Taxonomy & naming
Melanotaenia longispina was formally described in 2015 by Kadarusman, Avarre and Pouyaud as part of a larger paper — Nugraha, Kadarusman, Hubert, Avarre and colleagues, 'Eight new species of rainbowfishes (Melanotaeniidae) from the Bird's Head Region, West Papua, Indonesia' (Cybium 39(2): 99–130). That work split out a series of narrow-range Melanotaenia from the Bird's Head Peninsula, M. longispina among them alongside species such as M. susii, M. flavipinnis and M. pimaensis. Eschmeyer's Catalog of Fishes records the valid name as Melanotaenia longispina; because it was described in Melanotaenia, where it remains, no parenthetical authority is used.
The genus Melanotaenia is the largest in the family Melanotaeniidae, the true rainbowfishes, which radiate across New Guinea and northern and eastern Australia. Within that genus M. longispina is diagnosed less by colour than by a combination of meristic and morphometric characters. The type material gives fin-ray and scale counts of D IV–VI+I,13–14; A I,23–26; 37–40 lateral scales; and 19 gill rakers on the first arch. The most distinctive features, and the source of the name, are the elongated spines of the dorsal and anal fins together with the absence of stripes along the dorsal-fin margin — characters that separate it from its Bird's Head congeners.
The species was described from comparatively little material, and much of what is recorded about it derives from the original type series. As with several of the 2015 Bird's Head rainbowfishes, there is no significant body of subsequent field or aquarium study, so the formal description remains the principal source for the species' biology.
Morphology
Melanotaenia longispina is a small rainbowfish: the maximum recorded size is 3.5 in standard length in males or unsexed specimens and 3 in SL in females, both figures based on the type specimens rather than on a large measured sample. In overall build it follows the family pattern of a laterally compressed body bearing the characteristic divided dorsal fin — a short anterior dorsal followed by a longer posterior dorsal — though detailed body-depth proportions for this species are not well documented outside the original description.
The defining morphological character, reflected in the name longispina (Latin longus, 'long', plus spina, 'spine'), is the elongation of the spines in the dorsal and anal fins. Diagnostic counts from the type series are D IV–VI+I,13–14 and A I,23–26, with 37–40 lateral scales and 19 gill rakers on the first arch. A further distinguishing feature noted in the description is the absence of stripes on the dorsal-fin margin, which separates it from some related Bird's Head species.
Live colour is barely described in the available record. Like other Melanotaenia the species would be expected to show the genus's mood- and condition-dependent iridescence, displaying males brighter than subordinate or stressed individuals, but with no published colour notes or aquarium observation to go on, the point stays data sparse and is not asserted here.
Habitat
Melanotaenia longispina is known from the Bird's Head (Vogelkop) Region of West Papua, Indonesia. The type locality is the Klahfot River near Saluk, in the Sorong Regency. FishBase records the species as a freshwater, benthopelagic, tropical fish, with the type material collected from a river flowing through secondary rainforest.
The habitat at the type locality is described as a river over a gravel and sandy bottom, with turbid water and no aquatic vegetation. Recorded water parameters are sparse: a pH reading from 7.4 upward and a temperature from about 82 °F upward are noted, but the available data give lower bounds rather than a measured range, so any wider envelope would be speculative. The water is warm and at least neutral-to-slightly-alkaline at the point of collection.
Everything about this fish's distribution comes back to one point: it is confined, so far as anyone knows, to a single river system on the Bird's Head Peninsula, and that confinement is what earned it its conservation listing. As with the other narrow-range rainbowfishes from the same 2015 study, there is nowhere else it is currently known to occur, and its full range remains poorly resolved.
Feeding
There is no published diet study specific to Melanotaenia longispina. FishBase assigns the species an estimated trophic level of about 2.9, which is consistent with an omnivorous to weakly carnivorous diet drawn largely from small invertebrates and some plant or detrital material.
Rainbowfishes of the genus Melanotaenia are, as a group, opportunistic omnivores that feed in the upper and middle water column, taking small aquatic and terrestrial invertebrates — insect larvae, small crustaceans and surface-fallen insects — together with algae and plant matter. In the absence of species-specific observation, M. longispina is reasonably assumed to feed in this general manner within its turbid, vegetation-poor river habitat.
No aquarium feeding experience is on record for the species, and detailed wild gut-content or foraging data are not available, so its precise diet is best treated as data sparse.
Mating
No mating or courtship behaviour has been documented for Melanotaenia longispina. FishBase records no reproduction or spawning information for the species, and the original description does not report breeding observations.
Across the genus the pattern is egg-scattering, plant-spawning fishes with no parental care: males court with colour and fin displays, often most intensely in the early morning, and pairs release a few eggs at a time among vegetation. Whether M. longispina follows that pattern is unconfirmed, and its turbid, vegetation-poor type habitat means even the spawning substrate can't be assumed from the genus norm.
Given the lack of direct evidence, the species' mating behaviour is treated here as unknown rather than inferred.
Breeding
There is no captive or wild breeding record for Melanotaenia longispina. FishBase lists no spawning, fecundity or development data, and the species does not appear to be established in the aquarium hobby, so no first-hand breeding experience exists to draw on.
Most rainbowfishes that have been bred in captivity are egg-scattering plant spawners that deposit small adhesive eggs by fine filaments onto fine-leaved vegetation or spawning mops over a period of days, with no parental care and a hatching time on the order of one to two weeks depending on temperature. Whether M. longispina would behave similarly is plausible but unverified.
FishBase estimates a medium resilience for the species, with a minimum population doubling time of roughly 1.4 to 4.4 years — a life-history proxy rather than a breeding observation. Actual fecundity, egg size, incubation period and fry development remain undocumented.
In the aquarium
Melanotaenia longispina is not a part of the aquarium hobby in any meaningful sense. It carries no common name, is not known to be in trade, and there is no body of husbandry experience for it; it is a wild species of scientific record, not an ornamental fish.
What little can be said about likely requirements is extrapolated from its habitat and from the genus generally: warm water of at least about 82 °F and neutral-to-slightly-alkaline pH at the type locality, in a turbid river over sand and gravel. Like other rainbowfishes it would presumably be an active, shoaling, peaceful fish best kept in a group with open swimming space, but this is inference, not tested practice.
Given the species' Critically Endangered status and its absence from the trade, it should be regarded as a conservation and research subject rather than an aquarium candidate. Any keeping of it would properly fall to managed or scientific programmes rather than the general hobby.
Conservation
Melanotaenia longispina is assessed as Critically Endangered (CR) on the IUCN Red List, under criteria B2ab(v). The assessment (species ID 161100016) reflects the fish's extremely restricted range — currently known only from the Klahfot River system in the Bird's Head Region near Sorong — which leaves it with no large outlying population to buffer against local impacts.
A single-locality endemic of this kind is inherently vulnerable. Habitat disturbance, a shift in water quality, or the introduction of non-native fishes into the catchment could hit the entire known population at once. The same pressures threaten the other narrow-range Bird's Head rainbowfishes described in 2015, several of which sit in similarly precarious conservation status.
The species is Not Evaluated under CITES and CMS. With no established captive population to provide a safety net — unlike better-known rainbowfishes such as M. boesemani, which is widely tank-bred — the long-term security of M. longispina depends almost entirely on the integrity of its small native range.