Taxonomy & naming
Hemirhamphodon sesamum was described in 2013 by Heok Hui Tan and Kelvin K. P. Lim in the Raffles Bulletin of Zoology, in a paper erecting three new species of freshwater halfbeak in the genus Hemirhamphodon from Borneo. The species is placed in the family Zenarchopteridae within the order Beloniformes — the lineage of needlefishes, flyingfishes and halfbeaks — and specifically among the internally fertilising, often viviparous halfbeaks that form the Old World ecological analogue of the American livebearers. Eschmeyer's Catalog of Fishes treats Hemirhamphodon sesamum Tan & Lim, 2013 as the valid name.
The genus name Hemirhamphodon combines the Greek hemi (half), rhamphos (bill or beak) and odous (tooth), describing the toothed, half-elongated lower jaw shared by these halfbeaks. The species epithet sesamum is a noun in apposition referring to the seeds of the sesame plant (Sesamum, family Pedaliaceae): the small black spots and dashes pigmented between the rays of the male's dorsal fin resemble scattered black sesame seeds, which is the feature the describers chose to immortalise in the name.
The 2013 revision separated several Bornean populations that had previously been lumped under broader names, with sesamum diagnosed from its congeners by a combination of male fin coloration, the elongated dorsal-fin filaments, and the bright red flange on the lower jaw. As with most recently described nano-fishes of remote drainages, the species is known from relatively few collections, and its biology is documented far more thinly than its formal taxonomy.
Morphology
Hemirhamphodon sesamum is a small, elongate halfbeak reaching about 2 in in standard length. The body is slender and pencil-like, built for hanging just beneath the water surface, and carries the genus's characteristic beak — an elongated, toothed lower jaw projecting well beyond the short upper jaw. Meristic counts recorded for the species include 13 to 14 dorsal soft rays, 8 anal soft rays, and 38 to 41 vertebrae. The pelvic-fin origin lies anterior to the dorsal-fin origin.
Males carry the diagnostic ornamentation. The distal portions of the first six or seven dorsal-fin rays are drawn out into filaments that can reach up to twice the depth of the dorsal fin, and the dorsal fin itself is marked with yellow and an iridescent red margin, with black pigment laid between the rays — the scatter of dark dashes that gives the species its name. The lower jaw bears a bright red dermal flange with a blue lower margin and a black ventral edge, a striking flash of colour on an otherwise understated fish. The fourth anal-fin ray is enlarged, part of the modified male anal-fin apparatus associated with internal fertilisation in this family.
Females are described far more sparingly in the literature and lack the elaborated dorsal filaments and the vivid jaw flange of the males, conforming to the general pattern of sexual dimorphism seen across Hemirhamphodon. Quantitative female measurements beyond the shared body proportions are data-sparse, and finer details of growth and coloration in this species remain poorly documented.
Habitat
The species is endemic to Indonesian Borneo, where it is known from the Batulicin and Cantung river basins in the south-eastern part of Kalimantan (south Kalimantan). FishBase classifies it as a freshwater, pelagic, tropical species occupying the upper water column. Its range is small and tightly bounded, which is the central fact in its conservation assessment.
Like other Bornean Hemirhamphodon, sesamum is a fish of soft, slightly acidic forest streams — the tannin-stained blackwaters that drain lowland and peat-influenced forest. These are warm, gently flowing waters where the fish patrols the surface film among overhanging vegetation and leaf litter. Precise field measurements of temperature, pH and conductivity for sesamum's specific localities are not well documented in the accessible literature, so habitat chemistry is best given at the genus level: warm, soft and acidic.
Because the species is confined to two adjacent basins, it is inherently vulnerable to habitat change across that limited area. Deforestation, conversion of land, and the degradation of forest-stream water quality in south-eastern Kalimantan are the kinds of pressures that bear on a narrowly distributed surface specialist of this sort, even where direct, species-specific threat data are thin.
Feeding
Hemirhamphodon halfbeaks are surface-feeding micropredators, and sesamum fits this pattern. The upturned mouth and elongated lower jaw are adapted for taking small prey from the water surface and the film just below it — chiefly small insects, insect larvae and other tiny invertebrates that fall onto or drift at the surface of forest streams. The species' modelled trophic level of about 3.1 is consistent with a small carnivore rather than an omnivore or grazer.
In the aquarium these halfbeaks retain their preference for live and frozen foods of appropriate size: small live insects and crustaceans such as Daphnia, newly hatched brine shrimp, microworm, fruit flies and other tiny surface fare suit them best. They can be reluctant to take dry prepared foods, and a diet skewed too far toward flake or pellet is a common reason they fail to thrive. Because they feed at the surface, food that sinks quickly is largely wasted on them.
