Taxonomy & naming
Phalloptychus eigenmanni was described by Arthur W. Henn in 1916 from material collected by John D. Haseman in the rio Catu at Alagoinhas, in the Brazilian state of Bahia, which remains the type locality and the only place the species has ever been recorded. Eschmeyer's Catalog of Fishes treats Phalloptychus eigenmanni Henn, 1916 as the valid name, placing it in the genus Phalloptychus Eigenmann, 1907, whose type species is Girardinus januarius Hensel, 1868. The species epithet honours Carl H. Eigenmann, the American ichthyologist who erected the genus and whose work on South American fishes underpins much of the region's modern systematics.
The genus name Phalloptychus is built from the Greek phallos, "penis," and ptyx / ptychos, "a fold," referring to the folded structure of the male's gonopodium — the modified anal fin used for internal fertilisation in poeciliids. The genus is a small one: Lucinda's 2005 generic revision in Neotropical Ichthyology recognises just three valid species — P. eigenmanni, P. januarius and P. iheringii — all endemic to the coastal drainages of eastern Brazil.
The species' rank has not always been settled. Von Ihering (1931) treated it as a subspecies, P. januarius eigenmanni, folding it into its more widespread congener. Lucinda's revisionary work in the early 2000s re-validated it at full species rank, distinguishing it from the other two Phalloptychus by anal-fin and gonopodial ray counts. Catalog of Fishes follows this and recognises P. eigenmanni as a distinct, valid species.
Morphology
Phalloptychus eigenmanni is a very small livebearer. FishBase records a maximum standard length of about 1 in for males, while females, which are the larger sex, reach roughly 1.5 in total length — the female-larger pattern of sexual size dimorphism typical of poeciliids. Beyond these figures the published morphometric detail is sparse, and much of what is known of the fish's appearance comes from Henn's original description and Lucinda's later comparative work rather than from living, well-photographed material; FishBase carries only a placeholder drawing.
In terms of meristic characters, Lucinda's revision reports a dorsal fin set posterior to mid-body with 8–9 rays, pectoral fins with 11–12 rays, and pelvic fins with 4 rays in males and 6 in females. The species is separated from its two congeners chiefly by counts of the anal-fin and gonopodial rays rather than by obvious external colour or shape, which is consistent with the conservative, generally cryptic body plan of the genus.
The most distinctive structure is the male gonopodium. Across Phalloptychus the gonopodium is long and asymmetrical and bears spines along its length — the feature that gives the genus its name and that functions in transferring sperm during internal fertilisation. As in other small poeciliids, females are plainer and deeper-bodied, especially when gravid, while the reproductively mature male is recognised by the elongate intromittent organ formed from the modified anal fin.
Habitat
Phalloptychus eigenmanni is known from a single freshwater locality: the rio Catu at Alagoinhas, in the Catu River drainage of Bahia, eastern Brazil. FishBase classes it as a freshwater, pelagic, tropical species. The genus as a whole occupies coastal drainages along the eastern Brazilian seaboard from Bahia southward to Rio Grande do Sul, but P. eigenmanni itself sits at the northern end of that distribution and is, on present evidence, restricted to this one small coastal river system.
The IUCN assessment underlines just how confined the range is: the area of occupancy is estimated at only about 2 mi², and the species is treated as occurring at a single location, with its sole known subpopulation at the type locality. This is an extreme example of micro-endemism — a fish whose entire documented world is one short stretch of one Bahian river.
Detailed water-chemistry data for the rio Catu population are not available in the consulted sources, so specific temperature, pH and hardness values are reported here only as broad inferences from the species' tropical coastal-Brazilian setting rather than measured figures. The habitat is a warm lowland coastal drainage, the kind of small, vegetation-fringed freshwater that small poeciliids typically favour, but the consulted literature does not provide measured environmental parameters for this particular site, and the data are genuinely sparse.
Feeding
No dedicated dietary study of Phalloptychus eigenmanni appears in the consulted sources, and its feeding ecology has not been documented in detail. FishBase places the species at a modelled trophic level of roughly 3.0 — a value estimated from its phylogenetic position and body form rather than measured from gut contents — which is consistent with a small omnivore or micro-predator rather than a strict herbivore or piscivore.
On the basis of its membership in the Poeciliinae and its small size, P. eigenmanni is best understood as an opportunistic micro-omnivore in the manner of related small livebearers: a fish that would be expected to take small aquatic invertebrates, insect larvae, micro-crustaceans and algal or detrital material from the water column and substrate of its coastal stream. This is an inference from its relatives and its build, however, not a documented observation for this species.
Given the species' conservation status and probable rarity in the wild, there is effectively no aquarium-husbandry literature on its feeding either. Any captive maintenance would, by analogy with the better-known Phalloptychus januarius and other small poeciliids, rely on small live and frozen foods supplemented with fine prepared foods — but it must be stressed that this is general livebearer practice rather than anything established specifically for P. eigenmanni.
