Taxonomy & naming
Phalloceros leptokeras was described by Paulo H. F. Lucinda in 2008 as part of his sweeping systematic and biogeographic revision of the genus Phalloceros Eigenmann, 1907 (Cyprinodontiformes: Poeciliidae: Poeciliinae), a single monograph that recognised twenty-one new species where earlier workers had lumped almost everything under the wide-ranging Phalloceros caudimaculatus. The species sits in the family Poeciliidae, the New World livebearers, within the subfamily Poeciliinae. Eschmeyer's Catalog of Fishes treats Phalloceros leptokeras Lucinda, 2008 as the valid name (catalog spid 70285), and FishBase carries it under SpecCode 64067.
The specific epithet is built from Greek leptos (slender or narrow) and keras (horn), a noun in apposition alluding to the slender gonopodial appendix that helps diagnose the species. The genus name follows the same anatomical theme — Phalloceros from Greek phallos (penis) and keras (horn) — a reference to the conspicuous, horn-like terminal structures of the male copulatory organ that run through the whole genus. The English common name "Sapucaia toothcarp" derives from the type locality at Sapucaia in Rio de Janeiro state; the Portuguese vernacular "guaru" is applied broadly across small Brazilian poeciliids.
Within Phalloceros the species is one of a cluster of look-alikes, and its description rests on fine characters rather than obvious external features. Lucinda separated it from its congeners — except the closely similar P. tupinamba and P. aspilos — by the lateral ramus of the female urogenital papilla, the left-turned female papilla, a large sickle-like hook on the inner surface of the gonopodial appendix near its base, and the absence of vertical body bars. It differs from P. aspilos in possessing a lateral spot, and from P. tupinamba in the flattened shape of the female urogenital papilla and the broader (two-to-three-scale) lateral spot.
Morphology
Phalloceros leptokeras is a very small, sexually dimorphic livebearer. Males reach about 1 in in standard length and females are distinctly larger, to roughly 1.5 in SL — the larger-female pattern typical of poeciliids. Meristic counts recorded in the original description include eight dorsal-fin soft rays, eight to eleven anal-fin soft rays, and 32 to 33 vertebrae. The body is a plain, unbarred toothcarp form without the vertical bars seen in some congeners.
The single most useful field-and-microscope character is the lateral spot: a densely pigmented, rectangle-like mark sitting on the 14th–15th (rarely 16th) scale of the longitudinal series, spanning two to three scale rows. This autapomorphy, together with the absence of vertical bars, is what most readily separates the species from the otherwise near-identical P. aspilos (which lacks the spot) within the cluster of similar Phalloceros.
As in all poeciliids, the sexes are told apart at a glance by the male's gonopodium — the anal fin modified into a slender intromittent organ. In P. leptokeras the diagnostic detail lies in the gonopodial appendix, which is slender (the basis of the name leptokeras) and bears a large, sickle-shaped hook on its inner surface near the base. The female's urogenital papilla is likewise diagnostic, being flattened and turned to the left with a distinct lateral ramus. These reproductive-anatomy characters, rather than colour, do most of the work of identification in this group.
Habitat
The species is endemic to Brazil, restricted to the middle portions of the rio Paraíba do Sul drainage in Rio de Janeiro state, in the south-eastern Atlantic-Forest region. The IUCN describes its habitat simply as inland wetlands. The type locality is the rio São Francisco, a tributary of the rio Paquequer, at Fazenda São Francisco de Paula, Sapucaia, Rio de Janeiro — the source of both the scientific record and the "Sapucaia" common name. Recorded coordinates for the species fall near 22° S, 42° W, placing it squarely within the coastal Atlantic-Forest drainages.
Like other small Phalloceros, P. leptokeras is a freshwater, surface-and-midwater toothcarp of warm tropical waters, the kind of fish associated with shallow streams, marsh margins and slow vegetated waters rather than large rivers. The rio Paraíba do Sul basin is a heavily populated and industrialised Atlantic-coastal system, so the wetlands this fish occupies sit within a landscape under considerable human pressure, even though the species itself is not currently flagged as threatened.
Beyond the drainage-level range and wetland association, biotope detail specific to P. leptokeras is data sparse. Precise water chemistry, microhabitat preferences and seasonal behaviour have not been documented for this species in the accessible literature, and the figures given in this profile for temperature are inferred from the genus's general tropical-lowland tolerances rather than measured for the species itself.
Feeding
No diet study specific to Phalloceros leptokeras is available, and FishBase assigns it only a model-estimated trophic level of about 3.0 (± 0.4), derived from its size and from relatives rather than from gut-content data. On that basis the species is best characterised as a small omnivore.
Small Phalloceros, like the better-known P. caudimaculatus, are generalist micro-feeders that take a mix of algae, organic detritus, small aquatic invertebrates and insect larvae from shallow vegetated water. In the absence of species-specific data, P. leptokeras can reasonably be assumed to follow the same opportunistic, plant-and-invertebrate diet, foraging in the upper water column and among marginal vegetation.
