Taxonomy & naming
Melanotaenia susii was described in 2015 as one of eight new rainbowfishes from the Bird's Head Region of West Papua, Indonesia, in Cybium (volume 39, issue 2). Kadarusman, Hubert and Pouyaud get the credit, working from a study that paired morphology with DNA barcoding to untangle the dense, geographically structured diversity of Melanotaenia across the Vogelkop Peninsula. Eschmeyer's Catalog of Fishes, the authority for valid names, keeps the species in Melanotaenia, so no parenthetical authority applies.
The epithet susii points to the type locality, Susi Creek, the small watercourse the fish was pulled from. The genus name pairs the Greek for black or dark with the Greek and Latin for a band or stripe — a nod to the dark lateral markings common across the genus. Melanotaenia is the largest genus in the family Melanotaeniidae, the true rainbowfishes, which radiate across New Guinea and northern and eastern Australia. The Bird's Head species cluster tightly, each typically confined to a single creek or small drainage.
The fish is so newly described, and so nearly limited to its type material, that almost everything said about it traces back to the original paper rather than to any later study. FishBase flags the maximum-length figure as drawn from the type specimen and in need of a better reference — a sign that the biological record here is thin, and that several reported quantities should be read as preliminary.
Morphology
Melanotaenia susii is a moderately compressed, deep-bodied rainbowfish, and its comparatively long snout helps set it apart within the Bird's Head assemblage. Maximum recorded length is about 5 in in standard length, taken from the type specimen. As with many rainbowfishes, total length and the size of ordinary live fish drift somewhat from that benchmark, and the describers themselves flagged the measurement for refinement once more material turns up.
Colour and fin markings do the most work in the field. There is a clear red margin along the dorsal fins, and the flank carries roughly two to five vertical bars that run the full body depth rather than fading toward the belly. Meristic counts from the description give the first dorsal four to six spines, the second dorsal one spine and thirteen to sixteen soft rays, the anal fin one spine with twenty-three to twenty-seven soft rays, and a lateral scale series of thirty-eight to thirty-nine.
Like other Melanotaenia, it carries the family's divided dorsal — a short anterior section, then a longer posterior one — over a deep, laterally flattened body that tends to deepen with age in dominant males. What's missing is the living detail: how intense the colour runs, how males build display colour, how much the fish lights up in courtship. None of that is documented yet, so the honest description stays close to the preserved characters in the type material.
Habitat
Melanotaenia susii is a freshwater, benthopelagic fish endemic to the Bird's Head (Vogelkop) region of West Papua, Indonesia, known from the creek it is named for. The type habitat reads as a clear, shallow, slow-flowing creek over limestone boulders and mud, littered with dead tree branches and walled in by dense forest — a small, sheltered, structurally busy water, much like the confined creeks that hold many of the short-range Bird's Head rainbowfishes.
That limestone bed matters for water chemistry. FishBase reports the fish from tropical, warm water at around 26 degrees Celsius and above, with a pH at or above 7.7 — neutral to distinctly alkaline, which fits the mineral-rich runoff of a karstic catchment. Past those figures the published environmental data run thin: hardness, dissolved oxygen, and seasonal and depth measurements specific to the locality aren't well established in the accessible literature.
The range is the whole story. A creek endemic apparently boxed into a very small area, M. susii has nowhere else to be, and that confinement governs everything about it. The forested, slow-flowing, limestone-floored creek is its only documented home and the root of its vulnerability.
Feeding
No dietary study of Melanotaenia susii has been published, so its feeding ecology is read off the genus rather than from any direct observation. FishBase gives an estimated trophic level of about 2.9 — an omnivore leaning toward animal prey, which is the usual rainbowfish pattern.
Like other Melanotaenia, it almost certainly feeds as an opportunistic omnivore in the upper and middle water column: insect larvae, small crustaceans, insects fallen to the surface, plus algae and plant matter grazed off submerged surfaces. A creek full of dead branches and forest litter would furnish both the prey and the grazing surfaces.