Mating
Like all poeciliids, Phalloptychus eigenmanni reproduces by internal fertilisation rather than external egg-laying. The male transfers sperm to the female using the gonopodium, the intromittent organ formed from the modified rays of the anal fin. In Phalloptychus this organ is notably long and asymmetrical and bears spines along its length, and it is precisely this folded gonopodial structure that gives the genus its name (phallos, "penis," plus ptychos, "fold"). Differences in the gonopodial rays are among the characters Lucinda used to separate the three species of the genus.
The specifics of courtship and mating behaviour in P. eigenmanni have not been documented — the species is too rare, and quite possibly no longer extant at the only site it is known from, for behavioural study. What can be said is grounded in the general biology of small poeciliids and of the better-studied congener P. januarius: males of these fishes typically pursue females persistently and use the mobile, forward-directed gonopodium to achieve insemination, often with little or no elaborate display.
As in other livebearing poeciliids, females are expected to be able to store sperm internally, allowing successive broods to be produced from a single mating. This is a general feature of the family rather than a documented observation for P. eigenmanni specifically, and the consulted sources do not record reproductive details for this particular species.
Breeding
Phalloptychus eigenmanni is a viviparous (livebearing) poeciliid: after internal fertilisation the female carries the developing embryos and gives birth to free-swimming young rather than laying eggs. This much follows securely from its placement in the Poeciliinae and from the livebearing habit of the genus, even though the species' own reproduction has never been studied directly.
The precise reproductive parameters — gestation length, brood size, and whether the species shows superfetation (the simultaneous carrying of broods at different stages of development, which occurs in some poeciliids) — are not recorded in the consulted sources for P. eigenmanni. The systematic literature explicitly notes that these reproductive specifics are undocumented for this species, and FishBase lists its maturity as unknown. Where superfetation is concerned, it cannot be asserted for this fish; it is a trait that varies across the family and has simply not been examined here. The honest position is that the breeding biology is data-sparse.
What the available data do indicate is a high intrinsic capacity for population growth: FishBase rates the species' resilience as high, with an estimated minimum population-doubling time of under 15 months. That is a model-based figure typical of small, fast-maturing livebearers, and it implies the fish could in principle recover quickly given suitable habitat — a point of some importance for a species teetering at the edge of extinction.
In the aquarium
Phalloptychus eigenmanni is essentially absent from the aquarium hobby. It is a single-locality endemic, Critically Endangered and possibly extinct in the wild, and the consulted sources contain no husbandry account for it. It is not a fish that is traded, bred in captivity, or realistically obtainable, and nothing in this section should be read as encouragement to seek it out.
Were the species ever to be brought into managed care — most plausibly in a conservation-breeding rather than a hobby context — its requirements would be expected to resemble those of its small coastal-Brazilian relatives, particularly the more familiar Phalloptychus januarius: a small, warm, well-planted freshwater aquarium with gentle filtration and a varied diet of fine live, frozen and prepared foods. These are extrapolations from congeners and general small-poeciliid practice, not parameters established for P. eigenmanni itself.
The more useful point for an aquarist is the conservation framing. The greatest service the hobby can render a fish like this is not to keep it but to support the protection of its single Bahian river and the wider effort to document and conserve Brazil's narrowly endemic coastal livebearers. For the overwhelming majority of keepers, P. eigenmanni is a fish to read about rather than to own.
Conservation
Phalloptychus eigenmanni is assessed as Critically Endangered on the IUCN Red List, under criteria B2ab(ii,iii,iv,v). The assessment, carried out on 7 November 2018 by a team including S. M. Q. Lima and T. P. A. Ramos through Brazil's ICMBio process (the Livro Vermelho da Fauna Brasileira Ameaçada de Extinção, Vol. VI: Peixes), reflects an extremely restricted distribution — an area of occupancy of about 2 mi² at a single location — together with a continuing decline in the extent and quality of the species' habitat in the Catu drainage of Bahia.
The situation is graver still than the formal category alone conveys. The species is known only from its type locality, and repeated collecting efforts have failed to recover it; earlier Brazilian assessments (Rosa & Menezes, 1996; MMA, 2004) listed it among the country's threatened fishes and considered it seriously endangered and probably already extinct. It thus sits in the difficult category of a fish that may persist only as a memory of a few museum specimens, while not yet being formally declared extinct.
The threats are those of a micro-endemic in a developed coastal region: habitat degradation, water-quality decline, and the inherent vulnerability of a species confined to one small drainage with no known refuge populations elsewhere. CITES and CMS list the species as not evaluated. Conservation attention, if it is to be meaningful, must focus on the rio Catu itself — surveying for any surviving population, safeguarding the habitat, and treating this tiny livebearer as a flagship for the threatened freshwater fishes of eastern Brazil's coastal drainages.