Taxonomy & naming
Nomorhamphus hageni was described in 1912 by the Dutch zoologist Canna Maria Louise Popta, working from material collected on the Indonesian island of Sulawesi (Celebes). The original combination has since been moved into the genus Nomorhamphus, and the species is listed in Eschmeyer's Catalog of Fishes — the governing authority for valid fish names — as Nomorhamphus hageni (Popta, 1912), the parentheses recording that combination change. The species carries FishBase SpecCode 24751.
The fish belongs to the order Beloniformes and the family Zenarchopteridae, the internally fertilised halfbeaks. This family unites the live-bearing and egg-laying halfbeaks of fresh and brackish water across South-East Asia and is distinct from the marine, egg-scattering Hemiramphidae with which halfbeaks were historically lumped. Within Zenarchopteridae the genus Nomorhamphus is a Sulawesi-centred radiation of small, viviparous stream fishes, of which N. hageni is one of the lesser-studied members.
The genus name Nomorhamphus derives from Greek roots — nema, a filament or thread, and rhamphos, a bill or beak — referring to the produced lower jaw characteristic of halfbeaks. The species epithet hageni is an eponym honouring Dr Bernhard Hagen (1853–1919), a German physician and amateur naturalist who collected and supported natural-history work in the Indonesian archipelago.
Morphology
Nomorhamphus hageni is a small, elongate fish. FishBase records a maximum length of about 4 in SL, based on Kottelat and colleagues' 1993 survey of the freshwater fishes of western Indonesia and Sulawesi; this is the only firm size figure available for the species. The body is slender and built for life at the water's surface, the dorsal and anal fins set well back toward the tail in the streamlined manner typical of beloniform fishes.
The defining feature of the genus is the halfbeak jaw structure — an elongated, beak-like lower jaw paired with a short, mobile upper jaw, an arrangement suited to picking prey from the surface film. As an internally fertilising halfbeak, the male bears a modified, partly fused anal fin used in mating rather than the rod-like gonopodium of the New World poeciliids; this andropodium is the zenarchopterid counterpart to the livebearer's intromittent organ.
Species-specific colour and fin detail for N. hageni are sparsely documented. In Nomorhamphus generally, males tend to be smaller and more colourful than females, often carrying red or orange tones on the fins that intensify during courtship, while females are larger and plainer — but the precise pattern of this particular species is not well recorded in the available literature, and this profile errs toward saying less rather than inventing detail.
Habitat
The species is endemic to Sulawesi, Indonesia, in fresh water; FishBase classes it as a tropical, pelagic freshwater fish living in the upper part of the water column. The exact type locality is not stated on the available summary records, so its precise distribution within the island is uncertain.
Nomorhamphus halfbeaks as a group inhabit the slow-moving streams, rivers and lakes of Sulawesi, frequently in stretches with dense marginal and surface vegetation. They are surface-oriented fishes, hanging just beneath the film with their upturned mouths poised to take insects and small invertebrates from above. The waters they favour tend to be soft to moderately mineralised and slightly acidic to neutral.
Because N. hageni itself is Data Deficient, the finer points of its preferred habitat — flow, substrate, the specific water bodies it occupies — are not securely documented. The genus-level picture of a vegetated, gently flowing Sulawesi stream is the best available guide, and should be read as context drawn from congeners rather than measured fact for this species.
Feeding
Nomorhamphus halfbeaks are surface-feeding micro-predators. The upturned mouth and produced lower jaw are adaptations for taking prey from the water's surface, and the natural diet of the genus is dominated by insects and small aquatic invertebrates — terrestrial insects that fall onto the water, emerging aquatic larvae, and other small drifting items. FishBase assigns N. hageni a modelled trophic level of about 3.2, consistent with a carnivorous, invertebrate-eating habit.
No diet study specific to N. hageni is available, so the feeding picture is generalised from the genus and from close relatives such as N. liemi. In the aquarium, Nomorhamphus halfbeaks take floating flakes and micro-pellets but do markedly better with a strong component of live and frozen foods — bloodworm, daphnia, brine shrimp and mosquito larvae — offered at or near the surface where these fish naturally feed.
