Taxonomy & naming
Phalloceros spiloura was described by Paulo H. F. Lucinda in 2008 in his monographic revision of the genus, "Systematics and biogeography of the genus Phalloceros Eigenmann, 1907 (Cyprinodontiformes: Poeciliidae: Poeciliinae), with the description of twenty-one new species" (Neotropical Ichthyology 6(2):113–158). Before that work, almost every spotted southern-Brazilian Phalloceros was lumped under the single name Phalloceros caudimaculatus, and Lucinda's revision split that catch-all into a genus of more than twenty narrowly distributed species, of which spiloura is one. Eschmeyer's Catalog of Fishes treats Phalloceros spiloura Lucinda, 2008 as the valid name, placing it in the genus Phalloceros within the subfamily Poeciliinae of the family Poeciliidae, the New World livebearers.
The genus name Phalloceros combines the Greek phallos, "penis," with keras, "horn," a direct reference to the gonopodium — the modified, intromittent anal fin of the males — which is the structure on which much of the genus's taxonomy turns. The species epithet spiloura derives from Greek roots meaning "spotted tail," a pointer to the diagnostic dark mark the species carries low on the caudal peduncle. Because the species was erected from within a previously undivided complex, older aquarium and faunal literature referring to "Phalloceros caudimaculatus" from the rivers of Santa Catarina and Rio Grande do Sul may in fact have been reporting this fish or one of its newly recognised neighbours.
Lucinda distinguished P. spiloura from its congeners on a suite of skeletal and gonopodial characters rather than colour alone, which is typical of the genus: the species are externally similar small spotted livebearers, and reliable identification rests on internal anatomy and locality. This makes spiloura a good example of how the modern systematics of small Neotropical poeciliids has uncovered hidden diversity within what the hobby long treated as a single, variable species.
Morphology
Phalloceros spiloura is a small, slender livebearer with the marked sexual size difference usual in the genus: males reach only about 1 in standard length while females grow larger, to roughly 1.5 in SL. The body is the typical elongate, slightly compressed poeciliid shape, with a dorsally placed mouth suited to surface and mid-water feeding and a dorsal fin set well back over the anal region. Meristic counts recorded for the species are 7–9 soft dorsal rays, 9–11 soft anal rays, and 31–32 vertebrae.
The species was diagnosed by Lucinda on several autapomorphies — characters unique to it within the genus. The most visible is a rounded dark spot on the lower half of the caudal peduncle, near the base of the lowest caudal-fin rays, the feature that gives the fish both its epithet ("spotted tail") and its English name, Three point toothcarp. Females additionally carry a patch of dark pigment on the last rays of the anal fin. The remaining diagnostic traits are internal: the two halves of the paired gonopodial appendix are straight and set perpendicular to ray R3, the gonapophysis of the fourteenth vertebra is straight in adult males, and an anterior orbital bone is present.
In overall coloration the fish is modest — a pale to olive body scattered with small dark blotches and speckles, like its relatives — rather than vividly coloured. The males' defining external feature is the gonopodium, formed from elongated and modified anal-fin rays; in this genus its fine structure, including the form of the terminal hooks and appendices, is the primary tool ichthyologists use to tell the species apart. There is no record of ornamental finnage or selectively bred colour forms, as this species has not entered the broad aquarium trade in the way the guppy or the dusky millions fish have.
Habitat
Phalloceros spiloura is endemic to southern Brazil, where FishBase records it from the Rio Iguaçu and from a series of independent coastal drainages of Rio Grande do Sul and Santa Catarina — including the rio Tubarão, rio Itajaí-Açu, rio Itapocu, rio Mampituba, rio Cubatão (North) and rio Tramandaí systems. This is a comparatively small, well-defined range along the Atlantic slope and the upper Paraná tributary of the Iguaçu, the kind of restricted distribution that Lucinda's revision revealed for many of the genus's species once the old broad "caudimaculatus" concept was broken up.
Like other members of the genus, it is a freshwater, benthopelagic fish of warm, lowland to mid-elevation waters. Although habitat data specific to spiloura are sparse, Phalloceros species characteristically occupy slow-flowing, densely vegetated streams, river margins, ditches and ponds, where the dense aquatic and marginal plants provide both cover and the algal and invertebrate food that small livebearers depend on. Some members of the genus tolerate slightly brackish conditions in coastal reaches, consistent with the species' presence in coastal drainages, though the extent of that tolerance in spiloura specifically is not documented.
FishBase classifies the species as having high resilience, with an estimated population doubling time of under fifteen months, and a low vulnerability to fishing — figures that reflect the fast-breeding, opportunistic life history typical of small poeciliids rather than any specific study of this fish's populations. Beyond its drainage list and broad ecological category, detailed field information on water chemistry, microhabitat and seasonal behaviour for P. spiloura is essentially absent from the literature; data here are sparse and should be read as genus-level inference.
Feeding
No diet study exists for Phalloceros spiloura specifically; what is known is inferred from the genus, which FishBase places at a trophic level of roughly 3.0 — that of a small omnivore. Members of Phalloceros are generalist grazers and micro-predators, taking a mixed diet of algae and biofilm, small aquatic invertebrates, insects and insect larvae, and crustaceans, with the balance shifting according to what each weedy stream or pond margin offers. Their upturned mouths suit feeding at and just below the surface, where they pick at drifting items and graze soft-leaved plants and the aufwuchs that coats them.
