Taxonomy & naming
Nomorhamphus celebensis was described by Max Weber and Lieven Ferdinand de Beaufort in 1922, from material collected on Sulawesi — the island then known to European naturalists as Celebes, the source of the species epithet celebensis, "from Celebes." Eschmeyer's Catalog of Fishes treats Nomorhamphus celebensis Weber & de Beaufort, 1922 as the valid name and records an early orthographic-error spelling, Nomoramphus celebensis. The species is placed in the genus Nomorhamphus within the family Zenarchopteridae, the internally fertilising halfbeaks, in the order Beloniformes.
The Zenarchopteridae were historically lumped with the surface-dwelling marine and brackish halfbeaks of the family Hemiramphidae, but the internally fertilising freshwater genera — chiefly Nomorhamphus, Dermogenys and Zenarchopterus — are now recognised as a distinct lineage. Phylogenetic work on the viviparous halfbeak genera Dermogenys and Nomorhamphus (Meisner, 2001) clarified the relationships and morphology of these fishes, and the broader study of live-bearing in the family (Reznick, Meredith & Collette, 2007) placed N. celebensis among halfbeaks that evolved complex live-bearing life histories convergent with the Poeciliidae.
The genus name Nomorhamphus is built from Greek elements relating to the beak; the halfbeak group as a whole is named for the characteristically elongated lower jaw. Notably, N. celebensis is an exception within its genus, largely lacking that elongated lower jaw — a point repeatedly noted in the taxonomic and conservation literature and one that can make it look superficially unlike a typical halfbeak.
Morphology
Nomorhamphus celebensis is a small, slender, elongate fish, reaching a maximum length of about 3 in standard length (Kottelat, Whitten, Kartikasari & Wirjoatmodjo, 1993). Its most distinctive feature is what it lacks: unlike most members of the genus, it does not develop the characteristic elongated lower jaw, or "beak," that defines halfbeaks (Meisner, 2001). The body is built for life in flowing clear water, streamlined and adapted to holding position in stream currents.
Detailed published descriptions of colour and fine meristics for this species are sparse in the accessible literature, and specific data on sexual size differences are not well documented here. As in other internally fertilising halfbeaks, however, males are expected to carry a modified anal fin — the andropodium, the halfbeak counterpart of the poeciliid gonopodium — used to transfer sperm to the female, while females are typically the larger and deeper-bodied sex. Beyond these general expectations the published morphological detail is thin, and precise dimorphism figures for N. celebensis should be regarded as data-sparse rather than asserted.
Habitat
The species is endemic to the Lake Poso region of Central Sulawesi, Indonesia, occurring in the lake itself and in the surrounding clear-water rivers and rivulets (Meisner, 2001). It inhabits small streams with stone and muddy substrates and clear, flowing water near Lake Poso, and is recorded from permanent rivers, streams and creeks as well as the permanent freshwater lake. Lake Poso is an ancient, deep, clear-water lake, and its fauna includes a number of narrowly endemic fishes; N. celebensis is part of this distinctive lacustrine and stream community.
The geographic footprint of the species is very small. FishBase places it at roughly 1–3°S and 119–122°E, and the IUCN assessment estimates an extent of occurrence of only about 62–559 mi² — best estimated at under 311 mi² — across just one to five locations. This is a tropical, freshwater, pelagic-to-benthic fish of the upper few metres of the water column, and the clarity of its habitat is biologically important: Lake Poso is a clear-water system and is correspondingly sensitive to pollution and nutrient enrichment that would cloud it.
Feeding
Specific dietary studies of Nomorhamphus celebensis are not detailed in the available sources, and FishBase records a trophic level of approximately 3.1, consistent with a small carnivore or micro-predator rather than a herbivore. Internally fertilising halfbeaks generally feed at and just below the water surface, taking small invertebrates, insect larvae and terrestrial insects that fall onto the water, and there is no reason to think N. celebensis departs from this surface-oriented, insectivorous pattern in its clear streams and lake margins.
In aquarium care, halfbeaks of this group are usually maintained on small live and frozen foods — Daphnia, brine shrimp, mosquito larvae and similar items — supplemented by floating dry foods that they can take from the surface, where their upturned mouth is best suited to feeding. Beyond these general expectations, fine detail on the natural diet of this particular species is data-sparse.