Because the fish is not established in the aquarium trade, there is no body of hobbyist feeding experience to draw on. Were it to be kept, the husbandry would presumably mirror that of other small toothcarps — frequent small feedings of fine flake, micro-pellets and small live or frozen foods — but this is an extrapolation from congeners, not documented practice for this species.
Mating
Phalloceros leptokeras reproduces by internal fertilisation, the defining reproductive mode of the poeciliid livebearers. The male uses his gonopodium — the anal fin modified into a slender, mobile intromittent organ — to transfer sperm directly to the female, so that fertilisation occurs inside her body rather than over a spawning site. In this species the gonopodial appendix is notably slender and carries a large sickle-like hook near its base, structures that function in clasping and sperm transfer and that also serve the taxonomist as diagnostic characters.
The female's reproductive anatomy is correspondingly specialised: the urogenital papilla is flattened and turned to the left, with a lateral ramus, the configuration Lucinda used to separate this species from its closest relatives. As in poeciliids generally, females are able to store sperm internally, which allows a single insemination to fertilise more than one successive brood — though the specific storage duration and mating behaviour of P. leptokeras have not been studied.
Species-level detail on courtship, male competition and mate choice is data sparse for P. leptokeras. The general poeciliid pattern — persistent male pursuit combining brief display with attempted insemination, and no nest-building or territoriality — is the reasonable expectation, but it has not been documented for this fish in particular.
Breeding
Phalloceros is a viviparous, livebearing genus: females retain the fertilised eggs internally and give birth to free-swimming young rather than laying eggs. Development is lecithotrophic — the embryos are nourished by their own yolk reserves, with no placenta-like maternal provisioning — which distinguishes Phalloceros from the matrotrophic, placenta-bearing poeciliids. Importantly, Phalloceros is not among the genera (such as Poeciliopsis and Heterandria) known to practise superfetation, the carrying of multiple broods at different developmental stages at once; broods here develop as a single cohort.
For P. leptokeras specifically, gestation length and brood size are undocumented — data sparse. Congeners such as P. caudimaculatus are prolific, tolerant breeders that produce modest broods of fully formed fry and mature quickly, and FishBase rates this species' resilience as high, with an estimated population doubling time of under fifteen months, implying a comparably rapid reproductive turnover. These figures, however, are life-history estimates for the species' resilience category rather than direct brood observations.
Newborn poeciliid fry are miniature, independent fish able to swim and feed from birth, and in small livebearers like this they typically need dense cover to escape predation, including from adults. Whether and how P. leptokeras has ever been bred in captivity is not recorded; given its absence from the trade, any breeding account would be an extrapolation from the genus rather than a documented result for this species.
In the aquarium
Phalloceros leptokeras is effectively absent from the aquarium hobby. It is an obscure, recently described Atlantic-Forest endemic, not part of the ornamental trade, and there is no body of species-specific care experience to report. Any aquarist encountering it would most likely do so through a specialist livebearer collector or a biotope project rather than a shop.
If it were maintained, husbandry would presumably follow the well-worn template for small Phalloceros and similar toothcarps: a modest, densely planted aquarium with stable, warm tropical water, gentle filtration, and a varied diet of small foods. Such fish are generally hardy and peaceful, suited to a quiet species or biotope setup rather than a boisterous community, and — as livebearers — capable of building up numbers quickly if conditions suit them. All of this, though, is inference from congeners; it should not be read as tested guidance for this particular species.
The honest position for the atlas is that care-level specifics for P. leptokeras are data sparse. The value of the species lies in its place in the Phalloceros radiation and the Atlantic-Forest fauna of the rio Paraíba do Sul, not in an aquarium track record it does not have.
Conservation
Phalloceros leptokeras is assessed as Least Concern (LC) on the IUCN Red List under criteria version 3.1. The assessment was carried out on 7 November 2018 and published in the 2022-1 update. The species is endemic to the middle rio Paraíba do Sul drainage in Rio de Janeiro state, and at the time of assessment its population trend was unknown and the number of mature individuals was not estimated; the Least Concern listing reflects an absence of identified major threats rather than detailed population monitoring.
FishBase independently rates the species' intrinsic vulnerability as low (about 10 on a 100-point scale) and its resilience as high, consistent with a small, fast-reproducing livebearer. Such life-history traits tend to buffer a species against decline, but they do not offset the inherent risk that comes with a narrow, single-drainage range.
That narrow endemism is the real conservation consideration here. The rio Paraíba do Sul is one of south-eastern Brazil's most heavily used river systems, draining a densely populated and industrialised region of the Atlantic Forest — a biome that has lost most of its original cover. A wetland-dwelling fish confined to the middle portions of that basin is, in principle, exposed to habitat alteration, pollution and water management even while currently classed as Least Concern, which is why continued documentation of these small, easily overlooked endemics matters.