in) standard length (SL). Larger specimens can reach 7.4 cm (2.9 in) SL. The dorsal fin has 8 rays, the pelvic fin 5–6, and the anal fin 26–28. The head is large, with a blunt snout, and the body is generally robust. The base scale color is white ventrally, with a green-to-yellow predorsal region and a lateral stripe in silver or silver-blue. The humeral spot is usually blue, and is elongated vertically, as well as somewhat obscure. The pectoral and pelvic fins are pink or red with a dark margin. The other fins are different colors depending on whether the individual specimen is female or male. One species, Bryconamericus pectinatus, bears visual similarities to C. aurocaudatus, and was considered as a candidate for the genus upon its nomination; however, it remains in Bryconamericus.

Sexual dimorphism. Male and female specimens of C. aurocaudatus bear drastic differences to each other when it comes to coloration. In males, the anal and caudal fins are reddish, as is the caudal peduncle. The dorsal fin, with a dark margin, is white or yellow at the base. In females, the anal fin and the entire caudal region are yellow or yellow-gray as opposed to red. The dorsal fin still has a dark margin, but the base is often a dark yellow instead of pale. …

Carlastyanax aurocaudatus has a restricted distribution, endemic solely to the upper Cauca River basin of Colombia, where it is found in various creeks and tributaries. It was originally cited from the Boquia area, in the department of Quindío. Carlastyanax aurocaudatus has been described in areas with organic substrate. Further, it prefers clear waters, and a substrate of combined organic materials and stone is not uncommon in places it inhabits. It tends to linger close to the riverbed. Waters with a pH of 5.8–7.5 and a temperature range of 22 °C–25 °C appear to be its ideal habitat.

More than half of the diet of C. aurocaudatus consists of insects, with aquatic larvae being the most prominent. Benthic insects are also common. The other portion consists of plant material, the vast majority of it allochthonous, and this proclivity for adaptability means that it does not compete with similar species that live within the same range; for instance, though other fishes may also target invertebrates, C. aurocaudatus does not consume enough for it to be of competitive concern, given its taste for food items like seeds and fruits. Regardless of food availability, spawning is estimated to occur in the wet seasons (April to May, September to October) and dry seasons (January to February). Coloration of the anal and caudal fins becomes more vibrant during these stretches of time. Carlastyanax aurocaudatus has been observed living syntopically with Cetopsorhandia boquillae, Trichomycterus caliense, and Poecilia caucae.

Carlastyanax aurocaudatus is sometimes known as the gold-tailed tetra. In fact, its specific epithet means "gold tail"; "auro-" means "gold", and "caudatus" means "tail". The genus name "Carlastyanax" is to honor Carl H. Eigenmann and the original genus to which C. audocaudatus once belonged, Astyanax.