Taxonomy & naming
Phalloceros enneaktinos was described by the Brazilian ichthyologist Paulo Henrique Franco Lucinda in 2008, in his comprehensive revision of the genus published in Neotropical Ichthyology. That revision was a wholesale reworking of a group that the older literature had treated as essentially a single, hugely variable species — Phalloceros caudimaculatus. Lucinda recognised twenty-two species in the genus, twenty-one of them described as new, separated principally on details of the male gonopodium and gonopodial suspensorium together with female and meristic characters. Before this work, the populations now called P. enneaktinos would simply have been logged as P. caudimaculatus, and much of the scattered ecological and life-history information for the genus still travels under that older name.
The genus Phalloceros belongs to the family Poeciliidae, subfamily Poeciliinae — the New World livebearers that include guppies, mollies, swordtails and the mosquitofishes — within the order Cyprinodontiformes. Eschmeyer's Catalog of Fishes treats Phalloceros enneaktinos Lucinda, 2008 as the valid name. The genus name combines the Greek phallos, "penis," with keras, "horn," a reference to the conspicuous intromittent gonopodium that characterises these fishes. The specific epithet enneaktinos is a noun in apposition built from the Greek ennea, "nine," and aktinos, "ray," and alludes directly to the diagnostic count of nine dorsal-fin rays. In its Brazilian range the small Phalloceros are known by the vernacular names guaru, guaru-guaru and barrigudinho — the last meaning roughly "little pot-belly," a fitting label for a gravid livebearer.
Morphology
This is a very small fish. Lucinda's type material gives a maximum standard length of about 1 in in males and unsexed specimens and roughly 1.5 in in females, the females being the larger and deeper-bodied sex as is usual in the family. The body is the slender, slightly compressed, blunt-snouted form typical of the small Phalloceros, with the dorsal fin set well back on the body and the mouth angled upward for surface and mid-water feeding.
The single most reliable diagnostic feature, and the source of the species' name, is the dorsal-fin ray count: P. enneaktinos consistently has nine dorsal soft rays, whereas the great majority of its congeners — including P. mikrommatos, P. caudimaculatus, P. elachistos, P. titthos, P. harpagos, P. spiloura and P. reisi — carry eight, only very rarely nine. The anal fin has nine to eleven soft rays and the vertebral count is thirty-one to thirty-two. As in all poeciliids, the defining sexual structure is the male's gonopodium, a slender intromittent organ formed from modified anterior rays of the anal fin; the precise architecture of this organ and its supporting skeleton is what Lucinda used to separate the species of the genus, and it is the character to consult for any confident identification rather than colour, which in these small dark guarus is muted and variable.
Habitat
Phalloceros enneaktinos is a Brazilian endemic with one of the most restricted ranges of any described livebearer: it is known only from its type locality, the Córrego da Toca do Boi, a small coastal-forest stream near the Condomínio Laranjeiras in the municipality of Paraty (Parati), in the state of Rio de Janeiro. There is no confirmed record of the species from anywhere else, and the IUCN assessment explicitly notes that the limits of its distribution remain unknown and in need of further survey.
The habitat is freshwater wetland of the inland type — a permanent stream or creek draining the wet Atlantic Forest of the Serra do Mar coast. The fish has been reported to occur in relatively numerous shoals, aggregations of dozens of individuals, which is the social pattern typical of the small Phalloceros in their shallow, vegetated waters. Importantly for its conservation, the type locality falls within protected land: it lies inside the Área de Proteção Ambiental de Cairuçu (the Cairuçu Environmental Protection Area) and the Reserva Ecológica Estadual da Juatinga, so the species is documented from within at least one protected area. Detailed water-chemistry measurements specific to this stream are not recorded in the sources at hand; the broad picture is of warm, tropical, lowland freshwater, and precise temperature, pH and hardness figures for P. enneaktinos itself are data-sparse.
Feeding
The species is reported to be omnivorous, in keeping with feeding studies of the genus more broadly (work such as Mazzoni and colleagues and Casatti has documented an omnivorous, opportunistic diet in small Phalloceros). In practice this means a mixed intake of algae, organic detritus, diatoms and small aquatic invertebrates — micro-crustaceans, insect larvae and other tiny drifting and benthic prey — taken from the surface, the water column and the vegetated margins of the stream.
