Iriatherina werneri - photo© Hristo Hristov

Iriatherina werneri

Species Summary
In 1973, two visiting German aquarists collected a number of small freshwater fishes in a rice paddy field on the outskirts of the town of Merauke in West Papua by. 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.

The Threadfin Rainbowfish may grow to a maximum size of 5 cm, but are more commonly seen at around 3~4 cm. 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. Females 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. A yellow finned variety has been collected from a tributary of the Embley River in northern Queensland. Although it has been reported that they lose this yellow colouration after some time being maintained in the aquarium. The colour may well have something to do with environmental conditions rather than being a distinct colour form.

Iriatherina werneri [Myall Creek] - photo© Neil Armstrong

Distribution & Habitat
In New Guinea, Threadfin Rainbowfishes have been collected from several coastal rivers between the Merauke and the Fly Rivers. In the Fly River they have been collected in swampy lagoons along the mainstream of the river more than 500 km upstream from its mouth. In Australia, the Jardine River swamps were believed to be the only place where they could be found. However, in 1985 they were found in the Edward River catchment on the western side of Cape York Peninsula. More recent surveys have found that they have a much wider distribution in a number of rivers both on the western and eastern sides of Cape York Peninsula. They have also been collected from the Arafura Swamps and a number of streams in the Northern Territory. I suspect that they will eventually be found in all the river systems around the Gulf of Carpentaria.

Threadfin Rainbowfishes are generally found in clear, slow flowing streams, grassy wetland swamps and lagoons that have abundant vegetation. They are most commonly found along the heavily vegetated margins of lagoons and small streams at depths of 0.5 to 1.25 metres, and usually in open water not far from clumps of vegetation. Small schools of females and juveniles can be seen moving slowly through the water while the more mature males display around them. A temperature range of 22~30°C and pH values of 5.2~7.5 have been recorded in their natural habitats.

Iriatherina werneri habitat - Lagoon [Wenlock River] - photo© Graeme Finsen

Keeping & Caring
Very little is known about the natural life history and ecology of the Threadfin Rainbowfish in the wild. Most information is mainly based on aquarium observations. They have been a popular aquarium fish with Australian native fish enthusiasts for many years and are easy to maintain under standard aquarium conditions. Although they are a fish that requires a little more attention than most other rainbowfishes. They are just one of the many beautiful rainbowfish species of the family Melanotaeniidae that are available to aquarium hobbyists. They should not be kept in aquaria less than 60 cm (55 litres) and are best kept in groups of 5 or more individuals. They can be maintained and bred in water conditions that are suitable for most aquarium species. However, best results will be achieved if maintained at a temperature range of 23~28°C; pH 6.0~7.0; and water hardness of 10~70 ppm (mg/L).

The Threadfin Rainbowfish is one of the most peaceful of all aquarium fishes, almost to the point of exclusion as a potential "community" tank fish. Their timidity, gentle manner, small size and dainty feeding habits put them at a disadvantage in an aquarium with most other types of tropical fishes. This is probably why they are not readily available in aquarium stores and are mostly maintained by rainbowfish enthusiasts. Nevertheless, they can be kept in a mixed community aquarium containing other small native species such as Pseudomugil gertrudae. A breeding tank can be easily set up and the fish induced to spawn throughout the year. Although some aquarists have found this species has been a difficult challenge to breed and successfully raise the newly hatched larvae.

In their natural environment they feed mainly on large quantities of unicellular, planktonic algae and diatoms. However, in captivity they seem to thrive on live brine shrimp nauplii, daphnia, copepods, mosquito larvae, and worms such as grindal and microworm. The challenge of attacking larger live foods is usually declined. Floating dry foods, such as small bite-sized pellets or flake foods are acceptable as their dorsally projected jaws are designed for surface feeding.

Iriatherina werneri - photo© Hristo Hristov

In their natural environment spawning usually occurs during the warmer period of the year (Spring-Summer) when water temperatures are around 24~32°C. Spawning in captivity can be attempted in a number of ways. They can be placed in a specially set up breeding aquarium with a one male; two females combination. A bunch of aquatic moss or spawning mop is placed in the tank, on which the fish will readily spawn. The spawning medium, with attached eggs, can then be removed each day, and place in a special hatching container. Alternatively, in a permanent aquarium environment purposely set up for breeding the Threadfin Rainbowfish, a self-sustaining population can be maintained. In fact, this is probably the most reliable method of breeding for the general hobbyist. Such an aquarium need not be larger than 50 litres, although a 90x45x45 cm aquarium tastefully decorated with river stones, driftwood overgrown with aquatic mosses and some floating duckweed or water sprite can be very attractive. An aquarium designed accordingly and containing a school of displaying Threadfin Rainbowfish, is a sight not easily forgotten.

Literature
Allen, G.R. & D.F. Hoese (1980). A Collection of Fishes from the Jardine River, Cape York Peninsula, Australia. J. Roy. Soc. West. Aust. 63(2): 53 61.

Hansen, B. (1987). Werneri from the wild. Fishes of Sahul 4(2): 165-168.

Meinken H. (1974). Mitteilung der Fischbestimmungsstelle des VDA, Nr. 77. Aus Neu-Guinea kommt eine neue Gattung und Art der Ährenfische (Pisces, Atherinidae). Aquarium Aqua Terra v. 8 (no. 55). 9-11.

© Copyright Adrian R. Tappin
Updated December, 2008.