Taxonomy & naming
Phalloceros elachistos was described by Paulo H. F. Lucinda in 2008 as part of his comprehensive revision of the genus Phalloceros, a single monographic study (Neotropical Ichthyology 6(2): 113–158) that recognised 21 new species and transformed what had long been treated as one or two variable fishes into a diverse radiation of coastal and inland south-eastern South American livebearers. Eschmeyer's Catalog of Fishes treats Phalloceros elachistos Lucinda, 2008 as the valid name, placing it in the genus Phalloceros within the subfamily Poeciliinae of the family Poeciliidae — the New World toothcarps that include guppies, mollies and mosquitofishes.
The genus Phalloceros is built around the familiar type species P. caudimaculatus, the "dusky millions fish" of the hobby, but the great majority of its species — including P. elachistos — are little-known, narrowly distributed forms separated by fine details of the male copulatory anatomy and squamation. Lucinda distinguished P. elachistos from its congeners on a suite of characters: the female urogenital papilla runs straight along the body midline between the anus and the base of the first anal-fin ray, and the male gonopodial appendix carries only a very small simple hook, a minute lateral protuberance that is often absent altogether in juveniles. From the similar P. enneaktinos it is separated by its 7–8 dorsal-fin rays rather than nine.
The epithet elachistos comes from the Greek elachis (small, short, little) in its superlative form, meaning "the least" — a direct reference to the very small size of the type specimens. The genus name Phalloceros combines the Greek phallos (penis) and keras (horn), alluding to the horn-like gonopodium that defines these livebearing fishes.
Morphology
Phalloceros elachistos is an exceptionally small poeciliid. Males and unsexed specimens reach only about 1 in in standard length, while the larger females attain around 1 in SL, making this one of the most diminutive members of an already small-bodied genus — the basis for its "the least" epithet. The body is the slender, slightly compressed shape typical of Phalloceros, suited to the shallow flowing waters it inhabits.
Meristic counts place the species within the genus's range: the dorsal fin carries 7 to 9 soft rays, the anal fin 8 to 11, and the vertebral column comprises 31 to 33 vertebrae. The low dorsal-ray count — typically 7 to 8 — is one of the diagnostic features separating it from the nine-rayed P. enneaktinos. As in all Phalloceros, the sexes differ in the anal fin: in the male the anterior anal-fin rays are modified into the gonopodium, an intromittent organ, here ending in a gonopodial appendix that bears only a minute simple hook.
No detailed colour description is available for live specimens of this little-collected species, and the fine pigmentation pattern that often distinguishes Phalloceros species in life is not well documented for P. elachistos. The fish's identity rests chiefly on the anatomy of the reproductive structures and on counts rather than on conspicuous external colour.
Habitat
Phalloceros elachistos is endemic to the Atlantic coastal drainages of Espírito Santo State in south-eastern Brazil. It is recorded from the rio Doce system and from the smaller coastal rivers to its south — the Santa Maria da Vitória, Jucu and Timbuí drainages — a tight cluster of basins along a short stretch of the Brazilian seaboard. This is a genuinely range-restricted fish, known from a handful of localities within a single state rather than from any broad region.
The environment is freshwater, tropical and shallow. FishBase classifies the species as a pelagic freshwater fish of warm lowland and coastal streams; the small, clear-to-tea-coloured watercourses of the Espírito Santo coastal plain — slow margins, pools, vegetated edges and shallow runs — are the kind of habitat that suits a tiny surface-and-midwater livebearer of this sort. Precise water-chemistry measurements from the type localities are not published in the sources available, so the figures that can be given for temperature and pH are general expectations for tropical Brazilian coastal streams rather than recorded values for this species.
Because the fish occupies a small, low-lying and increasingly developed part of Brazil's Atlantic Forest seaboard — a region affected by agriculture, urban growth around Vitória, and the well-publicised pollution history of the lower rio Doce — its habitat is not pristine, even though the species itself is currently assessed as being of low conservation concern.
Feeding
No diet study specific to Phalloceros elachistos has been published, and FishBase carries no food-composition records for the species. Its feeding ecology must therefore be inferred from the biology of the genus and from a modelled trophic level of about 3.0, which places it as a small omnivore feeding low on the food web.
Small Phalloceros are, like most poeciliids of comparable size, opportunistic micro-omnivores. They graze the aufwuchs — the film of algae, diatoms and detritus coating submerged surfaces — and take small aquatic invertebrates, insect larvae and other tiny prey from the water column and surface. A fish of this size operates as a continual nibbler rather than a predator of large items, working the shallow margins and vegetated edges of its streams.