Mating
Like other members of the Zenarchopteridae, Nomorhamphus celebensis reproduces by internal fertilisation. In these internally fertilising halfbeaks the male's anal fin is modified into an intromittent organ — the andropodium, structurally and functionally analogous to the gonopodium of the poeciliid livebearers — which is used to transfer sperm directly to the female. This internal-fertilisation strategy is the defining trait that separates the freshwater Zenarchopteridae from the egg-scattering marine halfbeaks.
The evolution of live-bearing in these halfbeaks is a celebrated case of convergence with the New World Poeciliidae: two unrelated groups of small fishes independently arrived at internal fertilisation and viviparity, together with the suite of life-history adaptations that accompany it (Reznick, Meredith & Collette, 2007). Detailed observations of courtship behaviour specific to N. celebensis are not well documented in the accessible literature, so the precise mating displays of this species are best treated as data-sparse, even though the underlying internal-fertilisation mechanism is secure.
Breeding
Nomorhamphus celebensis is viviparous, giving birth to live, free-swimming young rather than laying eggs (Reznick, Meredith & Collette, 2007; the genus is described as live-bearing in the IUCN assessment). After internal fertilisation the female carries the developing embryos and delivers fully formed fry, mirroring the reproductive mode of the poeciliid livebearers with which the group is convergent. This live-bearing strategy is one of the features that makes the freshwater halfbeaks of Sulawesi of particular interest to evolutionary biologists.
Quantitative breeding data for this species — gestation length, brood size, frequency of broods and any occurrence of superfetation (the simultaneous carrying of embryos at different developmental stages) — are not reported in the sources at hand. Superfetation is known in some live-bearing halfbeaks, but it should not be asserted for N. celebensis without direct evidence, and the brood sizes of livebearing halfbeaks are generally modest compared with the large broods of a guppy. The honest position for this species is that the live-bearing mode is confirmed while the reproductive particulars remain data-sparse.
In the aquarium
Nomorhamphus celebensis enters the aquarium trade and is exported internationally as an ornamental fish, with United States import records dating to 1992 (IUCN). Halfbeaks as a group, however, are not beginner fish: they are surface-dwellers prone to startling and damaging their jaws against tank walls and covers, they appreciate gentle flow and cool, clean, well-oxygenated water reflecting their clear-stream origins, and they do best with calm tankmates and a securely covered, well-planted aquarium with open surface swimming space.
Given this species' clear-water, flowing-stream habitat near an ancient highland lake, it is best regarded as a fish for the experienced livebearer or halfbeak keeper rather than a community-tank staple. Because wild stocks are Endangered and the ornamental trade is itself listed among the species' threats, aquarists encountering it should prioritise captive-bred stock and avoid contributing to over-extraction from the wild Lake Poso population.
Conservation
Nomorhamphus celebensis is assessed as Endangered (EN) on the IUCN Red List under criterion B1ab(iii), in an assessment published in 2019 (assessment dated 22 January 2019; Mokodongan, assessor). The listing reflects its tiny range — an extent of occurrence best estimated at under 311 mi² across one to five locations, confined to the Lake Poso basin in Central Sulawesi — combined with an ongoing decline in habitat quality. Earlier assessments had ranked it Data Deficient (1996) and Rare (1990, 1994), so the 2019 Endangered listing represents a clearer, more concerning picture rather than a sudden change in status.
The threats are multiple and compounding. Invasive alien fishes — Common Carp (Cyprinus carpio) and Nile Tilapia (Oreochromis niloticus) — have been introduced to the lake; eutrophication and pollution from domestic waste and agricultural fertiliser runoff degrade a clear-water system that is inherently sensitive to nutrient loading; intensive fishing and over-extraction for the ornamental trade (Kusumah et al., 2018) draw directly on the population; and the basin's volcanic-tectonic setting carries the risk of gas-release fish-kill events of the kind recorded in 1983. Field collections illustrate the scarcity: a 2012 effort near Tentena yielded fewer than ten individuals, while a 2017 collection in a small stream west of the lake found 50–100. Conserving this fish is inseparable from protecting the water quality and stream connectivity of the Lake Poso system as a whole.