Taxonomy & naming
Phalloptychus januarius was first described by the German-Brazilian naturalist Reinhold Hensel in 1868 as Girardinus januarius, from material associated with the Rio de Janeiro region of south-eastern Brazil — the source of the epithet "januarius." The species later passed through several combinations and synonyms as nineteenth- and early-twentieth-century workers reassessed the small American livebearers: Girardinus iheringii Boulenger, 1869 and Poecilia januarius Eigenmann, 1907 both refer to this fish. Eschmeyer's Catalog of Fishes treats Phalloptychus januarius (Hensel, 1868) as the valid name, with the original Girardinus combination preserved in parentheses around the authority.
The genus Phalloptychus belongs to the family Poeciliidae, the New World livebearers, within the subfamily Poeciliinae of the order Cyprinodontiformes. It is a small genus — only two species are recognised, Phalloptychus januarius and its congener Phalloptychus eigenmanni — both confined to the Atlantic drainages of south-eastern South America. The distinctive structure of the male gonopodium is taxonomically informative and, in practical terms, isolates the genus reproductively: the species is thought unlikely to hybridise with anything but its single congener.
Morphology
Phalloptychus januarius is a small, elongate livebearer with marked sexual size dimorphism. Males are diminutive, reaching only about 1 in in standard length, while females are considerably larger and deeper-bodied, growing to roughly 1.5 in SL; a photographed specimen from the Santos channel in São Paulo State reached about 1.5 in SL. Older females tend to become noticeably high-backed with age. The dorsal fin carries about nine rays, and in overall proportion the fish has been likened to species of Poeciliopsis.
The colour pattern is the species' most recognisable feature. The body is brownish to silvery-grey with a faint bluish sheen, and each scale is edged in black, producing a fine net-like or reticulated effect across the flanks. Overlying this is a series of numerous vertical black bars that run along the sides and become longer and more pronounced toward the rear of the body — the basis of the hobby name "Barred Millionsfish." Some wild populations contain heavily black-speckled individuals, and in certain stocks males may be almost entirely black.
As in all poeciliids, the defining male character is the gonopodium, the intromittent organ formed from modified anal-fin rays. In Phalloptychus the males develop a very long, curved gonopodium whose folded form gives the genus its name (Greek phallos, penis, plus ptyx/ptychos, a fold). This elaborate structure is used to transfer sperm internally to the female.
Habitat
The species occupies fresh and occasionally brackish waters of the Atlantic coastal region of south-eastern South America, ranging from Rio de Janeiro south through Rio Grande do Sul in Brazil and into eastern Paraguay and Uruguay; broader compilations also list Argentina at the southern margin of the range. It is a fish of shallow, warm waters — both flowing and standing — that can heat up considerably in the sun, the kind of marginal pools, ditches, and slow stretches typical of small lowland livebearers.
Reported water conditions span a wide range. FishBase gives a pH window of roughly 6.8 to 8.0 and hardness up to about 25 dH, with tropical temperatures around 75–82 °F. Hobby accounts note that wild habitats range from soft and acidic to genuinely brackish, reflecting the variety of coastal environments the species inhabits. In the field it co-occurs with other livebearers, notably Poecilia vivipara and the goodeid-like Jenynsia lineata, a small assemblage of viviparous fishes sharing the same warm shallow waters.
Feeding
Phalloptychus januarius is a small omnivore, feeding in the wild on the mixture of micro-foods available to a shallow-water livebearer — algae and biofilm, fine detritus, and small aquatic invertebrates and insect larvae taken from the water column and surface. Its trophic level is estimated at around 3.0, consistent with a diet that combines plant and small animal matter rather than specialising on either.
In the aquarium the species accepts a varied small-particle diet. Hobby breeders report that the fry grow well on newly hatched Artemia (brine shrimp), reaching nearly a centimetre within about five weeks, and the adults take small flake, micro-pellets and a range of small live and frozen foods. As with most small, frequently grazing livebearers, several modest feedings suit it better than infrequent large meals.
Mating
Reproduction in Phalloptychus januarius is internal and viviparous, the hallmark of the poeciliid livebearers. Fertilisation occurs inside the female: the male uses his long, curved gonopodium — the modified anal fin that gives the genus its name — to transfer sperm directly to the female, dispensing entirely with the egg-laying and external fertilisation of most other fishes. Because that gonopodial structure is so distinctive, the species is considered very unlikely to cross with anything other than its single congener, Phalloptychus eigenmanni.
As in related poeciliids, females are able to store viable sperm internally and can therefore produce a succession of broods from a single mating. The pronounced size difference between the small males and the larger, deeper-bodied females is typical of the genus and reflects the live-bearing reproductive strategy, in which the female must accommodate the developing young internally through gestation.
Breeding
Phalloptychus januarius is a true livebearer that gives birth to free-swimming young rather than laying eggs. After a gestation of roughly 24 days the female produces a brood of about 10 to 30 young, the figure varying with her size and condition. In aquaria the species is reported to breed readily, and hobby accounts describe fry being born over a period of several weeks rather than all at once — possibly near-continuously, in the manner of Heterandria formosa — which suggests an extended or serial brooding pattern reminiscent of superfetation, in which a female carries broods at different stages of development simultaneously.
The newborn fry are fully formed miniature fish, able to swim and feed at once. Breeders raise them well on newly hatched brine shrimp and other fine foods, with the young reaching nearly a centimetre in about five weeks. As with most livebearers, dense planting and cover help fry survive, since adults will take small young given the chance; a well-planted, species-appropriate setup is the practical route to raising good numbers.
In the aquarium
Phalloptychus januarius is a peaceful but somewhat shy fish that does best in a well-planted aquarium offering plenty of cover. It is a specialist's livebearer rather than a general community fish — historically maintained by dedicated livebearer keepers, with documented trade imports to Germany as early as 1912 and later Brazilian imports reaching the European hobby in the 1990s. Earlier European stocks had largely disappeared by the early 1990s before fresh wild fish were brought in.
While wild habitats span soft acidic to brackish water, aquarium stocks of trade origin are reported to do best in neutral pH with moderate hardness, kept warm at around 75 °F (about 76 °F). Given the fish's shy temperament and small size, it is best kept either as a species-only group or with other small, gentle livebearers — companions such as Neoheterandria elegans or Quintana atrizona are suggested — rather than with boisterous or much larger tankmates that would intimidate it or outcompete it at feeding time.
Conservation
Phalloptychus januarius is assessed as Least Concern on the IUCN Red List, evaluated in November 2018. The species is widespread across the Atlantic coastal drainages of south-eastern South America, occupies a broad range of shallow-water habitats, and tolerates a wide span of water chemistry from soft and acidic to brackish — characteristics that, combined with a high estimated resilience, place it well clear of any species-level threat at present.
It has not been evaluated under CITES or CMS. As with many small, adaptable coastal livebearers, the main practical conservation considerations are local rather than global: the integrity of the shallow lowland waters it shares with Poecilia vivipara and Jenynsia lineata, and — as for any hardy live-bearing toothcarp — the avoidance of releasing aquarium stock into waters outside its native range.