Taxonomy & naming
Cairnsichthys bitaeniatus was described in 2018 by Gerald R. Allen, Michael P. Hammer and Tarmo A. Raadik as part of a broader revision of the Australian Wet Tropics endemic rainbowfish genus Cairnsichthys (Hammer, Allen, Martin, Adams, Ebner, Raadik & Unmack, 2018, Zootaxa 4413: 271–294). Before this work the genus was treated as a single widespread species, the Cairns rainbowfish (Cairnsichthys rhombosomoides); the revision recognised that the Daintree-region populations represented a distinct second species. Eschmeyer's Catalog of Fishes records the valid name as Cairnsichthys bitaeniatus Allen, Hammer & Raadik, 2018, in the family Melanotaeniidae, order Atheriniformes.
The specific epithet bitaeniatus is from the Latin bi- (two) and taenia (ribbon or band), referring to the two dark lateral stripes that run along the body — a black midlateral stripe plus a second, less distinct lower stripe. The genus name Cairnsichthys combines the north Queensland town of Cairns with the Greek ichthys (fish). The type locality is Little Cooper Creek in north Queensland (approximately 16°10.20′S, 145°24.84′E); the holotype is a 2.5 in male held at the Queensland Museum (QM I.39335).
The species is most readily separated from its close relative, the Cairns rainbowfish (C. rhombosomoides), by a stronger black stripe on the upper gill cover, a more intense black spot at the pectoral-fin base, yellow rather than orange-red fins in adult males, smaller scales, more vertebrae, and a shorter dorsal-fin base.
Morphology
Cairnsichthys bitaeniatus is a small, slender, laterally compressed rainbowfish, reaching about 2.5 in in standard length. It is more elongate than the deep-bodied Melanotaenia rainbowfishes, the streamlined shape suited to life in flowing water. Two dark stripes run lengthwise along the body — a well-defined black midlateral stripe and a second, fainter lower stripe — the pair that gives the species its name.
Additional distinguishing features include a short, broad rectangular dark patch on the upper part of the gill cover, set above a yellow spot; a yellow patch behind the base of the pectoral fin with a bluish area below it; and a prominent black spot at the pectoral-fin base. Adult males develop bright yellowish fins, the most obvious colour signal of breeding condition in this otherwise subtly marked fish.
Reported meristics from the original description are consistent across the species: dorsal fin V–VII spines plus 12–15 rays, anal fin I,19–23, pectoral fin 11–13, pelvic fin I,5, with 38–40 lateral scales and 37–40 vertebrae. The relatively small scales and high vertebral count are among the characters used to separate it from the Cairns rainbowfish.
Habitat
The Daintree rainbowfish is endemic to a very small area of the Daintree region in north-eastern Queensland, Australia — specifically the small tributary streams of the Cooper Creek system and the nearby Hutchinson Creek system. This narrow distribution within the Wet Tropics is the defining fact of its biology, and the reason for its precarious conservation status.
The streams it lives in are small, clear and somewhat shaded, flowing over a mixed bottom of rock, sand, gravel and log debris. The fish swims near the surface and in the midwater, often gathering in deeper pools where flow is strong, and it sometimes schools with blue-eyes (Pseudomugilidae) and other rainbowfishes. These are well-oxygenated, shaded headwater habitats typical of the Wet Tropics rainforest.
Detailed water-chemistry measurements for the species' streams are sparse in the available sources, so precise temperature, pH and hardness figures should be treated with caution. As a rainforest-stream rainbowfish of tropical north Queensland, it occupies warm tropical freshwater, but specific recorded ranges are not well documented here.
Feeding
Direct dietary studies for Cairnsichthys bitaeniatus are limited, and the available accounts describe its feeding only in general terms. It is presumed to feed mainly on terrestrial invertebrates taken from the water surface — insects and other small prey that fall or are washed in from the overhanging rainforest canopy. Its habit of swimming near the surface and in the midwater fits this surface- and drift-feeding strategy.
FishBase places the species at a low trophic level of around 2.9, indicating a diet weighted toward small invertebrates and some plant or detrital material rather than larger prey. As with rainbowfishes generally, it is best regarded as a micro-predator and opportunistic omnivore that takes whatever small invertebrate food the stream delivers.
