Iguanodectes geisleri

Iguanodectes geisleri is a small fish, reaching a maximum of 5.5 cm (2.2 in) standard length (SL).[1] It has a lateral stripe that is divided horizontally into three colors - red at the top, then yellow-green, then black - and the base scale color is yellowish.[2] The three colors of the lateral stripe can be used to differentiate it from similar congener Iguanodectes adujai, whose lateral stripe is only composed of red and yellow-green, lacking the black underline. [3]

Iguanodectes geisleri has several other identifying features. There are 20-25 anal-fin rays, fewer than other species, and the dorsal-fin origin is located in the middle of the body, which is unique to I. geisleri within Iguanodectes as a genus. The relatively short anal fin is also unique.[4][5]

Iguanodectes geisleri is a relatively recent addition to the genus Iguanodectes, named in 1970. The earliest member of the genus, Iguanodectes spilurus, was named in 1864 by Albert Günther, though under a different name at the time.[8] I. geisleri has retained its original name since description,[9] but has nonetheless changed taxonomic status; in 2011, researchers determined that the genus Iguanodectes, alongside Bryconops and Piabucus, belonged to a new family, Iguanodectidae, as opposed to their previous placement incertae sedis in Characidae.[10] Iguanodectes and Piabucus make up the subfamily Iguanodectinae.[11]

Iguanodectes geisleri is known from the basins of the Madeira, Negro, and Orinoco rivers; the former two are located in Brazil, and the latter in Venezuela.[1][4] The type locality is in Brazil, a small stream known as the Igarapé de Paricá, a tributary of the Rio Jufari.[15] I. geisleri is largely seen in blackwater habitats, a similarity it shares with other members of Iguanodectes. This is also seen in various members of related genus Bryconops.

The rio Negro specifically gets its name because it is a blackwater river, in which microbial decay releases tannins from leaf litter into the water. (This results in a low amount of available dissolved oxygen, as the microbes use a good deal of oxygen present.)[16] A large portion of rivers in the Orinoco basin are blackwater as well. The Madeira main is whitewater, but its tributaries are both clearwater and blackwater. Blackwater rivers are characterized by plentiful riparian vegetation, which serves as shelter for the river and a food source for the fauna therein.[4]

Many members of Iguanodectes are herbivorous.[17] I. geisleri is largely in line with this pattern, including leaf litter, fallen fruit, and filamentous algae in its diet,[4] but aquarists have reported omnivorous habits in captive specimens.[2][7] Aquarists also note that a captive I. geisleri does best if kept in a school ranging from 5 to 10 fish, which remains true of wild counterparts.[4] Wild fish are further known to peacefully coexist with species from related genera, including Hyphessobrycon melazonatus, Bryconops giacopinii, and Bryconops inpai, though a wider variety of species in one area does result in dietary shifts to maintain a specific ecological niche.[18]

Iguanodectes geisleri has not been evaluated by the IUCN. Its wide range offers it an advantage in terms of population survivability. Nonetheless, the freshwater inland habitats of South America are under constant pressure from anthropogenic sources, including infrastructure development like hydroelectric dams and chemical disruption from illegal mining operations. Specifically, wetlands like the ones I. geisleri prefers are known for their fragility.[19]