Melanotaenia australis

Melanotaenia australis [Drysdale River] - photo© Günther Schmida

(Castelnau, 1875)
Western Rainbowfish

Species Summary
Melanotaenia australis was first described by Castelnau as Neoatherina australis in 1875. The original type specimens were collected from Weeli Wolli, Hammersley Range, and Millstream homestead in Western Australia. In earlier days they were commonly known as the "Westralian Sunfish". In 1964 another species of rainbowfish collected from the Northern Territory was named Melanotaenia solata. After Gerald Allen's review of the rainbowfish family in 1980, these two species were considered as one and he placed them in the large "Splendida" group as a sub-species, and named them Melanotaenia splendida australis, but recent DNA studies indicate it is clearly distinct.

Their colouration, particularly the fins and intensity of the mid-lateral stripe, is highly variable depending on their geographic locality. This is mainly due to a rapid speciation in the rainbowfish group and is further enhanced by the degree of random morphological variation occurring among the various populations. Colouration generally consists of 1-2 broad, dark mid-lateral stripes and a series of narrow reddish stripes corresponding with each scale row. Fins range from nearly colourless to deep red, or clear with red or green flecks. Gerry Allen notes that the real trademark of this species is the zigzagging blackish lines on the lower sides, just above the anal fins. The black mid-lateral line may be very prominent or scarcely apparent. Males are easily distinguished from females by their brighter colours and longer and more elongated dorsal fin rays. Fish from the Drysdale River of Western Australia are generally smaller in statue and possess a distinctive double mid-lateral black stripe. Melanotaenia australis can grow to a length of around 10 cm, but are more common at 8 cm or less. Males are usually much larger and deeper bodied than females.

Melanotaenia australis [Blackmore River] - photo© Gunther Schmida

Distribution & Habitat
Melanotaenia australis have a restricted disjunct distribution in northern Western Australia and the Northern Territory. They are widespread throughout the Pilbara region of Western Australia between the Ashburton and DeGrey Rivers and in the Kimberley region in the extreme northern part of Western Australia between the Fitzroy River and the Northern Territory border. They also occur in streams of the north-western sector in the Timor Sea drainage of the Northern Territory, just east of Darwin.

Melanotaenia australis is a stream dwelling rainbowfish mainly found around sub-surface vegetation, submerged logs, or branches in small tributary streams, but can also occur in swamps and lagoons. They are most commonly found in backwaters or along the shoreline where there is minimal flow and grassy vegetation. Their natural environment is subjected to seasonal variations with water temperature (22~28°C), pH (6.5~8.0), and hardness levels varying considerably. There is often a large fluctuation in water conditions between the dry and wet seasons.

Melanotaenia australis [Carrawine Gorge] - photo© Neil Armstrong

Biology
Very little is known about the biology of this species in their natural environment. Most information is mainly based on aquarium observations. Spawning occurs from October to December, with females producing between 100 and 200 eggs. Spawned eggs are adhesive, negatively buoyant in freshwater and average 1.05~1.07 mm in diameter, are usually clear to light amber in colour and hang by a fine thread. Hatching occurs around 6 to 10 days depending on temperature. At 32 days after hatching, the length of the larval is about 10~11 mm and at 72 days 21~22 mm. Growth is fairly rapid reaching 2~3 cm in 10 to 12 weeks. Growth rates vary greatly with changes in temperature, feeding rate and densities.

Remarks
Melanotaenia australis show considerable variation in morphology across its known distribution. Genetic studies in 2000 found that Melanotaenia australis is represented by two distinct lineages. These lineages are geographically restricted to Western Australia and the Northern Territory. The difference between the two groups could indicate the presence of two distinct species. More recent studies (2004) have shown that there is also a very large genetic difference between the Pilbara and east Kimberley populations of Melanotaenia australis. These studies suggest that there maybe more than one species in the Melanotaenia australis complex. However, despite the research that has been undertaken to date, the specific status and distribution of Melanotaenia australis still remains unclear.

Literature
Allen, G.R. (1978). The rainbow fishes of northwestern Australia (Family Melanotaeniidae). Tropical Fish Hobbyist 26: 91-102.

Ivantsoff, W., L.E.L.M. Crowley, E. Howe and G. Semple (1988). Biology and early development of eight fish species from the Alligator Rivers Region. Technical memorandum 22, Supervising Scientist for the Alligator Rivers Region, AGPS, Canberra.

McGuigan, K., Zhu, D., Allen, G.R. and Moritz, C. (2000). Phylogenetic relationships and historical biogeography of melanotaeniid fishes in Australia and New Guinea. Marine and Freshwater Research 52: 713-23.

Phillips, R. (2004). Population Structure of the Western Rainbowfish, Melanotaenia australis in the East Kimberley. Submitted in partial fulfilment of the Bachelor of Science (Honours) Degree School of Animal Biology The University of Western Australia.

Zhu, D. Jamieson, B.G.M., Hugall, A. and Moritz, C. (1994). Sequence evolution and phylogenetic signal in control-region cytochrome b sequences of rainbowfishes (Melanotaeniidae). Molecular Biology and Evolution 11: 672-683.