Taxonomy & naming
Phalloceros lucenorum was described by Paulo H. F. Lucinda in 2008 as part of his comprehensive systematic revision, "Systematics and biogeography of the genus Phalloceros Eigenmann, 1907" (Neotropical Ichthyology 6(2):113–158). That single monograph transformed the genus: where the literature had long recognised essentially one wide-ranging species, Phalloceros caudimaculatus, Lucinda diagnosed and named twenty-one new species, of which P. lucenorum is one. Eschmeyer's Catalog of Fishes treats Phalloceros lucenorum Lucinda, 2008 as a valid name within the family Poeciliidae (subfamily Poeciliinae), the New World livebearers.
The genus name combines the Greek phallos (penis) and keras (horn), a reference to the prominent, hook-bearing gonopodium that characterises these fishes and that supplies most of the characters used to tell the species apart. The specific epithet lucenorum is a patronym honouring the ichthyologists Carlos A. S. de Lucena and Zilda Margarete Seixas de Lucena for their contributions to Neotropical ichthyology — the plural Latin genitive ("of the Lucenas") recognising both.
Because the species was erected on fine details of gonopodial and urogenital morphology rather than on conspicuous external colour, P. lucenorum is most reliably separated from its congeners by male anatomy. Lucinda's diagnosis cites an autapomorphy — a rectangular (rather than rounded) tip on the hook of the left half of the gonopodial appendix — together with the absence of the right gonopodial hook while the left is present, an asymmetrical gonopodial appendix whose right half is wider than the left, and a female urogenital papilla curved to the right and set laterally. A wide, square-shaped lateral spot in large specimens distinguishes it from P. uai, while the absence of a caudal-peduncle spot separates it from P. spiloura.
Morphology
Phalloceros lucenorum is a very small poeciliid. According to the type material reported by Lucinda (2008), males and unsexed specimens reach about 1 in standard length, while females are larger at up to roughly 1.5 in SL — the female-biased size dimorphism typical of the family. Meristic counts are modest and consistent with the genus: 7–8 dorsal soft rays, 9–11 anal soft rays, and 31–32 vertebrae.
As in all Phalloceros, the most diagnostically loaded structure is the male gonopodium, the intromittent organ formed from modified anal-fin rays. In P. lucenorum the gonopodial appendix is conspicuously asymmetrical, the right half wider than the left, and its hooks are unusual: the right hook is absent while the left is present, and the tip of that left hook is squared off rather than rounded — the autapomorphy on which the species was founded. Females carry a urogenital papilla curved to the right and positioned laterally, with the anal aperture border in contact with, or very close to, the first anal-fin ray.
In body markings the species is understated. Large specimens show a wide, square-shaped lateral spot on the flank, while in adult females the lateral spot is elliptical to roundish and never forms a vertical bar. There is no spot on the caudal peduncle. These pattern details are subtle but taxonomically useful, separating P. lucenorum from the several superficially similar congeners that share the Ribeira de Iguape and neighbouring coastal drainages.
Habitat
Phalloceros lucenorum is a narrow endemic. It is known only from the Rio Juquiá, a tributary of the Rio Ribeira de Iguape drainage in São Paulo state, south-eastern Brazil, which serves as both its type locality and its sole documented range. This is a single-drainage distribution — among the most restricted in a genus whose members are frequently localised to one river system or a cluster of adjacent coastal streams.
FishBase classifies the species as a freshwater, benthopelagic, tropical fish. Detailed in-situ ecological data specific to P. lucenorum are sparse, but the broader Phalloceros pattern is well established: these are fishes of slow-flowing streams, river margins, ponds and densely vegetated shallows, where small livebearers shelter among marginal plants and graze the surfaces around them. The Ribeira de Iguape basin drains the Atlantic Forest slope of São Paulo and Paraná, a region of high freshwater endemism, which makes a single-river restriction biologically unsurprising for a small, poorly dispersing livebearer.
Beyond its drainage and general stream habit, the specific water chemistry, depth profile and seasonal regime that P. lucenorum experiences in the wild have not been separately characterised in the accessible literature (data sparse), and care should be taken not to project the well-documented tolerances of the wide-ranging P. caudimaculatus directly onto this endemic.
Feeding
No diet study specific to Phalloceros lucenorum appears in the accessible literature, and FishBase records only a generic trophic level of about 3.0 for the species. The feeding biology must therefore be read at the genus level, where the pattern is consistent and well attested: Phalloceros are opportunistic omnivores and micro-grazers.
In the wild, congeneric Phalloceros take a mixture of algae and aufwuchs scraped from plant and substrate surfaces, small aquatic invertebrates, insect larvae and small crustaceans, with the balance of plant to animal matter shifting according to what each habitat offers. Like other small poeciliids, they are near-continuous foragers in vegetated shallows rather than pursuit predators.