In the absence of direct observation, any more specific account would be guesswork. The safe statement is that P. elachistos is a small omnivore of the same general feeding type as the better-known members of its genus, balancing plant and algal matter against small invertebrate prey according to what its coastal-stream habitat offers.
Mating
Phalloceros elachistos is a viviparous livebearer that reproduces by internal fertilisation, the defining reproductive mode of the Poeciliidae. The male transfers sperm to the female using the gonopodium — a slender intromittent organ formed from the modified anterior rays of the anal fin. In P. elachistos this structure ends in a gonopodial appendix carrying a very small simple hook, one of the fine anatomical characters Lucinda used to diagnose the species; in females the matching urogenital papilla sits straight along the midline between the anus and the first anal-fin ray.
As in other small poeciliids, mating is internal and does not involve nest-building or territory defence. Males pursue females and attempt to insert the gonopodium; in many livebearing toothcarps females are able to store viable sperm internally for extended periods, allowing successive broods to be produced from a single insemination, though this has not been measured in P. elachistos specifically.
No species-specific account of courtship behaviour, mate choice or mating frequency exists for this fish. What can be said confidently is anatomical — the presence and form of the gonopodium and the female papilla — while the behavioural detail is genus-level inference. The data here are sparse, and the specifics of how P. elachistos pairs and mates in its native streams remain undocumented.
Breeding
Like all Phalloceros, P. elachistos is a livebearer: after internal fertilisation the female carries the developing embryos and gives birth to free-swimming young rather than laying eggs. The embryos of this poeciliine lineage are lecithotrophic — nourished by their own yolk reserves through development, without the true matrotrophic placenta seen in some other poeciliids — so the female provisions her young largely before fertilisation rather than feeding them across gestation.
No gestation length or brood-size figures have been published for Phalloceros elachistos; FishBase carries no reproduction records for the species. Given its very small body — females reach only about 1 in — broods are necessarily modest, as in other tiny livebearers, but any specific number would be invented rather than reported. Superfetation, the simultaneous carrying of broods at different developmental stages found in some poeciliids, has not been documented for this species and should not be assumed. The genus's resilience is nonetheless rated high, with an estimated population doubling time of under fifteen months, consistent with the rapid, continuous breeding typical of small livebearers.
The honest summary is that the breeding of P. elachistos is known only in outline, by inference from its genus: internal fertilisation, yolk-nourished embryos, live birth of small numbers of fully formed fry. The measured details — gestation, brood counts, breeding season — have simply not been recorded for this particular fish.
In the aquarium
Phalloceros elachistos is not an aquarium-trade species. It is a tiny, range-restricted Brazilian coastal-stream fish described in 2008 and effectively unknown in the hobby; the American Livebearer Association and general livebearer literature cover only the common members of the genus, such as P. caudimaculatus, and not this taxon. There is therefore no body of hobbyist experience to draw on, and the species should be regarded as one encountered in the field and in collections rather than in home aquaria.
Were it ever maintained, the husbandry would follow the logic of its habitat and its genus: warm, clean tropical freshwater; gentle flow; a well-planted small aquarium with abundant cover; and a varied micro-diet of small live, frozen and prepared foods to match its omnivorous, fine-feeding habits. A fish of under three centimetres needs only tiny tankmates and would be lost or outcompeted among larger, boisterous fishes.
The responsible position is that this account is offered for reference, not as a care guide — the species is not in trade, and any specific stocking, temperature or breeding advice for P. elachistos would be extrapolation from its relatives rather than tested practice. Aquarists interested in the genus will find P. caudimaculatus the realistic and well-documented option.
Conservation
Phalloceros elachistos is assessed as Least Concern on the IUCN Red List (assessed 7 November 2018; IUCN species ID 187244). Despite its narrow range, the species has not been flagged as threatened: it is not targeted by any fishery — its fishing vulnerability is rated very low — and as a small, fast-reproducing livebearer it has the demographic resilience to withstand ordinary disturbance.
That said, a Least Concern rating for a fish confined to a few coastal drainages of a single Brazilian state carries an implicit caution. The Espírito Santo seaboard is a developed and modified landscape, and the lower rio Doce in particular has a well-known history of pollution; range-restricted endemics in such settings can be more vulnerable to habitat degradation than a broad-scale assessment captures. The species is not listed under CITES or the CMS.
For a fish absent from the aquarium trade, the conservation message is straightforward: its future depends entirely on the health of its native Atlantic coastal streams. Protecting the water quality and riparian vegetation of the rio Doce, Santa Maria da Vitória, Jucu and Timbuí drainages is what safeguards P. elachistos, since there is no captive population or hobby interest to fall back on.