Because the fish is so new and barely in the hobby, there's no pool of aquarium feeding experience to draw on. With the data this sparse, the honest move is to report the genus-typical omnivory and the estimated trophic level, not to invent a natural diet no one has documented for this species.
Mating
No one has described the reproductive behaviour of Melanotaenia susii in any detail, so the picture comes from the genus rather than from this fish. Melanotaenia rainbowfishes are egg-scattering, plant-spawning, and give no parental care; males court with intense colour displays in and around fine vegetation, most actively in the first hours after light.
Dominant males stake out display areas and flare fins and colour at receptive females, and a flickering pale band along the head and nape is a common courtship signal. How strongly M. susii works through that repertoire is anyone's guess — there is no field account of it spawning.
Without species-specific data, any claim about pair formation, display intensity, or spawning timing here would be guesswork. The honest position: it is presumed to follow the genus pattern, and its actual courtship remains unstudied.
Breeding
There is no published captive-breeding record for Melanotaenia susii. In hobby circles it comes up only as a new, at-risk fish, not as an established aquarium breeder, so its breeding biology has to be read cautiously off the genus.
Melanotaenia rainbowfishes spawn continuously or nearly so under good conditions, laying small adhesive eggs that hang by fine filaments from fine-leaved plants or spawning mops. Females drop a few eggs at a time over a long spawning stretch rather than one big clutch. The adults give no care and will eat their own eggs, so keepers usually breed the genus by moving egg-laden mops to a separate rearing tank. Eggs typically hatch in roughly one to two weeks depending on temperature, and the tiny fry need very small first foods before they can take newly hatched brine shrimp.
None of this is confirmed for M. susii — no incubation period, fecundity, or fry-rearing figures exist for it in the accessible literature. The safe read is that it shares the genus's egg-scattering, plant-spawning mode, with its actual parameters still unrecorded.
In the aquarium
Melanotaenia susii barely registers in the hobby. Rainbowfish keepers mention it as Susi Creek Rainbow — a new, good-looking, conservation-sensitive fish that draws interest precisely because of its Critically Endangered wild status. Firm husbandry experience is scant, and what notes exist are low-confidence social-media observations, not settled care records.
Where it is kept, its creek implies the conditions: warm, neutral-to-alkaline water, around 26 degrees Celsius and above, pH at or above roughly 7.7. That tracks with the hard, alkaline leanings of several Bird's Head and Vogelkop rainbowfishes, and runs against the soft-water defaults some aquarists assume for tropical fish. As with the genus, expect it to do best as an active, sociable schooler in a group, with a long open swimming lane, stable conditions, and good filtration.
Rare and threatened, M. susii is no routine commercial fish, and any specimens in the trade carry conservation weight. The responsible path where it is kept is breeding to sustain the line rather than leaning on wild stock — though the practical husbandry remains poorly documented.
Conservation
Melanotaenia susii is assessed as Critically Endangered (CR) on the IUCN Red List under criterion B2ab(v), in an assessment dated 6 December 2019. The listing turns on an extremely small area of occupancy: a restricted-range Bird's Head endemic known from essentially one creek, which leaves it acutely exposed to any damage to that single habitat.
With no outlying population to absorb local losses, a fish like this has no margin. For the Bird's Head rainbowfishes, the recognised threats to such creek endemics are habitat alteration in their tiny catchments and the fallout from introduced fishes — and a point-endemic has little to push back with. A minute range plus identifiable habitat pressure is exactly what a B2 Critically Endangered assessment is built on.
The species has not been evaluated under CITES or the Convention on Migratory Species. For a fish in this position, the conservation work that counts is protecting the specific creek and keeping captive lines going, so the species isn't wholly tied to one vulnerable wild population. So little is documented beyond the description and the Red List entry that even basic monitoring of the wild population would be a real step toward safeguarding it.