Species-specific feeding observations for sesamum in the wild are sparse; the account above draws on the genus's well-established surface-micropredator ecology rather than on dedicated dietary study of this particular halfbeak.
Mating
The Zenarchopteridae practise internal fertilisation, and Hemirhamphodon is one of the genera in which this is well established — the family is the Old World counterpart to the live-bearing poeciliids, having evolved internal fertilisation and (in many members) viviparity independently. Rather than the rod-like gonopodium of New World livebearers, male zenarchopterid halfbeaks transfer sperm using a modified anal fin; in sesamum the enlarged fourth anal-fin ray is part of this male copulatory apparatus. Sperm is introduced into the female internally, dispensing with the external egg-laying of most fishes.
Male Hemirhamphodon are known to be territorial and to spar with one another, fencing and wrestling with their elongated beaks in contests that hobbyists describe vividly. The bright dorsal filaments and the red jaw flange of male sesamum are almost certainly signals used both in male-male competition and in courtship, advertising condition to rivals and to females in the dim, tannin-shaded water where these fish live.
Detailed courtship behaviour specific to sesamum has not been documented in the accessible literature. What can be said with confidence is the reproductive mode — internal fertilisation via a modified anal fin, with males ornamented and combative — which is consistent across the genus and supported directly by the species' described male anatomy.
Breeding
Hemirhamphodon are live-bearers: after internal fertilisation the female retains the developing embryos and gives birth to free-swimming young rather than laying eggs, the hallmark reproductive strategy of these zenarchopterid halfbeaks. Broods tend to be small — a handful of relatively large, well-developed fry — and are typically delivered over an extended period rather than all at once, so that a female may release young in a trickle across days. This low-fecundity, large-offspring strategy is the opposite end of the livebearer spectrum from the prolific guppies and mollies.
The fry are comparatively large and capable at birth and can take small live foods such as newly hatched brine shrimp and microworm almost immediately. Dense surface and floating vegetation gives them cover and gives the keeper a chance of raising them, since adults will opportunistically take fry. Quantitative figures for sesamum specifically — exact brood size, gestation length, size at birth — are data-sparse and should be treated as genus-level expectations rather than measured values for this species.
Whether sesamum exhibits superfetation — the simultaneous carrying of broods at different developmental stages, which occurs in some internally fertilising fishes — is not documented for this species and should not be assumed; the reliably established facts are viviparity and small, well-developed broods characteristic of the genus.
In the aquarium
Hemirhamphodon sesamum is a rare and somewhat specialist aquarium fish rather than a beginner's livebearer. It is a nano halfbeak suited to a well-planted setup that recreates its soft, slightly acidic blackwater home, with calm surface water and plenty of floating plants. The single most important piece of equipment is a tight-fitting lid: these are skittish, surface-oriented fish that jump readily and are easily lost, and they are also sensitive to disturbance of the water surface, so vigorous surface agitation should be avoided.
Males spar with one another, so keeping more than one male requires enough swimming length and visual barriers — line-of-sight breaks created by plants and hardscape — to defuse aggression; in too small or too bare a tank a dominant male can harass rivals persistently. The fish do best on small live and frozen foods taken at the surface and may snub dry food, so a keeper should be prepared to supply appropriate micro-prey. Tankmates, if any, should be small, peaceful and not surface-competing.
This is a fish for the experienced keeper of oddball livebearers: jump-prone, particular about water and food, and uncommon in the trade. Much of the practical husbandry guidance available is genus-level, drawn from the better-known Bornean redline halfbeaks, and applies to sesamum by extension rather than from species-specific aquarium experience.
Conservation
Hemirhamphodon sesamum is assessed as Near Threatened (NT) on the IUCN Red List, under criterion B1b(iii), with the assessment published on 14 January 2020. The listing is driven by the species' restricted geographic range: confined to the Batulicin and Cantung basins of south-eastern Kalimantan, it has a small extent of occurrence, and the criterion reflects a continuing decline in the quality of its habitat.
The pressures implied by that listing are those that bear on small forest streams in Borneo — deforestation, land conversion, and the degradation of water quality in lowland and peat-forest drainages. For a fish endemic to two adjacent basins and dependent on soft, tannin-stained, vegetation-shaded surface water, even localised habitat change can affect a meaningful fraction of the global population, which is why a narrow range alone is enough to push the species above Least Concern.
FishBase records a low fishing vulnerability for the species, and there is no indication that collection for the aquarium trade is a significant pressure — the fish is rare in the hobby. The conservation concern is therefore habitat-based rather than exploitation-based, and the appropriate posture for aquarists is to source any specimens responsibly and to recognise that the long-term security of the species rests on the integrity of its small Bornean range.