Cairns Rainbowfish, Cairnsichthys rhombosomoides (Nichols & Raven 1928)

Other Names: Cairns Rainbow Fish, Northern Soft-spined Sunfish, Northern Soft-spined Sun-fish

Cairns Rainbowfish, Cairnsichthys rhombosomoides. Source: Brad Pusey. License: All rights reserved


A small yellowish-brown rainbowfish with with a narrow dark mid-lateral stripe, and a large silvery to yellowish spot on the gill cover. The more brightly coloured males are deeper bodied than females and have elongate dorsal and anal fins with yellowish-orange fin margins, and the caudal fin also has a yellow-orange margin. 

Cite this page as:
Gomon, M.F. & Bray, D.J. 2022, Cairnsichthys rhombosomoides in Fishes of Australia, accessed 01 Apr 2023,

Cairns Rainbowfish, Cairnsichthys rhombosomoides (Nichols & Raven 1928)

More Info


Endemic to the wet tropics in northern Queensland, from the Daintree - Cape Tribulation area, 120 km north of Cairns to the Innisfail region. 

Inhabits shallow swiftly flowing and heavily shaded granite boulder, sand or alluvium dominated streams at the base of mountain ranges or hills in high rainfall areas. The streams usually have shelter provided by tree roots, woody debris, leaf litter, undercut banks and aquatic vegetation.


Dorsal fin V-VII + I, 11-14; Anal fin 1,17-21; Pectoral fin 11-13
Body relatively elongate, laterally compressed; lateral line absent; greatest body depth (adults > 50 mm SL): males 25.7-30.1, females 21.6-28.0; head length 24.6-27.9 in SL; snout length 7.2-8.7 in SL; eye large, diameter 7.3-10.3 in SL; interorbital width 9.9-11.1 in SL; jaws equal; mouth terminal, initially horizontal becoming oblique caudally, large, reaching back to anterior region of eye; outside of mouth with several rows of small conical teeth.
Scales: vertical scale rows 36 to 38; predorsal scales 15 or 16; preopercle-suborbital scales 4 to 8.
Two separate dorsal fins, anal fin long-based; caudal fin emarginate to falcate.


To 8.5 cm SL, commonly to 6 cm. Males larger than females.


Usually yellowish-green with a relatively thin black mid-lateral stripe, pale below. Males more brightly coloured with a yellow margin on the dorsal anal and caudal fins.


Feeds on insects and insect larvae, microcrustaceans, small shrimps and algae.


Spawning occurs from April to December with peak spawning from August to October. Females are egg-scatters, depositing their eggs in batches amongst aquatic plants, root-masses and leaf litter. The eggs are small and numerous, and likely to be somewhat adhesive.

The larvae are small at hatching, with little yolk, well-developed eyes, a well-developed feeding apparatus and gut - they commence feeding shortly after hatching.


Rare in aquaria.


Populations of Cairns Rainbowfish are highly fragmented and threatened by extended drought and habitat disturbances such as pollution, land clearing and water extraction.

Similar Species

The similar Daintree Rainbowfish, Cairnsichthys bitaeniatus, differs in having a more robust black stripe across the upper operculum, a more intense black spot on the pectoral fin base, a pronounced yellow patch on the anteroventral body, and usually a more conspicuous second dark stripe on the lower body (vs. the second stripe absent or diffuse in C. rhombosomoides. Adult males also have yellow fins (vs. orange-reddish in C. rhombosomoides). The Daintree Rainbowfish also has smaller scales, more vertebrae and a shorter dorsal-fin base than the Cairns Rainbowfish.


The species is named rhombosomoides due to its resemblance to juvenile Rhomobosoma novaeguineae: 'It looks very like the young of Rhombosoma novaeguineae ..., but none of the fine rays are pungent.' 

Species Citation

Rhadinocentrus rhombosomoides Nichols & Raven, 1928, Am. Mus. Novit. 296: 1. Type locality: Babinda Creek drainage, Queensland.


Gomon, M.F. & Bray, D.J. 2022


Atlas of Living Australia

Cairns Rainbowfish, Cairnsichthys rhombosomoides (Nichols & Raven 1928)


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Allen, G.R. & Cross, N.J. 1982. Rainbowfishes of Australia and Papua-New Guinea. New Jersey : T.F.H. Publications 142 pp. figs.

Aarn & Ivantsoff, W. 1997. Descriptive anatomy of Cairnsichthys rhombosomoides and Iriatherina werneri (Teleostei: Atheriniformes), and a phylogenetic analysis of Melanotaeniidae. Ichthyological Exploration of Freshwaters 8(2): 107-150.

Ebner, B., Brown, C., Hammer, M., Unmack, P. & Martin, K. 2019. Cairnsichthys rhombosomoides. The IUCN Red List of Threatened Species 2019: e.T3461A123377657. Accessed on 22 February 2022.

Hammer, M.P., Allen, G.R., Martin, K.C., Adams M., Ebner, B.C., Raadik, T.A. & Unmack, P.J. 2018. Revision of the Australian Wet Tropics endemic rainbowfish genus Cairnsichthys (Atheriniformes: Melanotaeniidae), with description of a new species. Zootaxa 4413(2): 271-294

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Martin, K.C. & Barclay, S. 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 

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Pusey, B.J., Kennard, M.J. & Arthington, A.H. 2004. Freshwater Fishes of North-eastern Australia. Collingwood, Victoria : CSIRO Publishing 684 pp. 

Sparks, J.S. & Smith, L.W. 2004. Phylogeny and biogeography of the Malagasy and Australasian rainbowfishes (Teleostei: Melanotaenioidei): Gondwanan vicariance and evolution in freshwater. Molecular Biology and Evolution 33: 719–734.

Thuesen, P.A. 2004. Biological barriers to gene flow and population genetic structure of an endemic rainbow-fish, Cairnsichthys rhombosomoides [Honours Thesis]. Cairns: James Cook University. 89 pp.

Thuesen P.A., Pusey, P.J., Peck, D.R., Pearson, R. & Congdon, B.C.. 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.

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Quick Facts

CAAB Code:37245002

Conservation:IUCN Endangered

Fishing:Aquarium fish

Habitat:Clear freshwater streams

Max Size:8.5 cm SL; 10 cm TL


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