Iriatherina werneri, are commonly known as the 'Threadfin' or 'Featherfin' Rainbowfish. It is a rainbowfish, the only species in the genus Iriatherina. It is characterized by long beautiful fins, and is among the most attractive of the rainbowfishes.It is native to freshwater swamps and demersal or thickly vegetated areas of flowing waters, in tropical northern Australia and New Guinea.
It grows up to 5 cm (2.0 in) in length, but this does not include the Threadfin's long tail. Sexing is easy by examining fins: males have larger, gaudier finnage than females. Males also have more intense colours.
In 1973, two visiting German aquarists collected some small freshwater fishes in a rice paddy field on the outskirts of the town of Merauke in New Guinea. They were transported back to Europe and a number of them were given to Herman Meinken, a well known aquarist and ichthyologist, who realised that they were an undescribed species. In 1974, Meinken published the scientific description of the fish in the German aquarium magazine Das Aquarium (Aqua Terra) and they were named Iriatherina werneri after one of the collectors, Arthur Werner. They are commonly known as the 'Threadfin' or 'Featherfin' Rainbowfish. In Australia the 'Standard Names of Australian Fishes' published by the CSIRO lists them as 'Threadfin Rainbowfish'.
Mature males have a first dorsal fin that is fan shaped, while the second dorsal fin has exceptionally long filaments. The anal fin is similarly extended. This elegant finnage is used in a remarkably vivacious display for females and rival males. The body is slender, laterally compressed and general metallic silver with slightly visible dark vertical bars. The colours of the dorsal, anal and pelvic fins are black with a reddish-tan wash. The tail fin is deeply forked, transparent, and edged in a rustic red colouration. There are also small differences in fin shape and colouration of male specimens from different locations. Some males have a narrow and high sail-like first dorsal fin, while this fin is lower and broader in other males. Specimens from the Cadell River in the Northern Territory often have a more lyre-tail shaped tail fin, and the fins may be darker and longer. Females however, pale in comparison to the males, although their tail fins are adorned with a pinkish margin and the edges of the pelvic and anal fins are edged with black. The colouration of the fish in their natural habitat is usually much more intense than specimens maintained in the confines of an aquarium. Specimens found in New Guinea are usually darker than the Australian variety.