No aquarium-specific feeding regime is documented for P. enneaktinos, which is essentially absent from the ornamental trade. By analogy with its close relatives and the small livebearers generally, it would be expected to take small live and frozen foods such as microworm, newly hatched brine shrimp and finely chopped frozen items, alongside grazing on biofilm and algae, with several small feedings suiting a fish of this size better than a single large meal. These husbandry inferences should be read as reasonable extrapolation from the genus rather than as documented practice for this particular species.
Mating
Like all poeciliids except the egg-laying Tomeurus, Phalloceros enneaktinos reproduces by internal fertilisation. The male uses the gonopodium — the modified anal fin acting as an intromittent organ — to transfer sperm directly to the female; there is no external spawning of eggs and no nest. In the small livebearers this is generally accompanied by males pursuing and attempting to inseminate females more or less continuously, and by the female's capacity to store sperm internally, so that a single mating can fuel more than one brood. Species-specific courtship behaviour has not been described for P. enneaktinos, and what can be said about its mating system is inferred from the well-studied biology of the family and of its congeners.
The fine structure of that gonopodium is more than a reproductive detail here: it is the very character on which the species was erected. Lucinda's 2008 revision delimited the twenty-two species of Phalloceros chiefly on male gonopodial and suspensorial morphology together with female characters, which means that the intromittent organ doing the work of insemination is also the diagnostic feature distinguishing this fish from its near-identical relatives.
Breeding
Phalloceros enneaktinos is viviparous, as is the entire subfamily Poeciliinae apart from Tomeurus. The female carries the developing embryos internally and gives birth to free-swimming, fully formed young rather than laying eggs. The IUCN assessment, drawing on life-history work for the genus, records that Phalloceros nourishes its embryos lecithotrophically — that is, the young develop on the yolk supplied within the egg rather than being fed through a placenta-like connection (the matrotrophy seen in some other poeciliids). For this species the evidence points to yolk-based, lecithotrophic development rather than placental nourishment.
A notable feature of the genus, and one attributed to P. enneaktinos in its conservation assessment, is superfetation: the female gestates more than one brood simultaneously, carrying clutches of embryos at different developmental stages at the same time. The practical effect of superfetation is a more continuous trickle of young rather than discrete large broods, spreading reproduction across time. Specific figures for brood size, gestation length and frequency in P. enneaktinos are not recorded in the available sources and are best described as data-sparse; what is documented is the reproductive mode — viviparous, lecithotrophic, with superfetation — shared with its close relatives in the genus.
In the aquarium
Phalloceros enneaktinos is essentially not an aquarium fish. It is a recently described, single-locality endemic with no established presence in the ornamental trade, and there is no published husbandry record specific to it. The small Phalloceros that do reach hobbyists — generally circulating under the old name P. caudimaculatus, including the spotted "dusky millions fish" — are hardy, undemanding, peaceful little livebearers, and any keeping of this species would reasonably follow the same lines: a well-planted aquarium with gentle filtration, warm tropical temperatures, stable neutral to slightly hard water, and small foods offered often.
Because the wild population is confined to a single protected stream, the more important point for an aquarist is conservation-minded rather than husbandry-minded: this is not a fish to source from the wild casually, and any small Phalloceros in the hobby should be valued and bred responsibly rather than treated as disposable. The care notes here are an extrapolation from the genus, not a documented protocol for P. enneaktinos; the honest summary is that its aquarium requirements are unstudied.
Conservation
Phalloceros enneaktinos is assessed as Least Concern on the IUCN Red List. The assessment was prepared by the Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio), dated to its November 2018 evaluation and published in 2022, under assessment e.T187107A1823224. That a fish known from a single stream should be rated Least Concern rather than threatened reflects the assessors' judgement that no significant threats currently act on it: the population trend is recorded as unknown, but the type locality lies within protected land — the Cairuçu Environmental Protection Area and the Juatinga State Ecological Reserve — and no active pressures were identified.
The caveat behind that rating is the species' extreme range restriction and the thinness of the data. The assessment itself flags the need for research on the true limits of its distribution, on population trends, and on its life history, all of which are presently unknown. A fish confined to one small coastal-forest creek is, by its nature, vulnerable to any localised change — habitat alteration, water-quality decline or the arrival of introduced species — even if no such threat has yet been documented. The responsible reading is that P. enneaktinos is secure for now on present evidence, but that its narrow endemism makes continued protection of its single known stream the key to keeping it that way.