Taxonomy & naming
Phalloceros tupinamba was described by Paulo H. F. Lucinda in 2008, in his comprehensive revision of the genus published in Neotropical Ichthyology (6(2):113–158). That revision was a watershed for the group: where earlier workers had lumped most of these fishes under the wide-ranging Phalloceros caudimaculatus, Lucinda recognised that the complex contained many narrowly distributed species and erected twenty-one new ones, of which P. tupinamba is one. Eschmeyer's Catalog of Fishes treats Phalloceros tupinamba Lucinda, 2008 as the valid name, placing the species in the family Poeciliidae, subfamily Poeciliinae, within the order Cyprinodontiformes.
The genus name combines the Greek phallos, "penis," with keras, "horn," a reference to the prominent gonopodium that defines male poeciliids. The specific epithet tupinamba honours the Tupinambá, the indigenous people who inhabited this part of the south-eastern Brazilian coast in pre-Cabralian times, before Portuguese contact. The fish carries the English book name "Tupi toothcarp."
Diagnostically, Lucinda separated P. tupinamba from its congeners — except the closely allied P. leptokeras and P. aspilos — by a suite of reproductive characters: the presence of a left-turned lateral ramus on the female urogenital papilla, and a large, sickle-like hook on the inner surface of the gonopodial appendix near its base, together with the absence of the inconspicuous vertical bars that mark many relatives. From the very similar P. aspilos it is told apart by possessing a lateral spot and by the longer predorsal length of males. The lateral spot is vertically elongated, about one scale long, and sits on the sixteenth to nineteenth scale of the longitudinal series.
Morphology
Phalloceros tupinamba is a small fish showing the marked sexual size dimorphism typical of poeciliids, in which the female is the larger sex. Males reach roughly 1 in in standard length, while females grow noticeably larger, to about 1.5 in SL. The body is the slender, slightly compressed form usual for the genus, with the dorsal fin set well back and a rounded caudal fin. Meristic counts are modest: seven to eight soft dorsal rays, nine to eleven soft anal rays, and thirty-one to thirty-two vertebrae.
Colour is understated rather than showy. The most reliable external marking is a small lateral spot, vertically elongated and roughly one scale long, positioned on the sixteenth to nineteenth scale of the longitudinal line; this spot is one of the features separating the species from the otherwise very similar P. aspilos. Unlike several of its congeners, P. tupinamba lacks the faint vertical bars that can run along the flanks, which is part of how Lucinda diagnosed it.
The sexes differ most clearly in the anal fin. In the male, the anterior anal-fin rays are modified into a gonopodium, the intromittent organ used to deliver sperm to the female. In P. tupinamba this structure carries a distinctive large, sickle-shaped hook on the inner surface of the gonopodial appendix near its base — a fine anatomical detail, but one of the key characters by which the species was defined. The female bears a urogenital papilla whose lateral ramus turns to the left, another of the diagnostic reproductive traits.
Habitat
The species is endemic to small coastal drainages of south-eastern Brazil, in the states of São Paulo and Rio de Janeiro. The type material and recorded distribution centre on the rio Itamambuca and rio Macacu drainages, with the species occurring more broadly in the minor coastal river systems of this stretch of the Atlantic seaboard. Its range is therefore narrow — a restricted-range coastal fish rather than a wide-ranging one.
Ecologically it is a freshwater, benthopelagic species of tropical waters, living in the small streams and associated still waters of these coastal catchments. Phalloceros as a genus favours quiet, shallow, often vegetated margins of streams and pools, and there is no reason to think P. tupinamba departs from that pattern, though detailed habitat measurements specific to this species are sparse in the literature.
Because the data on water chemistry for this particular species are limited, precise temperature, pH and hardness figures for its native waters are best treated as approximate; the broad picture is of warm, lowland coastal-stream habitat in the Atlantic Forest region of south-eastern Brazil.
Feeding
Direct dietary studies of Phalloceros tupinamba are not available, so its feeding ecology is inferred from its relatives and its position in the food web. FishBase places the genus around a trophic level of roughly 3.0, consistent with an omnivorous to micro-carnivorous habit. In practice this means a diet of small invertebrates, insect larvae, micro-crustaceans, algae and detritus taken from the substrate and vegetation of its coastal streams.
Small poeciliids of this kind are typically opportunistic grazers and pickers, working the margins and surface film for whatever animal and plant matter is available and shifting the balance of their diet with the season and the habitat. Their small mouths and continuous feeding suit a steady intake of tiny items rather than large prey.