For a fish this small and this little-kept, captive feeding can only be inferred from its relatives. Phalloceros generally accept fine flake and micro-pellets readily and benefit from small live and frozen foods — Daphnia, newly hatched brine shrimp, microworm — alongside the algal and vegetable component their grazing habit implies. These are reasonable husbandry expectations rather than species-specific records for P. lucenorum.
Mating
Like all Poeciliinae, Phalloceros lucenorum reproduces by internal fertilisation. The male transfers sperm to the female using the gonopodium, the rod-like intromittent organ built from modified anal-fin rays — the structure that gives the genus its name (phallos + keras, "penis-horn") and that supplies the species' diagnostic characters. The asymmetry of that organ in P. lucenorum, with its single left hook and squared-off hook tip, is exactly the kind of fine genital detail that distinguishes closely related livebearer species and that is thought to function in the mechanics of mating.
Following the general poeciliid pattern, males are expected to pursue females persistently and to attempt copulation by swinging the gonopodium forward to make brief internal contact, rather than defending territories or building any structure. Females of livebearing poeciliids commonly store viable sperm internally, allowing successive broods to be produced from a single insemination, although this has not been documented specifically for P. lucenorum.
The species-specific courtship behaviour, mate-choice criteria and any display repertoire of P. lucenorum are undescribed (data sparse). What can be stated with confidence is the reproductive mode — internal fertilisation via the gonopodium — which is invariant across the subfamily and central to the genus's taxonomy.
Breeding
Phalloceros lucenorum is a viviparous livebearer: rather than scattering eggs, the female carries developing young internally and gives birth to free-swimming fry. Within the Poeciliinae this development is lecithotrophic — the embryos are nourished mainly by yolk laid down before fertilisation rather than by a placental supply — and there is no parental care after birth.
Species-specific brood data for P. lucenorum are not available, so the quantitative picture comes from the genus. In Phalloceros generally, gestation lasts on the order of four weeks and is temperature-dependent, and broods range up to several dozen fry, with young or small females producing fewer. The newborns are fully formed miniature fish that swim and feed at once, and — as is usual for livebearers — adults will readily predate fry, so dense vegetation or floating cover is what allows young to survive. FishBase rates the species' resilience as High, with a minimum population doubling time of under fifteen months, consistent with the fast, fecund life history of small poeciliids.
Superfetation — the simultaneous carrying of broods at different developmental stages — occurs in several poeciliid lineages but has not been reported for Phalloceros lucenorum, and should not be assumed for it. As with mating, the breeding account here rests on genus-level biology plus the species' FishBase life-history scores rather than on direct observation of P. lucenorum (data sparse).
In the aquarium
Phalloceros lucenorum is essentially an unkept species in the ornamental trade. It is a small, narrowly endemic Brazilian livebearer described from preserved material, and its specific aquarium parameters are not separately documented (data sparse). The species should not be confused with its much better-known congener Phalloceros caudimaculatus, the "dusky millions fish," whose husbandry is widely published but whose tolerances cannot simply be transferred onto this São Paulo endemic.
What little practical guidance exists is therefore genus-level and provisional. Phalloceros are generally peaceful, undemanding small livebearers suited to well-planted aquaria kept in groups, with several females per male to spread out male attention — the standard precaution against the persistent courtship of poeciliid males. They favour clean, stable, vegetated conditions, and at least some Phalloceros prefer distinctly cooler water than tropical guppies, which is a relevant caution when guessing at conditions for a subtropical-to-tropical Atlantic-slope species.
Given the absence of species-specific records, the responsible position is restraint: a keeper encountering genuine P. lucenorum (as opposed to a mislabelled caudimaculatus) should treat published care numbers as congener proxies, prioritise habitat-style cleanliness, planting and a small social group, and avoid asserting precise temperature or chemistry targets that the literature does not support for this fish.
Conservation
Phalloceros lucenorum is assessed as Least Concern (LC) on the IUCN Red List, in an assessment dated 7 November 2018 (assessment 186498/1813974). The species is not listed by CITES or the CMS. Notably, the LC rating stands despite a very narrow distribution: the fish is known only from the single Rio Juquiá drainage, the kind of restricted range that frequently underlies a threatened listing in other freshwater fishes.
That combination — a single-drainage endemic nonetheless judged Least Concern — means the assessment rests on the population appearing stable and not facing a documented, range-wide threat at the time of evaluation, rather than on the fish being widespread. For a narrowly endemic species, however, that security is conditional: any localised pressure on the Rio Juquiá or the wider Ribeira de Iguape basin — habitat alteration, pollution, water abstraction or the spread of non-native fishes within the Atlantic Forest drainages — would fall disproportionately on a fish with nowhere else to live.
From an aquarist's standpoint the practical conservation note is simple. P. lucenorum is not a trade staple and is unlikely to be a routine purchase; where small Brazilian Phalloceros do enter collections, the relevant responsibilities are accurate identification (so that genuine endemics are not lost under a generic caudimaculatus label) and never releasing livebearers into local waters, since this lineage's fecundity and adaptability make escaped poeciliids a recurring invasive problem worldwide.