Taxonomy & naming
Brachyrhaphis olomina was described by the American ichthyologist Seth Eugene Meek in 1914 from Costa Rican material, and the species is conventionally cited with its author in parentheses — (Meek, 1914) — reflecting a change of genus since the original description. Eschmeyer's Catalog of Fishes, the governing authority for valid fish names, recognises it within Brachyrhaphis, a genus of small live-bearing toothcarps in the family Poeciliidae, order Cyprinodontiformes. The species is treated as endemic to Costa Rica.
The genus name Brachyrhaphis is built from the Greek brachys (short) and rhaphis (needle), a reference to the short, needle-like gonopodium — the modified anal-fin intromittent organ — that characterises the males of this group and distinguishes them from relatives with longer gonopodia. Within the Poeciliidae, Brachyrhaphis belongs to the subfamily Poeciliinae, the New World livebearers that also include the guppies, mollies, platies and mosquitofishes. The species epithet olomina derives from "olomina," a Costa Rican vernacular term applied broadly to small native livebearers, and it survives in the fish's English book name, the Olomina toothcarp.
Morphology
Brachyrhaphis olomina is a small fish with an elongated body and an oval cross-section. According to Bussing's standard reference on the freshwater fishes of Costa Rica, males and unsexed individuals reach about 2.5 in in standard length, while females are recorded to roughly 1.5 in SL — an unusual pattern in which males are described as the larger of the cited measurements, though most poeciliids show the reverse and the figures here reflect the available specimen data rather than a firm rule.
The most consistent marking is a series of about ten dark vertical bars along the sides, more pronounced in males than in females. The fins carry distinctive colour: the dorsal fin has jet-black interradial membranes at its base with a yellowish to orange distal half edged in black, present in both sexes; the anal fin is yellow to yellow-orange with a black blotch running from base to tip; the pelvic fins are yellowish; and the caudal fin often shows a brilliant yellow distal margin. A notable exception to the banded pattern is the headwater population of the Río Virilla near Santa Ana and Villa Colón, where individuals grow larger and most lack the vertical bars altogether — a local form that has prompted comment in the descriptive literature.
Habitat
The Olomina toothcarp is a freshwater stream fish endemic to Costa Rica, where it occupies waters of low to moderate current velocity across a wide elevational span — from roughly 10 metres up to about 900 metres above sea level. It is most often found at the surface or in the midwaters of brooks and streams, and occasionally in the shallow margins of rivers, favouring clear, flowing tropical waters rather than still pools.
FishBase characterises the species as a tropical, freshwater, pelagic and non-migratory fish tolerant of a broad temperature range, from about 19 to 34 degrees Celsius — a span that mirrors its occurrence across lowland and highland streams of differing thermal regimes. No species-specific data on pH, hardness or detailed water chemistry are documented in the cached sources, and beyond temperature and elevation the fine-scale habitat preferences of B. olomina remain sparsely recorded.
Feeding
Brachyrhaphis olomina is insectivorous, taking small invertebrate prey from the surface and water column of the streams it inhabits. Its trophic level is estimated at about 3.2, consistent with a diet dominated by animal prey — chiefly insects and their larvae — rather than algae or detritus. This places it among the more predatory of the small poeciliids, a feeding habit shared across much of the genus Brachyrhaphis, whose members are generally small stream predators.
No detailed dietary study specific to this species is recorded in the cached sources, so the picture is necessarily broad: a surface- and midwater-feeding micro-predator of flowing Costa Rican waters. In the rare event it is kept in captivity, a member of this genus would be expected to accept small live and frozen invertebrate foods, in keeping with its wild insectivory, but no documented aquarium feeding regime for B. olomina exists to draw on.
Mating
Like all poeciliids, Brachyrhaphis olomina reproduces by internal fertilisation. The male transfers sperm to the female using the gonopodium, a slender intromittent organ formed from modified rays of the anal fin; in Brachyrhaphis this structure is comparatively short, the feature that gives the genus its name ("short needle"). Courtship and mating in this group typically involve the male approaching and positioning to make contact with the female, after which fertilisation occurs within her body rather than in open water.
Species-specific details of B. olomina's mating behaviour — display, female choice, sperm storage — are not documented in the cached sources, and the species is essentially unstudied in this respect. The general poeciliid pattern is the safest frame: internal insemination via the gonopodium, with females of many livebearers able to store sperm and produce successive broods from a single mating. Whether and to what degree those traits hold in this particular species is not established here, and the data are sparse.
Breeding
Brachyrhaphis olomina is viviparous, a true livebearer that gives birth to free-swimming young rather than laying eggs — the reproductive mode shared across the Poeciliidae. After internal fertilisation the female carries the developing embryos and delivers fully formed fry. FishBase provides no species-specific figures for gestation length, brood size or fecundity, and the cached research notes explicitly caution against inventing such values.
What genus-level literature suggests is that Brachyrhaphis tend toward lecithotrophy — the developing young drawing largely on yolk reserves rather than on substantial post-fertilisation maternal nutrient transfer (matrotrophy) — placing them at the low-provisioning end of the poeciliid spectrum rather than among the strongly placental livebearers. Superfetation, the carrying of multiple broods at different developmental stages simultaneously, is documented in some poeciliids but is not recorded for B. olomina in the cached sources. The species is rated as having high reproductive resilience, with an estimated population doubling time of less than 15 months, implying a productive breeder despite the absence of precise brood data.
In the aquarium
Brachyrhaphis olomina is essentially absent from the aquarium trade. It is a wild Costa Rican stream poeciliid, and the cached research located no species-specific care guide, breeding account or husbandry source for it — only generic livebearer-breeding pages. Any keeping notes are therefore extrapolated from the genus rather than documented for the species itself, and should be read as general expectation rather than tested practice.
As a small, insectivorous, flowing-water fish tolerant of a wide temperature range (about 19 to 34 degrees Celsius in the wild), a member of this genus would in principle suit a well-oxygenated aquarium with gentle current and small invertebrate foods. Brachyrhaphis are, however, known as comparatively pugnacious small livebearers, so cautious tankmate selection would matter. Given the lack of any real hobby record for B. olomina, the honest summary is that its aquarium requirements are undocumented and it is not a fish encountered in the trade.
Conservation
Brachyrhaphis olomina is assessed as Data Deficient (DD) on the IUCN Red List, with the assessment dated 31 December 2019. A Data Deficient listing means there is not enough information on the species' distribution, population status or threats to assign it to a threat category — it is neither a confirmation of safety nor of risk, but a statement that the evidence is insufficient. The species is corroborated as Data Deficient by other aggregators drawing on the same global assessment.
As a Costa Rican endemic with a restricted national range, B. olomina's fortunes are tied to the condition of its stream habitats, and the headwater Río Virilla populations near the densely settled central valley sit in a catchment exposed to urban and agricultural pressures. The species is listed as having low fishing vulnerability and high resilience, but those metrics speak to productivity rather than to habitat security. It is not evaluated under CITES or CMS. The clearest conservation message is the one implied by the Data Deficient status itself: the species needs basic distributional and population study before its standing can be judged.