Taxonomy & naming
Phalloceros buckupi was described by Paulo H. F. Lucinda in 2008 as part of his comprehensive revision of the genus, published in Neotropical Ichthyology (6(2):113–158). Before that revision, most southern-Brazilian Phalloceros populations were lumped under the single name Phalloceros caudimaculatus; Lucinda's work split that catch-all into about two dozen species distinguished largely by the fine architecture of the male gonopodium and the female urogenital region. Eschmeyer's Catalog of Fishes treats Phalloceros buckupi Lucinda, 2008 as the valid name, placing it in the family Poeciliidae, subfamily Poeciliinae — the New World livebearers that include guppies, mollies and mosquitofishes.
The holotype (MCP 30472) was collected at the rio Jacareí where it crosses the BR-277 road on the border of Paranaguá and Morretes municipalities, Paraná, Brazil, in July 1988 by Paulo A. Buckup and colleagues. Paratypes extend the known material to artificial ditches near Guaraguaçu in Paraná and a stream affluent to the rio Parati in Araquari, Santa Catarina. The species is diagnosed within the genus by details of the female urogenital papilla — curved to the right and positioned laterally — by the anal aperture's border contacting the first anal-fin ray, and by features of the gonopodial appendix, the kinds of microscopic characters on which the whole revised genus turns.
Morphology
Phalloceros buckupi is a very small fish even by livebearer standards, and it shows the strong female-biased size dimorphism typical of the genus. Females reach about 1.5 in in standard length (the type series ran from roughly 1–1.5 in SL), while males are markedly smaller, only about 0.5–1 in SL. Meristically it carries 7 to 8 dorsal soft rays, 9 anal rays in males and 10 in females, five pelvic rays, 11 to 12 branched caudal rays, 28 to 30 scales in the longitudinal series, and 32 vertebrae — a compact, lightly built poeciliid body plan.
Colour, as recorded from alcohol-preserved type specimens, is a pale brown ground that darkens over the upper half of the body, overlaid by a reticulate pattern produced by brown chromatophores concentrated at the scale borders — the net-like marking that runs through much of the genus. A lateral dark-brown blotch sits over roughly the sixteenth scale of the longitudinal series, about one scale wide and three scales tall, and faint light-brown vertical bars may run down the flanks, though these are variable and absent in some individuals. A band of dark chromatophores marks the distal edge of the dorsal fin and the area near its base. Live colour and any breeding intensification are not documented in the cited sources, so beyond this preserved-specimen description the appearance of fresh fish is data sparse.
Habitat
The species is restricted, as far as the type material shows, to the rio Jacareí drainage and neighbouring streams that flow into the Baía de Paranaguá on the Atlantic coast of Paraná State, southern Brazil, with paratype localities reaching into adjacent Santa Catarina. These are low-elevation coastal drainages of the Brazilian Atlantic Forest belt — the holotype came from the river where it crosses a highway near sea level, at roughly 25°35′S 48°43′W, and other specimens from artificial ditches and small stream affluents.
FishBase classifies Phalloceros buckupi as a freshwater, benthopelagic, tropical species. That ecological profile — small coastal-stream and ditch habitats — fits the genus as a whole, whose members occupy slow margins, vegetated shallows and disturbed waters such as drainage channels. Specific physical and chemical data for the water it lives in (temperature, pH, flow) are not given in the available sources, so any figures beyond "warm tropical coastal freshwater" would be guesswork.
Feeding
No diet study specific to Phalloceros buckupi is available in the cited sources; FishBase lists only an estimated trophic level of about 3.0 (±0.4), consistent with a small omnivore that takes a mix of animal and plant matter rather than a strict carnivore or herbivore.
By genus-level analogy, Phalloceros are opportunistic micro-omnivores of vegetated shallows and stream margins, grazing on algae, biofilm and detritus and picking off small aquatic invertebrates, insect larvae and other tiny drifting items. Several species in the genus have, like the better-known livebearers, been noted to take mosquito larvae. In the absence of a published gut-content analysis for this species, that general picture is the most that can responsibly be said.
Mating
Phalloceros buckupi is a viviparous poeciliid, so fertilisation is internal. As in all the New World livebearers, the male's anal fin is modified into a gonopodium — a slender, mobile intromittent organ built from elongated and specialised anal-fin rays — through which sperm is transferred to the female. Much of the taxonomy of the genus, including the features that separate this species from its congeners, rests precisely on the fine structure of that gonopodium and its terminal appendix, underlining how central the male copulatory apparatus is to both the biology and the classification of these fishes.
The complementary female character used to diagnose the species — a urogenital papilla curved to the right and set laterally, with the anal aperture's border touching the first anal-fin ray — reflects the close mechanical fit between the sexes that internal fertilisation demands. Beyond this morphology, no courtship behaviour, mate-choice or sperm-storage data specific to Phalloceros buckupi are recorded in the available sources; in many poeciliids females can store sperm and produce successive broods from a single mating, but whether and to what degree this species does so is undocumented here.
Breeding
As a livebearing poeciliid, Phalloceros buckupi gives birth to free-swimming young rather than laying eggs, with the female carrying the developing embryos internally until they are born fully formed. This much follows securely from its placement in the genus and family. The original description, however, does not detail the reproductive mode in numbers, and FishBase carries no populated reproduction data — there are no published gestation periods, brood sizes or notes on whether this species shows superfetation (the simultaneous carrying of broods at different developmental stages, known in some poeciliids).
Where figures are needed, only cautious genus-level analogy is possible: small Phalloceros are generally lecithotrophic to weakly matrotrophic livebearers that produce modest broods, on the order of a few to a few dozen fry, with the small adult size capping how many young a female can carry at once. These are general expectations for the genus, not measured values for Phalloceros buckupi, and should be read as such — the species-specific breeding data are simply sparse.
In the aquarium
Phalloceros buckupi has little presence in the aquarium trade. It is a small, recently described coastal-Brazil livebearer, and no dedicated species-level husbandry account could be found; what little can be offered comes from how the genus behaves in tanks rather than from records of this fish specifically.
Genus-wide, Phalloceros are kept as small, peaceful, hardy livebearers well suited to densely planted aquaria with gentle filtration, where their grazing habits and constant activity show to advantage. They are best maintained in groups, tolerate a fair range of ordinary tropical conditions, and — as with most poeciliids — breed readily enough that the newborn fry need cover such as floating or fine-leaved plants to escape being eaten by the adults. Anyone keeping a species this poorly documented should treat care parameters conservatively and watch the fish rather than rely on published numbers, because for Phalloceros buckupi in particular those numbers do not yet exist.
Conservation
Phalloceros buckupi is assessed as Least Concern on the IUCN Red List (assessment 1814882, species id 186555), evaluated on 18 May 2022. Despite its narrow known range in the coastal drainages of Paraná and adjacent Santa Catarina, the species has not been flagged as threatened at the global level, and FishBase rates its resilience as high — a population doubling time of under fifteen months — and its fishing vulnerability as low. It is harmless to humans and not listed under CITES or CMS.
That said, narrowly distributed coastal-stream fishes of the Brazilian Atlantic Forest live in a region under heavy pressure from urbanisation, agriculture and habitat alteration, and several of the type localities are already artificial ditches and roadside crossings rather than pristine streams. The Least Concern listing should therefore be read as the current global verdict on a species whose detailed population trends are not closely monitored, rather than as evidence that its habitat is secure.