No aquarium-feeding observations specific to this species are on record. By analogy with congeners and other small livebearers, it would be expected to accept the usual small foods — fine flake, micro-pellets, and small live or frozen items such as Daphnia and brine shrimp — but this is extrapolation rather than documented husbandry, and the data here are sparse.
Mating
Like all Poeciliidae, Phalloceros tupinamba reproduces by internal fertilisation. The male does not scatter milt over eggs; instead he transfers sperm directly to the female using the gonopodium, the modified anterior rays of the anal fin that give the family its livebearing biology and the genus its name. In P. tupinamba the gonopodium carries a species-specific feature — the large sickle-like hook on the inner surface of the appendix near its base — that helps anchor the organ during mating and serves as a diagnostic character.
The corresponding female structure is the urogenital papilla, whose lateral ramus in this species turns to the left. Such fine, sex-specific differences in the genital apparatus are a recurring theme in poeciliid taxonomy: closely related species that look almost identical externally are often separated chiefly by the shape of the male gonopodium and the female papilla, and Phalloceros is a classic example, with Lucinda leaning heavily on these characters to define the twenty-one species he described in 2008.
No detailed courtship observations are published for P. tupinamba specifically. In small poeciliids generally, males pursue females persistently and combine brief displays with sneak copulation attempts, and females are often able to store sperm internally for some time — but applying those generalisations to this particular species goes beyond what the sources document.
Breeding
Phalloceros tupinamba is viviparous, as are all members of the family — the female carries the developing embryos internally and gives birth to free-swimming young rather than laying eggs. The nourishment of the embryos is lecithotrophic, meaning the developing young are sustained by the yolk of the egg rather than through a placenta-like connection to the mother; in this respect Phalloceros differs from the matrotrophic, placenta-bearing poeciliids such as Heterandria and Poeciliopsis, where the mother actively provisions the embryos during gestation.
The species is rated as having high biological resilience, with an estimated minimum population doubling time of under fifteen months — the kind of rapid turnover expected of a small, early-maturing livebearer. That figure implies sizeable, frequently produced broods and quick maturation, the reproductive strategy that lets small poeciliids persist in unstable coastal-stream habitats.
Specific figures for gestation length, brood size and the frequency of broods in P. tupinamba are not recorded in the available sources, and superfetation — the simultaneous carrying of broods at different developmental stages, known in some other poeciliids — has not been documented for this species. Here the data are genuinely sparse, and it is better to note the gap than to borrow numbers from other livebearers.
In the aquarium
Phalloceros tupinamba is essentially a museum and field species rather than an aquarium-trade fish, and there is little or no published husbandry literature dealing with it by name. What can be said is general: it is a small, peaceful, hardy livebearer from warm coastal streams, and on the evidence of its relatives it would be undemanding to keep, suited to a small, well-planted aquarium with gentle filtration and the dense cover that small livebearers use for refuge.
As with other small Phalloceros, a group rather than a pair would be the natural way to keep it, with floating and marginal plants giving the fish security and providing cover for any fry, which adult livebearers will otherwise eat. Tankmates should be small and equally peaceful; the fish is far too slight to share a tank with anything boisterous.
These remarks are extrapolated from the biology of the genus, not from documented experience with this species — keepers should treat them as a starting point and watch the fish itself. Given its narrow native range and its near-absence from the hobby, anyone who did obtain P. tupinamba would be handling an uncommon fish, and accurate record-keeping of its requirements would be a genuine contribution.
Conservation
Phalloceros tupinamba is assessed as Least Concern on the IUCN Red List (assessment dated 7 November 2018; species ID 186925). Despite being a restricted-range fish confined to small coastal drainages of south-eastern Brazil, it was judged not to face major widespread threats, and its high resilience and low vulnerability support that conclusion. It is not listed by CITES or the CMS.
The Least Concern rating should not, however, obscure the inherent fragility of a narrowly distributed coastal species. The Atlantic Forest catchments of São Paulo and Rio de Janeiro are among the most densely populated and heavily altered landscapes in Brazil, and small endemic stream fishes in such regions are exposed to habitat loss, pollution and water-course modification even where they are currently secure. A species known from only a handful of small drainages has little geographic buffer against localised damage.
For a fish this poorly represented in aquaria, the most useful conservation contribution is documentary: good locality data, careful habitat description, and accurate identification, the same painstaking work that underlay Lucinda's 2008 revision. The species is secure for now, but its long-term outlook is tied to the fate of a small set of south-eastern Brazilian coastal streams.