As surface specialists they are reluctant to forage from the bottom, so food that sinks past them is largely wasted. Feeding to the fish's strengths — small, frequent, surface-presented meals — suits both their anatomy and their disposition.
Mating
Nomorhamphus hageni is internally fertilised, the trait that places its family among the few beloniform groups to have abandoned external egg-scattering. Fertilisation occurs inside the female: the male transfers sperm using a modified anal fin (the andropodium), the zenarchopterid equivalent of the rod-like gonopodium that New World poeciliids evolved independently for the same purpose. This is a case of convergent evolution — two unrelated lineages of small fishes arriving separately at internal fertilisation and live birth.
Courtship in the genus involves the smaller, more colourful males approaching and displaying to the larger females, with fin colour — often red or orange — heightened during these encounters. As with other internally fertilising livebearers, the female may be able to store sperm and produce successive broods, though the details of sperm storage and superfetation (the carrying of broods at more than one developmental stage simultaneously, documented in some halfbeaks) have not been specifically established for N. hageni.
Species-specific reproductive behaviour for N. hageni is undocumented in the available sources; the account here reflects the general biology of Nomorhamphus and the internally fertilised halfbeaks rather than direct observation of this species.
Breeding
As an internally fertilised, viviparous halfbeak, Nomorhamphus hageni gives birth to live, fully formed young rather than laying eggs — the developing embryos are carried by the female and released as free-swimming fry. FishBase provides no species-specific brood size or gestation figures for N. hageni, and those numbers should not be invented; the data are simply sparse.
In the genus more broadly, and in the much better-studied N. liemi, gestation runs on the order of several weeks (roughly four to six), after which the female produces a modest number of relatively large, well-developed fry. The newborns of livebearing halfbeaks are large for the parent's size and are capable swimmers and feeders from birth, an advantage of the live-bearing strategy. Adults, including the mother, may predate the fry, so in captivity dense planting or separation is used to raise good numbers.
Because these figures derive from congeners, they describe what an aquarist might reasonably expect from a Nomorhamphus halfbeak rather than confirmed measurements for N. hageni itself. The honest summary is that the species breeds by live birth following internal fertilisation, and that the quantitative detail remains to be documented.
In the aquarium
Nomorhamphus hageni is rarely seen in the trade, and dedicated husbandry information for the species is effectively absent; what follows is drawn from the keeping of its congeners, principally N. liemi, and should be treated as genus-level guidance. These are surface-dwelling oddball livebearers best kept in a long, well-covered aquarium — they sit near the top and can be startled into jumping, so a tight-fitting lid and calm conditions matter.
Reported genus husbandry favours warm water around 72–82 °F with a pH near 6.5–7.5, gentle filtration and low surface agitation, and floating plants such as water lettuce or duckweed to provide overhead cover and security for fishes that spend their lives at the surface. A modest group does well in a tank of roughly 15–20 US gal or larger. Males can be mildly territorial toward one another, but the fish are otherwise peaceful; their delicate beaks and surface habit make boisterous or fin-nipping tankmates a poor match.
The overriding caution is availability and data: this is a poorly documented Sulawesi endemic, and an aquarist taking it on is working largely from the example of related halfbeaks. Stable, clean water and surface-presented live and frozen foods are the foundation of keeping any Nomorhamphus well.
Conservation
Nomorhamphus hageni is assessed as Data Deficient (DD) on the IUCN Red List, assessed on 16 January 2020 (species ID 90981483). The Data Deficient category means there is not enough information about the species' range, population or threats to judge its risk of extinction — it is a statement about knowledge gaps rather than a finding of either safety or peril.
As a Sulawesi freshwater endemic with an uncertain distribution, the species shares the general vulnerabilities of the island's stream and lake faunas, which face pressures from habitat alteration, water extraction and introduced species. Whether and how strongly these affect N. hageni specifically is unknown, precisely because the species is so little studied. It has not been evaluated under CITES or CMS.
For a fish this poorly documented, the most useful conservation contribution an aquarist can make is careful record-keeping — accurate locality data, husbandry notes and breeding observations — and never releasing aquarium stock into wild waters. Filling the data gap is itself the conservation task here.