Taxonomy & naming
Jenynsia diphyes was described by Paulo Lucinda, Michael Ghedotti and Vinícius Abilhoa da Graça in 2006, in the journal Copeia, from material collected in the upper Rio Iguaçu drainage of Paraná, southern Brazil. The original paper — "A new Jenynsia species (Teleostei, Cyprinodontiformes, Anablepidae) from southern Brazil and its phylogenetic position" — both named the species and placed it within a phylogenetic framework for the genus. Eschmeyer's Catalog of Fishes treats Jenynsia diphyes Lucinda, Ghedotti & da Graça, 2006 as the valid name.
The species sits in the family Anablepidae, the order Cyprinodontiformes — the same broad assemblage of toothcarps that contains the killifishes and the New World livebearers, but a separate family from the Poeciliidae of guppies and mollies. Anablepidae unites the four-eyed fishes (Anableps), the genus Jenynsia, and Oxyzygonectes; Jenynsia and its relatives are sometimes called the "onesided livebearers" for a peculiarity of their reproductive anatomy. Within this family J. diphyes is one of a growing number of described Jenynsia species, many of them restricted to single drainages in subtropical South America.
The genus name commemorates the Reverend Leonard Jenyns (1800–1893), the English clergyman-naturalist who described the fishes collected on the voyage of the Beagle. The specific epithet diphyes derives from a Greek adjective meaning "of double nature" or "of double form," a reference to the species' chromatic sexual dimorphism — males and females are differently coloured. Lucinda and colleagues distinguished the species on a suite of osteological and pigment characters, including the absence of mandibular canal pore W, long epiotic processes in adults that extend beyond the first vertebra, a single configuration of anal-fin rays, and a midlateral stripe broken into a series of blotches rather than running continuously.
Morphology
Jenynsia diphyes is a small fish that shows pronounced size dimorphism between the sexes, with females noticeably larger than males — a common pattern among livebearing fishes. Reported maximum sizes are about 2 in standard length for males and unsexed individuals and roughly 3 in standard length for females, so a large female is appreciably bigger and deeper-bodied than a mature male.
Meristic counts recorded in the original description and compiled by FishBase give 9 to 11 soft rays in the dorsal fin, 10 soft rays in the anal fin, and 33 vertebrae. Several of the species' diagnostic features are osteological and require dissection — the long epiotic processes, the large third hypobranchial, the presence of teeth on the fourth ceratobranchial of adults, and the absence of one of the mandibular sensory-canal pores. These are the characters Lucinda, Ghedotti and da Graça used to separate it from other members of the genus.
In external pattern the species is marked by discontinuous dark stripes: the midlateral and dorsolateral stripes are each formed by a series of blotches rather than an unbroken line, and the midlateral stripe is interrupted ahead of the origin of the dorsal fin. This blotchy, broken striping, combined with the colour difference between the sexes that gives the fish its name, is the most useful field impression of an otherwise plain, small-bodied livebearer. Detailed colour-in-life notes for this narrow-range endemic are sparse in the accessible literature.
Habitat
Jenynsia diphyes is a narrow-range endemic known from a very small area in the upper Rio Iguaçu drainage of Paraná, in southern Brazil. The type material came from the Córrego Passo do Aterrado and the Rio das Torres, both left-margin tributaries of the Rio Jordão, which is itself a tributary within the Rio Iguaçu basin. The species' entire documented distribution therefore falls within a single river system in the subtropical highlands of southern Brazil.
FishBase classes the species as a freshwater, benthopelagic fish of tropical waters, occupying the water column and the vicinity of the bottom in these small streams. The Iguaçu basin above its great falls is a well-known centre of endemism for small fishes, isolated from the lower Paraná system, and J. diphyes is one of many species confined to its headwaters. Specific measured water parameters — temperature ranges, pH, hardness — are not reported for this species in the accessible sources, so any husbandry figures must be inferred from the genus and the subtropical-highland setting rather than stated as fact.
The restricted, headwater nature of the habitat is central to the species' conservation status: a fish confined to a couple of small tributaries is inherently vulnerable to local changes in water quality, land use and flow. That confinement, rather than any documented population crash, is the basis on which the species has been assessed as threatened.
Feeding
No detailed dietary study of Jenynsia diphyes appears in the accessible literature, so its feeding ecology is inferred from its relatives and from general modelling. FishBase places the species at a trophic level of about 2.9, an estimate consistent with an omnivore that takes a mix of small animal and plant or detrital material rather than a strict carnivore or herbivore.
Members of the genus Jenynsia are generally small, opportunistic feeders of shallow streams and stream margins, taking aquatic and terrestrial invertebrates, insect larvae, micro-crustaceans, algae and organic detritus as they are encountered. A fish of this size in headwater tributaries would be expected to forage along the bottom and in the water column for small drifting and benthic prey, supplementing animal food with whatever algal and detrital matter is available.