Because the species is rare in the wild and seldom kept, captive feeding notes are scarce. By analogy with related rainbowfishes it would be expected to accept small live, frozen and prepared foods, but no specific aquarium feeding regime is documented in the sources used here.
Mating
Reproductive behaviour in Cairnsichthys bitaeniatus is little known and has not been studied in detail. The clearest signal of breeding condition is the bright yellowish fins of adult males, which presumably function in courtship as they do in other rainbowfishes, where males flush colour and flare fins to court females.
Based on observations of the closely related Cairns rainbowfish, breeding is presumed to take place between August and October. During this period females are thought to deposit batches of eggs among submerged root masses in the stream, a spawning substrate consistent with the species' habitat of log debris and bank-side roots.
Beyond this seasonal and substrate information, the specifics of pairing, courtship sequence and spawning frequency are not documented for the species. The sources describe its biology as little known, so any finer account of its mating behaviour would be speculative.
Breeding
As with mating behaviour, captive and field breeding data for the Daintree rainbowfish are sparse. The species is presumed, like the Cairns rainbowfish, to breed in the August-to-October window, with females depositing batches of eggs amongst submerged root masses rather than producing a single large clutch — the staggered, mop- or root-spawning pattern typical of rainbowfishes.
FishBase classifies the species as having High resilience, with an estimated population doubling time of less than 15 months. That points to a productive strategy — frequent spawning and reasonable fecundity — typical of small rainbowfishes that release eggs in batches over a long season. The figure reflects intrinsic reproductive capacity, though; it does nothing to offset the threat posed by such a tiny geographic range.
No documented aquarium breeding protocol exists in the sources consulted here. By analogy with other rainbowfishes, the species would likely spawn over fine-leaved plants or spawning mops in captivity with adhesive eggs hatching after one to two weeks, but this is an inference rather than a recorded observation for C. bitaeniatus.
In the aquarium
Cairnsichthys bitaeniatus is essentially not an aquarium fish in any ordinary sense. It is a Critically Endangered species with an extremely restricted natural range in protected Wet Tropics rainforest, and it is not a part of the ornamental-fish trade. The few keeping notes that exist come by analogy with its relative the Cairns rainbowfish and with rainbowfishes in general, rather than from established hobby experience with this species.
Were it ever maintained — say, in a conservation or research program — it would call for the conditions of its wild home: clear, well-oxygenated, gently to strongly flowing freshwater over a bottom of rock, gravel, sand and submerged wood, in a long tank that allows the active surface-and-midwater swimming it favours. Being a small, social rainbowfish, it would do best in a group, where males show their yellow-finned breeding colour.
Given its status, the relevant point for aquarists is not how to keep it but to recognise that it should not be collected from the wild. Its closely related congener, the Cairns rainbowfish, is the species the hobby actually encounters; the Daintree rainbowfish is best appreciated as a wild endemic rather than a candidate for the home aquarium.
Conservation
Cairnsichthys bitaeniatus is assessed as Critically Endangered (CR) on the IUCN Red List, under criteria B1ab(i,ii,iii,v), in an assessment dated 4 February 2019 (Martin, Hammer & Ebner, 2019; The IUCN Red List of Threatened Species 2019: e.T129041926A129041938). This is among the highest threat categories the Red List applies, reflecting an extremely small and fragmented range.
The listing follows directly from the species' biology: it is confined to a tiny cluster of small tributary streams in the Cooper Creek and Hutchinson Creek systems of the Daintree region. A fish restricted to so small an area has almost no buffer against localised threats — habitat disturbance, altered stream flow, water-quality decline, or the arrival of introduced fishes could each hit a large share of the entire global population at once. Field surveys in the Daintree have highlighted the species as a conservation concern precisely because of this narrow endemism.
The relatively recent recognition of the species, in 2018, means it has only lately been treated as a distinct conservation unit rather than as part of a more widespread Cairns rainbowfish. Its protection is tied to the integrity of the Daintree rainforest streams it depends on. For aquarists and naturalists alike, the responsible course is to leave wild populations undisturbed and to support habitat protection in the Wet Tropics.