Taxonomy & naming
Phalloceros anisophallos was described by Paulo H. F. Lucinda in 2008 as part of his comprehensive systematic and biogeographic revision of the genus Phalloceros (Cyprinodontiformes: Poeciliidae), a monographic work that erected twenty-one new species at a stroke. Before that revision the genus had been treated as essentially monotypic — almost every population across south-eastern South America was filed under the catch-all name Phalloceros caudimaculatus — so the description of P. anisophallos and its siblings represented the first fine-grained sorting of a long-conflated complex. Eschmeyer's Catalog of Fishes treats Phalloceros anisophallos Lucinda, 2008 as the valid name, placing it in the genus Phalloceros within the family Poeciliidae, subfamily Poeciliinae, the New World livebearers.
The species belongs squarely to the poeciliid radiation of internally fertilising, live-bearing toothcarps of the Americas. Within that family Phalloceros is a genus of small, sexually dimorphic fishes endemic to the Atlantic and La Plata drainages of south-eastern South America. The generic name itself is a description of male anatomy, and the specific epithet sharpens that theme: where most of the genus shares a broadly similar gonopodial appendix, P. anisophallos is distinguished by the asymmetry of that organ, the character Lucinda chose to enshrine in its name.
Morphology
Phalloceros anisophallos is a very small fish even by livebearer standards. Males reach only about 1 in in standard length, while females are larger and deeper-bodied at up to roughly 2 in SL — the marked size difference between the sexes typical of the family. The body carries 32 vertebrae, with seven to eight soft rays in the dorsal fin and nine to eleven in the anal fin. In overall build it is an unremarkable, silvery, slightly elongate toothcarp of the kind that is hard to identify on colour alone, which is exactly why Lucinda's revision leaned on internal and microscopic characters.
The most reliable external marking is the flank spot. In P. anisophallos this spot is elongate and oriented vertically, forming a narrow bar — roughly two scales wide horizontally and two to four scales tall — that reaches both the dorsal and ventral profiles of the body in adult females; there is no separate spot on the caudal peduncle. The species' true autapomorphy, however, is in the male gonopodium: the hook on the gonopodial appendix lies on the left half of the structure near its tip (in the distal quarter to distal third of its length), and the appendix as a whole is asymmetrical, its right half wider than its left. The female urogenital papilla is correspondingly curved to the right and set laterally. These are the characters that diagnose the species and give it its name.
Habitat
The species is endemic to Brazil, where it is known only from small coastal drainages in the state of Rio de Janeiro — the rivers Parati, Barra Grande, São Roque, Taquari and Itinguçu among them. These are short Atlantic-slope streams draining directly to the sea, the kind of modest, warm, often forested lowland and foothill watercourses that characterise much of the south-eastern Brazilian coast. FishBase classifies the fish as a freshwater, benthopelagic species of tropical waters.
Beyond this narrow distributional picture the published ecological detail is sparse. Phalloceros species as a group are surface-and-midwater inhabitants of slow margins, vegetated pools and small flowing channels, and there is no reason to think P. anisophallos departs from that pattern, but specific measurements of the temperature, pH and chemistry of its home streams are not recorded in the sources at hand. As a small coastal toothcarp with a very restricted range, the species' fortunes are tied closely to the integrity of these few Rio de Janeiro drainages.
Feeding
No diet study specific to Phalloceros anisophallos is available, and FishBase lists only an estimated trophic level of about 3.0, consistent with a small omnivore that takes a mix of animal and plant material. Phalloceros species in general feed as opportunistic micro-omnivores of the stream surface and water column, grazing algae and detritus and taking small aquatic invertebrates, insect larvae and whatever drifting items they can catch — the same broad foraging mode seen across the small poeciliids.
In the absence of population-specific data the honest position is that the feeding ecology of this species is data-sparse and inferred from its genus and body form rather than directly documented. What can be said is that, like its relatives, it is a small grazing-and-gleaning generalist rather than a specialist predator, and that its trophic role is that of a minor consumer near the base of its coastal-stream food web.
Mating
Like all poeciliids, Phalloceros anisophallos reproduces by internal fertilisation. The male transfers sperm to the female using the gonopodium, an intromittent organ formed from modified rays of the anal fin; in this species that organ is notably asymmetrical, with a hook positioned on the left side of the gonopodial appendix near its tip and the appendix's right half broader than its left. The female's urogenital papilla is correspondingly curved to the right, a matching asymmetry that almost certainly relates to how the sexes couple during mating. This lock-and-key configuration of the genitalia is the very feature that defines the species.
Detailed observations of courtship and mating behaviour in this particular species are not on record. By extension from the genus and family, males are expected to pursue females and attempt insemination more or less continuously, with little in the way of territoriality or nest-building, and females are expected to be capable of storing sperm internally to fertilise successive broods. Beyond that general poeciliid template, the specifics of mate choice and male display in P. anisophallos remain undocumented.
Breeding
Phalloceros anisophallos is viviparous, in the manner of poeciliid livebearers: after internal fertilisation the female carries the developing embryos and gives birth to free-swimming young rather than laying eggs. FishBase rates the species' resilience as high, with an estimated minimum population doubling time of under fifteen months — the kind of rapid turnover expected of a small, early-maturing livebearer that breeds readily and repeatedly.
Species-specific figures for gestation length, brood size and the frequency of reproduction are not reported in the available sources, and there is no documentation of superfetation (the simultaneous carrying of broods at different stages) for this fish, so any such claim would be guesswork. What the genus and family tell us is the general shape of its breeding: short gestation measured in weeks, modest broods of fully formed fry that are independent at birth, the capacity for several broods in succession from stored sperm, and no parental care — adults of small livebearers commonly prey on their own young. The precise numbers for this species are, at present, data-sparse.
In the aquarium
Phalloceros anisophallos is not a fish of the general aquarium trade. It is a small, narrowly distributed Brazilian endemic of interest chiefly to livebearer specialists and to keepers who maintain wild Phalloceros for conservation and study rather than colour, and it is unlikely to be encountered in a typical shop. Where the genus is kept, the fishes are valued precisely as wild forms — small, peaceful, undemanding stream livebearers maintained in dedicated species or biotope set-ups.
No husbandry protocol specific to P. anisophallos is established in the sources here, so recommendations can only be generalised from the genus: a small, well-planted aquarium with gentle flow, clean stable water and temperatures in the tropical range suits Phalloceros, and as prolific livebearers they need cover for fry if any are to survive alongside the adults. Because precise water parameters for its native Rio de Janeiro streams are not recorded, a keeper would be working from genus-level norms rather than measured field values — a reason to treat this species as a subject for the experienced specialist rather than the beginner.
Conservation
Phalloceros anisophallos is assessed as Least Concern on the IUCN Red List (assessment of 7 November 2018). Despite its very restricted distribution in a handful of small Rio de Janeiro coastal rivers, the species was judged not to face a level of threat warranting a higher category at the time of assessment, and it is recorded as harmless to humans, not evaluated under CITES or the CMS.
That said, a narrow-range endemic confined to short, coastal Atlantic-slope drainages in a heavily populated and developed part of south-eastern Brazil is inherently more exposed than its Least Concern status alone might suggest — such streams are vulnerable to pollution, abstraction, channelisation and habitat loss, and a species known from so few rivers has little geographic buffer. The responsible reading is that the fish is presently secure but that its security rests on the continued health of a small set of drainages, making the integrity of those Rio de Janeiro coastal streams the key to its long-term survival.