Taxonomy & naming
Cairnsichthys rhombosomoides was originally described as Rhadinocentrus rhombosomoides by the American ichthyologists John Treadwell Nichols and Henry Cushier Raven in 1928, in the American Museum Novitates (296:1). It was later transferred to its own genus, Cairnsichthys, of which it was for decades the sole species, so the original authority is cited in parentheses. The type locality lies in the Babinda Creek drainage of northern Queensland, Australia. Catalog of Fishes (Eschmeyer, CAS) governs the valid name as currently combined.
The genus name Cairnsichthys joins the north Queensland town of Cairns with the Greek ichthys, 'fish' — a geographic patronym for the region the species inhabits. The specific epithet rhombosomoides reflects a perceived resemblance of the fish to a juvenile Rhombosoma novaeguineae, a rainbowfish then placed in that genus. The species sits within the family Melanotaeniidae, the rainbowfishes and blue-eyes of the order Atheriniformes, which radiates across New Guinea and northern and eastern Australia.
For most of its taxonomic history Cairnsichthys was monotypic. That changed in 2018, when Hammer and colleagues revised the genus in Zootaxa (4413) and described a second, sister species — Cairnsichthys bitaeniatus, the Daintree rainbowfish — distinguishing the long-recognised C. rhombosomoides from its northern relative.
Morphology
The Cairns rainbowfish is a small, comparatively slender rainbowfish, reaching about 3.5 in standard length (roughly 4 in total length) in large males, though it is commonly seen at around 2.5 in. As with rainbowfishes generally it carries the family's divided dorsal fin — a short spinous first dorsal followed by a longer soft-rayed second dorsal. Reported fin counts are a first dorsal of V–VII spines, a second dorsal of one spine and 11–14 rays, an anal fin of one spine and 17–21 rays, and pectoral fins of 11–13 rays. The lateral line is absent, and there are about 36–38 vertical scale rows.
The colour is restrained, not gaudy. The body is yellowish-green, paler beneath, with a thin dark stripe running the length of the flank and a silvery-to-yellowish spot on the gill cover. That muted patterning, against the saturated colours of many Melanotaenia, is part of why it so seldom turns up in the trade.
The sexes differ in both size and finnage. Males grow larger and become noticeably deeper-bodied than females, and they develop yellow-orange margins on the dorsal, anal, and caudal fins. Females remain plainer and more slender — a pattern of sexual dimorphism familiar across the rainbowfishes.
Habitat
Cairnsichthys rhombosomoides is endemic to the Wet Tropics of northern Queensland, Australia. Following the 2018 split of the northern populations as the separate Daintree rainbowfish (C. bitaeniatus), its range is now defined from the Mulgrave River catchment in the Cairns region southward to the Liverpool Creek and Hull River catchments near Tully, with much of the habitat falling within the Bellenden Ker Range and protected national park land. This is a narrow distribution in a high-rainfall coastal rainforest belt.
The species is a fish of small, swiftly flowing, heavily shaded rainforest creeks in hilly terrain, where rapids alternate with rocky pools. Depths range from just a few centimetres in the riffles to one to three metres in the deeper pools. Stream beds are granite boulders, cobbles, gravel, sand, and alluvium with minimal aquatic plant growth, and the fish shelters among tree roots, woody debris, leaf litter, undercut banks, and what vegetation is present.
Water conditions match these clear upland streams: FishBase reports a tropical range of about 70–77 °F, with pH around 7.2–7.6 and a hardness of roughly 10 dH and upward. Everything about the fish — and most of what makes it vulnerable — comes back to its need for cool, clear, well-shaded flowing water in one small corner of the country.
Feeding
The Cairns rainbowfish is an omnivore that forages in the flowing waters of its rainforest creeks, with a trophic level estimated at around 2.9. Its natural diet comprises aquatic insects and insect larvae, microcrustaceans, and small shrimps, supplemented by algae grazed from the stream bed and submerged surfaces. Much of this prey is the drift and benthic invertebrate life characteristic of fast, boulder-bottomed streams.
Feeding in the upper and middle water column, it takes both invertebrates carried on the current and food gleaned from the substrate. The mix of animal prey and algae is typical of rainbowfishes, which are opportunists rather than dietary specialists. No detailed dietary study exists for this species, so the picture is pieced together from the broad foraging habits recorded for it and its relatives.
Mating
Like other rainbowfishes, Cairnsichthys rhombosomoides is an egg-scattering spawner that deposits its eggs among vegetation and other submerged cover rather than guarding a nest. Spawning is seasonal, taking place from April to December with a peak from August to October — a pattern tied to the warm, wetter months in its tropical Queensland range.
Courtship specific to this species is barely documented. In rainbowfishes generally, males court with fin-flaring displays in and around the spawning substrate, but how C. rhombosomoides displays goes unrecorded in the available sources. What is recorded: females deposit eggs in batches among aquatic plants, root masses, and leaf litter — a spawning strategy built around the cover of its stream habitat.
Breeding
The Cairns rainbowfish is an egg-scatterer with no parental care. Spawning females release their eggs in successive batches rather than a single clutch, depositing them among aquatic plants, root masses, and leaf litter where they lodge among the cover. The eggs are small, numerous, and somewhat adhesive, consistent with the filament-bearing, attaching eggs of rainbowfishes generally. A fecundity figure of around 131 has been reported for the species.
The larvae hatch small, with little yolk reserve but well-developed eyes and a functional gut, and they begin feeding shortly after hatching — an early-feeding strategy that suits the flowing-water environment. The species is assessed as having medium resilience, with an estimated population doubling time of roughly 1.4 to 4.4 years.
It is rarely kept or bred in aquaria, so dedicated captive-breeding accounts are thin next to the popular Melanotaenia rainbowfishes. The biology here comes mainly from Australian field sources rather than hobbyist breeding reports.
In the aquarium
You will almost never see this fish in the trade — its tiny range and its plain colours next to the showier rainbowfishes both work against it. So there is little real husbandry to draw on, and most of what can be said is read off the wild habitat rather than from anyone's tank.
The ecology points to a fish built for cool, clear, well-oxygenated flowing water: temperatures in the low-to-mid 20s Celsius, neutral to slightly alkaline pH around 7.2–7.6, and moderately hard water. Copy the native creeks — strong flow, good oxygenation, a bed of rounded gravel and cobbles, structure from roots and leaf litter. As a stream fish from hilly country, it will likely want cooler, more current-rich conditions than the typical community rainbowfish.
It is a peaceful, harmless shoaler and belongs in a group. Treat it as a specialist's fish, and — given its Endangered status in the wild — be careful where any stock comes from.
Conservation
Cairnsichthys rhombosomoides is assessed as Endangered (EN) on the IUCN Red List, under criteria B1ab(i,ii,iii)+2ab(i,ii,iii). The assessment was carried out on 15 February 2019 and published in the 2019-3 version of the Red List (assessment e.T3461A123377657), by Ebner, Brown, Hammer, Unmack, and Martin.
The listing reflects a species with highly fragmented populations confined to a narrow band of rainforest streams in the Wet Tropics. The principal threats are extended drought, which can dry or fragment its small-stream habitat, and habitat disturbance from pollution, land clearing, and water extraction. A naturally restricted, fragmented range gives the species little capacity to absorb such pressures: localised losses are not readily buffered by outlying populations.
One thing works in its favour: much of the habitat sits within national park land in the Bellenden Ker Range, which protects part of the range. The fish has little value to fisheries and is harmless to people; the worry is the integrity of its rainforest streams, not exploitation.