Taxonomy & naming
Phalloceros megapolos was described by Paulo H. F. Lucinda in 2008 as part of a comprehensive revision of the genus Phalloceros (Neotropical Ichthyology 6(2): 113–158). Before that revision, the small spotted livebearers of south-eastern South America were lumped almost entirely under the single name Phalloceros caudimaculatus, and Lucinda's work split that broad concept into twenty-one species, of which megapolos was one of the newly described forms. Eschmeyer's Catalog of Fishes treats Phalloceros megapolos Lucinda, 2008 as a valid name, placing it in the genus Phalloceros within the family Poeciliidae, the New World livebearers, in the order Cyprinodontiformes.
The genus name Phalloceros is built from the Greek phallos, "penis," and keras, "horn," a reference to the male's gonopodium — the modified, intromittent anal fin that defines the live-bearing poeciliids. The species is distinguished within the genus by features of that gonopodium and the male skeleton: Lucinda's diagnosis notes that the appendix of the gonopodium is greatly expanded into wing-like projections, and that adult males have the seventh pleural rib longer than the eighth. The common name recorded in FishBase is "Lindo toothcarp," a nod to the coastal region where the species occurs.
Morphology
Phalloceros megapolos is a very small fish, in keeping with its genus. According to Lucinda's description, males reach about 1 in in standard length and females are larger, attaining roughly 1.5 in SL — the female-larger size dimorphism that is typical of poeciliid livebearers. The meristic counts given in the original description are eight dorsal soft rays, nine to ten anal soft rays, and thirty-two vertebrae.
Like other members of the genus, it is a slender, laterally compressed toothcarp with a small upturned mouth suited to surface and mid-water feeding. The most diagnostic structures are in the male: the gonopodium carries an appendix that is greatly expanded into wing-like projections, a feature Lucinda treated as an autapomorphy of the species, and the seventh pleural rib is elongated relative to the eighth. Detailed published notes on live colouration are sparse; congeners in the genus are generally olive to grey-bodied fishes that may carry dark flank or caudal spotting, but specific pigment patterns for megapolos are not well documented in the accessible literature, so this is an area where the record is genuinely thin.
Habitat
The species is endemic to Brazil, and within Brazil to a compact set of coastal drainages in Paraná State. FishBase records it from the drainages of the rio São João and the rio Cubatão (Norte), together with small adjacent streams flowing into the Baía de Guaratuba, the region that anchors its type locality. This is part of the Atlantic Forest coastal slope of south-eastern Brazil, an area of high freshwater endemism where many short, independent rivers drain the Serra do Mar directly to the sea.
FishBase classes the species as a freshwater, benthopelagic fish of tropical climate. Beyond that broad characterisation, precise field measurements of the waters it inhabits — temperature ranges, pH, hardness — are not published in the sources at hand; small coastal Atlantic-Forest streams of this type are typically warm, soft and slightly acidic, but applying those generic figures to megapolos specifically would be an inference rather than a measured fact. The narrow, fragmented coastal distribution is itself the most notable feature of its habitat, and is the reason the species was not separated from its relatives until the genus was revised.
Feeding
No diet study specific to Phalloceros megapolos has been located, and FishBase assigns it an estimated trophic level of about 3.0, consistent with a small omnivore that takes a mixture of animal and plant material. Members of the genus are surface- and mid-water foragers with small upturned mouths, and in their native streams such fishes typically feed on aquatic and terrestrial invertebrates, insect larvae, small crustaceans, algae and organic detritus.
In the absence of species-level data, the safest statement is that megapolos is an opportunistic micro-omnivore in the mould of its congeners and of livebearers generally. The very small adult size means it is a near-continuous grazer on small items rather than a pursuit predator. Because the species is essentially absent from the aquarium trade, there is no body of hobbyist feeding experience to draw on either; this is a case where the honest answer is that the diet is known only in broad outline.
Mating
Phalloceros megapolos reproduces by internal fertilisation, the defining reproductive mode of the Poeciliidae. The male carries a gonopodium — a slender intromittent organ formed from modified rays of the anal fin — which he uses to transfer sperm directly to the female. In megapolos the gonopodial appendix is notably modified into wing-like projections, a structural detail that helped distinguish the species in Lucinda's revision; in poeciliids such species-specific gonopodial morphology is often important in how males clasp and inseminate females.
As in other small livebearers, mating is expected to be more or less continuous rather than seasonal and tied to a nest or territory, with males pursuing females and attempting insemination. Detailed behavioural observations — courtship displays, the balance of display versus sneak copulation, or whether females exercise mate choice as they do in better-studied poeciliids such as the guppy — have not been published for this species. What can be stated with confidence is the mechanism: internal fertilisation via the gonopodium, with the female then carrying and nourishing the developing young.
Breeding
Like other Phalloceros and the great majority of poeciliids, megapolos is viviparous: the female retains fertilised eggs internally and gives birth to free-swimming, fully formed young rather than laying eggs. The genus is understood to be lecithotrophic — the developing embryos are nourished mainly by yolk supplied in the egg rather than by an extensive placental connection — and no specialised matrotrophy or confirmed superfetation has been documented for Phalloceros, in contrast to some other poeciliid lineages where superfetation (carrying broods at several developmental stages at once) is well established.
Species-specific reproductive figures — gestation length, brood size, the number of broods a female can produce from stored sperm — have not been published for megapolos, and the cache turns up no brood-size or gestation data for it. By analogy with small congeners, broods are likely to be modest, on the order of a handful to a few dozen fry, with young born able to swim and feed immediately. FishBase rates the species' resilience as high, with a minimum population doubling time of under fifteen months, which is consistent with the rapid, repeated reproduction typical of livebearers but is a modelled estimate rather than a direct measurement of its fecundity.
In the aquarium
Phalloceros megapolos is, for practical purposes, absent from the aquarium hobby: no dedicated care or biotope profile for the species could be found, and it does not circulate in the trade. Any keeping advice is therefore extrapolated from the genus and from small Atlantic-Forest coastal livebearers in general, and should be read as informed guesswork rather than tested practice.
A fish of this size and origin would, by the usual standards for small South American livebearers, suit a modest, well-planted aquarium with gentle filtration, warm water and a quiet community of similarly small, peaceful tankmates. As with all livebearers, a sex ratio skewed toward females reduces harassment, and dense planting or floating cover lets fry survive among adults that will otherwise eat them. The genuinely important point for an aquarist is the opposite of husbandry detail: this is a narrowly distributed endemic with little or no presence in captivity, so it is far more a subject for catalogue and field study than a candidate for the home tank.
Conservation
Phalloceros megapolos is assessed as Least Concern on the IUCN Red List. The assessment was carried out by the Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio) and dated 7 November 2018; the population trend is listed as unknown. FishBase similarly flags the species as having low vulnerability to fishing pressure.
That Least Concern listing should be read against the species' narrow, fragmented range in the coastal drainages of Paraná. The Atlantic Forest of south-eastern Brazil is one of the most heavily impacted biomes in South America, and small endemic stream fishes there can be sensitive to local habitat loss, water abstraction and pollution even when the overall assessment is reassuring. With the population trend unknown and the distribution restricted to a handful of small systems around the Baía de Guaratuba, the species is best regarded as currently secure but worth monitoring rather than demonstrably abundant.