Cairnsichthys rhombosomoides - photo© Gunther Schmida

Cairnsichthys rhombosomoides

(Nichols & Raven, 1928)
Cairns Rainbowfish

Species Summary
Cairnsichthys rhombosomoides were originally collected by Henry Raven in October, 1921 from Babinda Creek, a tributary of the Russell River in north Queensland. They were scientifically described in the American Museum Novitates Nr. 296, in 1928 by John T. Nichols and Henry C. Raven and placed in the genus Rhadinocentrus. Gerald Allen's revision of the family Melanotaeniidae in 1980 placed them in the monotypic genus Cairnsichthys in recognition of its distinctiveness. Body colouration is yellow-brown on the dorsal half of body and silvery-white below with a continuous thin dark mid-lateral band. A large silvery or yellowish spot can be seen on the operculum. Males have a yellow-orange coloured outer margin on the dorsal and anal fins. The caudal fin has a similarly coloured fan. They may reach a maximum body size of 10 cm, but are usually around 7 cm. Males can be distinguished from females by their elongated dorsal and anal fins and brighter colours - females have shorter and more rounded fins. Males are also larger and deeper bodied than females.

Cairnsichthys rhombosomoides is believed to be a very old isolated species, whose current distribution is the remnant of a previously larger distribution range that has been reduced due to climatic changes. It is possible that they were the original rainbowfishes in rivers of northern Queensland.

Distribution & Habitat
Cairnsichthys rhombosomoides is restricted to a few coastal streams in northern Queensland from Cape Tribulation to Innisfail. They have been collected from Cooper Creek and Hutchinson Creek in Cape Tribulation, and the Daintree, Hull, North and South Johnstone, Moresby, Mulgrave, Russell and Tully river systems, plus Liverpool and Maria Creeks. They also occur in some small isolated coastal streams around the Innisfail region and may also occur in freshwater streams draining into Trinity Inlet. They have a very limited distribution range and as such, have a "restricted" conservation status listing which means "a species which is not presently in danger but which occurs in restricted areas and/or are uncommon".

Cairnsichthys rhombosomoides are generally found in small schools swimming above or among aquatic plants, woody debris and leaf litter. They can be found in both lowland and headwater tributary streams in water depth ranging from a few centimetres to about three metres. They are however, more commonly found in water between 30-50 cm deep. Upland streams generally have higher water flow over a substrate composed mainly of large rocks and bedrock with sand and fine gravel with good riparian cover (remnant rainforest) and minimal aquatic plants. Small lowland streams usually have mud, sand and fine gravel substrates with abundant leaf litter. They are often found in company with Melanotaenia splendida, Melanotaenia maccullochi and Melanotaenia utcheensis.

Very little is known about the natural life history and ecology of Cairnsichthys rhombosomoides. Water conditions recorded in their natural habitat are: temperature 15-29°C; pH 4.5-8.5 (lowland 4.5-6.8, upland 7.5-8.5); conductivity 0-91 mS/cm. Their diet is not well documented but appears to be dominated by small terrestrial insects and spiders. Spawning has been observed from April-December with a peak during August-October. Like all members of the rainbowfish family, Cairnsichthys rhombosomoides are egg-scatterers and generally spawn amongst aquatic plants and leaf litter.

Cairnsichthys rhombosomoides habitat (Russell River)

In captivity, I have on a regular basis observed this species spawning in the gravel substrate. Sexual maturity occurs at about 3-4 cm for both sexes. Fish commence spawning in their first year and mature females produce between 40 and 200 eggs. The number of eggs shed by a single female is directly related to the size of the female. Eggs adhere to water plants or artificial spawning medium and hatch after 5-9 days depending on temperature. Egg size is around 1.139 ± 0.021 mm with larvae hatching at about 3.46-5.46 mm. Larval development is complete at around 14-15 mm body length. I found this species seems to ignore free-swimming larvae in their aquarium. Although the larvae always stay close to some form of cover and generally avoid open areas of the aquarium.

Although an attractive species if kept under suitable conditions, Cairnsichthys rhombosomoides are rarely seen in the aquarium hobby and are mainly kept by a few aquarists who are principally interested in Australian native fishes.

Burrows D.W. (1998) FNQ 2010 Regional Environment Strategy - Key Waterways Report. ACTFR, Townsville, Qld.

Martin K.C. and S. Barclay (2013) New Distribution Records for the Cairns Rainbowfish Cairnsichthys rhombosomoides (Melanotaeniidae): implications for conservation of a restricted northern population. aqua, International Journal of Ichthyology, 19(3): 155-164.

Nichols J.T. and H.C. Raven (1928) A New Melanotaeniin fish from Queensland. American Museum Novitates Nr. 296, American Museum of Natural History.

Pusey B.J., M.J. Kennard and A.H. Arthington (2004) Freshwater Fishes of North-Eastern Australia. CSIRO Publishing, Victoria.

Thuesen P.A., B.J. Pusey, D.R. Peck, R. Pearson and B.C. Congdon (2008) Genetic differentiation over small spatial scales in the absence of physical barriers in an Australian rainforest stream fish. Journal of Fish Biology 72(5): 1174-1187.

Updated November, 2013

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Cairnsichthys rhombosomoides