Because there is no published diet analysis specific to J. diphyes, these statements are general expectations for the genus and habitat rather than documented observations for this species. Data here are sparse, and the responsible reading is that it is a small omnivorous micro-predator typical of its lineage.
Mating
Like all members of the Anablepidae, Jenynsia diphyes reproduces by internal fertilisation and is viviparous — the female is fertilised internally and gives birth to live young rather than laying eggs. In Jenynsia the male's anal fin is modified into an intromittent organ used to transfer sperm to the female, the functional equivalent of the gonopodium of the poeciliid livebearers, though the two families evolved their copulatory organs independently.
The genus is famous for a striking laterality in its reproductive anatomy: the male's copulatory organ is asymmetrical, capable of moving to one side or the other, and the female genital aperture is likewise restricted, so that "right-handed" males can effectively mate only with "left-sided" females and vice versa. This is the basis of the family's informal name, the "onesided livebearers." The chromatic sexual dimorphism that gives diphyes its name — differently coloured males and females — is the kind of trait that, in livebearers generally, is bound up with mate recognition and mate choice, though no specific mating-behaviour study has been published for this species.
Beyond these family-level traits, the specific courtship behaviour of J. diphyes is undocumented in the accessible sources. What can be stated with confidence is the mode of reproduction — internal fertilisation via a modified anal fin, leading to live birth — rather than the fine detail of how individuals pair and court.
Breeding
Jenynsia diphyes is a livebearer: after internal fertilisation the female carries the developing embryos and gives birth to free-swimming young. The genus Jenynsia is notable for matrotrophy — the mother actively provisions her developing young during gestation through placenta-like maternal tissues, rather than the embryos relying solely on a yolk supply laid down before fertilisation. This matrotrophic, placental-type viviparity is one of the features that distinguishes the anablepid livebearers from the more familiar lecithotrophic (yolk-feeding) reproduction of many poeciliids.
The marked size difference between the sexes — females reaching roughly 3 in against about 2 in in males — is consistent with the demands of livebearing: a larger female body provides room to carry and nourish a developing brood. Where it has been studied in the genus, gestation produces a modest brood of well-developed young that are independent at birth, but specific figures for brood size, gestation length and the frequency of broods in J. diphyes are not reported in the accessible literature, so superfetation (carrying overlapping broods at different stages) cannot be confirmed for this species.
FishBase rates the species as having high resilience, with an estimated population doubling time of under 15 months — a figure that reflects the genus's capacity for rapid reproduction under good conditions. Beyond that life-history modelling, the breeding biology of this particular species remains thinly documented.
In the aquarium
Jenynsia diphyes is essentially unknown in the aquarium hobby. As a recently described, narrow-range endemic of a single drainage in southern Brazil, it has no established presence in the ornamental trade, and there are no widely circulated husbandry accounts for it specifically. Anything written about keeping it is therefore extrapolation from the wider genus rather than experience with this species.
In general, Jenynsia species kept by specialist livebearer enthusiasts are treated as hardy fishes of cooler, well-oxygenated water, suited to a planted tank with steady, clean conditions and a varied omnivorous diet of small live, frozen and prepared foods. Their subtropical-highland origins mean many do better without tropical-warm temperatures, and the size dimorphism and one-sided mating biology make them an interesting but more specialised subject than the everyday guppies and platies.
Given its Endangered status and its absence from the trade, J. diphyes is best regarded as a fish of scientific and conservation interest rather than a hobby animal. The honest position is that captive-care detail for this species is not available, and an aquarist encountering the genus should look to documented husbandry of better-known Jenynsia species while treating any wild-origin stock as a conservation responsibility.
Conservation
Jenynsia diphyes is assessed as Endangered (EN) on the IUCN Red List, under criterion B2ab(iii) — a category that reflects a small area of occupancy combined with a continuing decline in the quality of its habitat. The assessment was carried out on 7 November 2018 and treats the species as a global, single-population endemic of the Rio Iguaçu basin. CITES and CMS list it as Not Evaluated.
The driver of the listing is geography rather than a measured collapse in numbers: the species is known only from a couple of small tributaries within one drainage, and a fish with so restricted a range is acutely exposed to any deterioration in water quality, changes in land use, or alteration of stream flow within that small area. The upper Iguaçu, isolated above its waterfalls, holds many such micro-endemics, and threats to the headwaters there translate directly into threats to the species that live nowhere else.
For a fish of this kind the conservation priority is the integrity of its tiny home range — the streams of the upper Rio Jordão and their catchment — rather than any captive or trade-based intervention. With no aquarium presence and a single known distribution, J. diphyes is a reminder of how much endemic diversity is concentrated in small South American headwaters, and how a narrow range alone is enough to place a species at risk.