Taxonomy & naming
Pseudoxiphophorus obliquus was described by the American ichthyologist Donn Eric Rosen in 1979. Rosen originally placed the species in the genus Heterandria, and for much of its history the fish circulated under that combination; the parenthetical authority — (Rosen, 1979) — reflects the later transfer of the species to Pseudoxiphophorus, a genus that had earlier been treated as a synonym of Heterandria before being resurrected. The work supporting the modern placement is the revision of the tribe Heterandriini by Morales-Cazan and Albert (2012, Neotropical Ichthyology 10(1):19–44), which examined the monophyly of the group and confirmed obliquus among the Pseudoxiphophorus. Eschmeyer's Catalog of Fishes, the authority for valid names, carries the species as Pseudoxiphophorus obliquus.
The family Poeciliidae unites the New World live-bearing toothcarps — guppies, mollies, platies, swordtails, mosquitofishes and their relatives — within the order Cyprinodontiformes. Pseudoxiphophorus is a chiefly Mexican and Central American genus of small, internally fertilising livebearers; its best-known member in the wider literature is Pseudoxiphophorus bimaculatus (the twospot livebearer, formerly Heterandria bimaculata), with which P. obliquus is recorded as occurring. The single widely used English name for obliquus is the Usumacinta toothcarp, after the river basin from which it is known.
Morphology
Pseudoxiphophorus obliquus is a small fish. FishBase gives a maximum size of about 2.5 in standard length for males or unsexed specimens and roughly 2.5 in for females, figures typical of the smaller Central American poeciliids. Beyond overall size, detailed published descriptions of its colour and meristics are sparse outside Rosen's original account and the genus revision, and this profile does not attempt to reconstruct fine pigment or fin-ray detail that the readily available sources do not state.
As in all poeciliids, the sexes differ in the structure of the anal fin: males bear a gonopodium, a slender intromittent organ formed from modified anal-fin rays and used to transfer sperm internally, while females retain a normal fan-shaped anal fin. Females of the genus also tend to be larger and deeper-bodied than males, and gravid females typically show a darkened patch toward the rear of the abdomen — the developing brood visible through the body wall. These are genus-level generalisations applied to obliquus; species-specific dimorphism notes for this fish are not well documented (data sparse).
Habitat
Pseudoxiphophorus obliquus is a freshwater, benthopelagic, tropical fish of Central America. Its recorded distribution is the Usumacinta River basin in Guatemala — a narrow range within one of the great rivers of the Maya lowlands, draining the highlands of Guatemala and the Mexican state of Chiapas before reaching the Gulf of Mexico. FishBase records it as occurring alongside Pseudoxiphophorus bimaculatus, indicating that it shares lowland stream and river habitat with at least one congener.
No dedicated ecological study of obliquus' preferred water conditions appears in the readily available literature, so the water values given here are inferred from the genus and from the lowland Central American setting rather than measured for this species: warm, tropical fresh water, broadly in the range that suits small benthopelagic poeciliids of Guatemalan rivers and streams. Pseudoxiphophorus as a group tolerates a range of conditions, and the Usumacinta lowlands are warm year-round; precise temperature, pH and hardness preferences for obliquus itself are not recorded (data sparse).
Feeding
No species-specific diet study for Pseudoxiphophorus obliquus is available in the sources at hand. FishBase assigns the species an estimated trophic level of about 3.0, derived from a model rather than from stomach-content analysis, which places it as an omnivore or micro-predator — the typical feeding mode of the genus and of small poeciliids generally.
On that basis, the fish can reasonably be expected to take a mixed diet of algae and detritus together with small aquatic invertebrates, insect larvae and other small drifting or benthic items, shifting opportunistically with what its stream habitat offers. Anything more precise — the balance of plant to animal matter, or seasonal diet shifts — is not documented for obliquus and is not asserted here.
Mating
Like all poeciliids, Pseudoxiphophorus obliquus reproduces by internal fertilisation. The male transfers sperm to the female using the gonopodium, the modified anal fin that is the defining feature of the family; there is no external spawning of eggs. In the better-studied members of the genus, males pursue females and attempt to inseminate them, and females are able to store sperm internally and produce successive broods from a single mating — a pattern general to the livebearing toothcarps.
Direct observations of courtship behaviour, male display, or mate choice in obliquus specifically are not recorded in the available literature. The account here is therefore drawn from the reproductive biology of the genus and family, applied to obliquus with the caveat that its own mating behaviour has not been described in detail (data sparse). Superfetation — the simultaneous carrying of broods at different developmental stages, seen in some poeciliids — is not documented for Pseudoxiphophorus and is not claimed for this species.
Breeding
Pseudoxiphophorus obliquus is viviparous: after internal fertilisation the female carries the developing young and gives birth to free-swimming fry rather than laying eggs. Reproduction in the genus is generally lecithotrophic — the embryos are nourished chiefly by yolk provisioned in the egg, with minimal placental transfer — rather than strongly matrotrophic, so a female's brood develops largely on its own reserves.
No brood size or gestation period has been recorded for obliquus itself. In the well-studied congener Pseudoxiphophorus bimaculatus, gestation runs roughly six to eight weeks depending on the female's age and size, with broods of about twenty to a hundred young; these figures are offered only as a genus-level reference and should not be read as measurements for obliquus, whose own reproductive output is unrecorded (data sparse). As in other livebearers, the fry are born fully formed and able to swim and feed at once.
In the aquarium
Pseudoxiphophorus obliquus is essentially absent from the aquarium hobby. There is no FishBase aquarium-maintenance sheet for it, and specialist livebearer resources — the American Livebearer Association and biotope sites — cover its relatives P. bimaculatus and P. jonesii but not obliquus. Practical, tested husbandry information for this species does not exist in any source consulted, so no care rating is asserted.
What can be said is general: as a small, warm-water Central American livebearer of the Usumacinta lowlands, it would be expected to need warm, well-filtered fresh water and a varied omnivore's diet, with the usual livebearer management of separating or providing cover for fry, which adults may eat. Anyone encountering this fish should treat care guidance as extrapolated from the genus rather than established for the species, and should record their own observations — there is real value in documenting a livebearer this poorly known.
Conservation
Pseudoxiphophorus obliquus is assessed as Data Deficient (DD) on the IUCN Red List, with the assessment dated 15 October 2019. The Data Deficient category means there is insufficient information to make a direct or indirect assessment of its risk of extinction — a reflection of how little is known about the species rather than a statement that it is secure.
The fish has a narrow recorded range within the Usumacinta basin of Guatemala, and many of its congeners are likewise poorly assessed. A narrowly distributed freshwater fish in a region subject to land-use change, pollution and water extraction warrants attention, but in the absence of population data or threat assessment the responsible position is the one the Red List takes: the species is too poorly documented to evaluate. Better field survey and basic natural-history work — distribution, abundance, habitat requirements — are what this fish most needs.