This omnivorous, opportunistic feeding is part of why the genus is so successful in vegetated still and slow waters, and why its better-known member, the dusky millions fish, was historically spread around the world as a mosquito-larva control agent. Larval insects, including mosquito larvae, are a natural component of the diet of small Phalloceros, and the fish's near-constant grazing keeps it well fed in the productive, plant-rich habitats it favours.
In captivity — where this species is rarely kept — a Phalloceros of this type would be expected to take small flake, micro-pellets and a range of small live and frozen foods such as Daphnia and brine shrimp, alongside a useful share of algal and vegetable matter. As with other small livebearers, frequent small feedings suit it better than infrequent large ones.
Mating
Phalloceros spiloura is a viviparous livebearer with internal fertilisation, the defining reproductive mode of the family Poeciliidae. Fertilisation is achieved by the male's gonopodium, an intromittent organ built from modified rays of the anal fin: the male manoeuvres to bring the gonopodium alongside the female and transfers sperm directly, rather than fertilising eggs externally as most fishes do. In Phalloceros the precise architecture of this organ — the orientation of the paired appendices and the form of its terminal elements — is so distinctive between species that it serves as a primary taxonomic character, and indeed several of the autapomorphies Lucinda used to define spiloura are features of the gonopodial skeleton.
As in related poeciliids, courtship is expected to be male-driven and more or less continuous, with the smaller males pursuing the larger females; many livebearers combine display with sneak-mating attempts, and females of the family are commonly able to store sperm internally so that a single mating can fertilise several successive broods. Detailed observations of courtship behaviour specific to P. spiloura have not been published, so these patterns are drawn from the well-studied biology of the genus and family rather than from this species directly.
The marked size dimorphism — females reaching about 1.5 in against the males' 1 in — is the externally obvious correlate of this internal-fertilisation, live-bearing system: the female must carry and nourish developing embryos, while the male is small and specialised around the delivery of sperm via the gonopodium.
Breeding
Like all members of the family, Phalloceros spiloura gives birth to free-swimming young rather than laying eggs. No brood data have been published for this species in particular, but the reproductive biology of the genus is well enough known to sketch the likely picture: after internal fertilisation the female carries the developing embryos for a gestation of roughly four weeks, the exact length depending on temperature, and then releases fully formed, independent fry. In the closely related dusky millions fish (P. caudimaculatus) broods can run to several dozen fry in a well-grown female and fewer in younger ones, and a comparable, though probably smaller, output would be expected in this more diminutive species.
There is no parental care: as in guppies and other small poeciliids, the newborn fry must fend for themselves from birth and are at immediate risk of predation by adults — including their own parents — and by other fishes. In the wild the dense vegetation of the streams and ponds these fish inhabit provides the cover that lets a share of each brood survive, and any captive breeding would likewise depend on heavy planting or separation of the fry. Superfetation — the carrying of more than one brood at different developmental stages simultaneously — occurs in some poeciliid lineages but has not been documented for Phalloceros, and should not be assumed for this species.
The combination of live birth, fast maturation and the sperm-storage capacity typical of the family underlies the high resilience FishBase attributes to the genus, with population doubling times estimated at under fifteen months. In practical terms it means a small founding group in suitable habitat can build a population quickly — the same trait that has made other Phalloceros species such effective colonisers where they have been introduced.
In the aquarium
Phalloceros spiloura is essentially an ichthyologist's fish rather than a hobby staple: it is not a fixture of the ornamental trade, and reliable, species-specific husbandry information is not available. What guidance exists has to be borrowed cautiously from its better-known relatives in the genus, while remembering that spiloura is smaller and more geographically restricted than the widely kept dusky millions fish.
The genus as a whole is undemanding and peaceful, suited to a well-planted aquarium with gentle flow and stable, clean water; small livebearers of this type are best kept in groups, typically with more females than males to spread out male attention. Phalloceros caudimaculatus, the congener most often kept, is notably a cool-water fish, comfortable around 63–72 °F, and tolerant of a broad pH range — a useful reminder that this southern-Brazilian genus is not a tropical hothouse group and that spiloura, from the temperate-influenced drainages of Santa Catarina and Rio Grande do Sul, may share that preference for cooler conditions.
Anyone keeping this fish should treat husbandry numbers as provisional and aim for stability and dense planting above precise targets, and — as with any livebearer from a restricted range — should never release surplus fish into local waters, since the genus's fecundity and adaptability make it a capable invader outside its native streams.
Conservation
Phalloceros spiloura is assessed as Least Concern on the IUCN Red List (species ID 186358), an assessment dated 7 November 2018 and current under Red List Version 2025-2. The fish is not listed on CITES or the CMS and is harmless to humans. The Least Concern rating reflects the species' broad occurrence across several southern-Brazilian drainages and the fast-breeding, resilient life history shared by small poeciliids, rather than any indication that it is rare or declining.
That said, the species' value is partly as a marker of how much unrecognised diversity the genus held: until Lucinda's 2008 revision, fishes like spiloura were invisible within the broad "Phalloceros caudimaculatus" concept, and several of its newly described congeners have much narrower ranges that warrant closer conservation attention. The coastal Atlantic-slope drainages of Santa Catarina and Rio Grande do Sul where spiloura lives face the usual pressures on small-stream habitats — deforestation, agriculture, urban runoff and water abstraction — so continued monitoring of these endemic livebearers is sensible even where the current assessment is reassuring.
For the genus more broadly there is also a flip side to the conservation ledger: relatives such as P. caudimaculatus have been introduced well beyond their native range and become invasive in places, so the conservation concern for Phalloceros is as much about keeping these adaptable fish out of foreign waters as about protecting them at home.