Taxonomy & naming
Jenynsia sanctaecatarinae was described by Michael Ghedotti and Stanley Weitzman in 1996, and Eschmeyer's Catalog of Fishes treats the name as valid (genus Jenynsia, the governing authority for the binomial). The species sits in the family Anablepidae — the four-eyed fishes and onesided livebearers — within the order Cyprinodontiformes, alongside the better-known anablepids and the genus Jenynsia itself. Unlike the guppies, mollies and swordtails of the hobby, which belong to the family Poeciliidae, Jenynsia represents a separate New World live-bearing lineage with its own distinctive reproductive anatomy.
The genus name Jenynsia honours the Reverend Leonard Jenyns (1800–1893), the English naturalist who described the fishes of Darwin's Beagle voyage and after whom the type species of the genus was named. The specific epithet sanctaecatarinae refers to Santa Catarina, the southern-Brazilian state from whose upland drainages the species was described. Its English common name, the Meleiro livebearer, points to the same region. The species was placed and re-examined within the broader phylogenetic framework Ghedotti developed for the Anablepidae, published in 1998.
Morphology
This is a small fish. Recorded maxima are roughly 1.5 in standard length in males and unsexed specimens and about 1.5 in SL in females, the larger female size being typical of livebearers where the female must carry developing young. The body is slender and the fins modest; the dorsal fin carries about 8 to 10 soft rays and the anal fin about 10.
The most useful field marks are the markings on the flanks: distinct horizontal, dash-shaped streaks roughly one to three scales long, arranged into four or five irregular rows along the sides, together with a rounded spot on the upper base of the pectoral fin. Females of this species lack the distinct swelling between the urogenital opening and the front of the anal-fin base that helps separate them from some congeners. As in other Jenynsia, the sexes differ in the anal fin: males bear a modified, asymmetrical anal fin (the gonopodium) used in mating, while females retain an unmodified fin. Colour in life is otherwise subdued, in keeping with a small stream fish.
Habitat
Jenynsia sanctaecatarinae is a freshwater, benthopelagic fish of subtropical streams. It is native to southern Brazil, where it occupies the upland drainages of the Santa Catarina region — a restricted range that is central both to its biology and to its conservation status. FishBase records a lower temperature reference of about 77 °F for the species, consistent with a warm-temperate to subtropical setting rather than a tropical lowland one.
Beyond the broad picture of small upland streams, fine-grained habitat data for this species are sparse. Its congeners in the genus span fresh and even brackish coastal waters in southern South America, but J. sanctaecatarinae itself is tied to inland stream habitats within a small geographic area. That narrow distribution is exactly what underlies its Endangered assessment: a species confined to a limited extent of occurrence is vulnerable to localised habitat change in a way that a wide-ranging fish is not.
Feeding
Direct dietary studies of Jenynsia sanctaecatarinae are not available, so its feeding is inferred from its size, its stream habitat and the trophic position estimated for it (a trophic level around 2.9, the band occupied by small omnivores and invertebrate feeders). On that basis it is best treated as a small omnivore, taking aquatic invertebrates, insect larvae, small crustaceans and associated algal and detrital material from the streambed and water column.
In the aquarium — for the rare keeper who encounters this species — a Jenynsia of this size would be expected to accept the standard small-livebearer fare: good-quality flake and micro-pellets supplemented with small live and frozen foods such as Daphnia, brine shrimp and finely chopped bloodworm, with some vegetable or algal content. Because hard species-specific data are thin, husbandry here is reasoned from the genus rather than documented for the species itself.
Mating
Jenynsia are internally fertilising livebearers, and their mating is shaped by an unusual and strict anatomical asymmetry that gives the group its name — "onesided livebearers." The male's anal fin is modified into a gonopodium that is bent permanently to one side, either left or right, while the female's genital opening likewise faces one side. As a result a given male can only successfully mate with females whose genital orientation is the mirror image of his own gonopodium; left-sided and right-sided morphs coexist in populations, and the geometry of mating enforces a kind of lock-and-key pairing absent from the symmetrical gonopodium of poeciliids like the guppy.
Fertilisation is internal: the male transfers sperm via the gonopodium, after which the female carries the developing embryos. Detailed courtship behaviour specific to J. sanctaecatarinae has not been documented, and the account here draws on genus-level reproductive biology — including work on the congener J. multidentata — rather than on species-specific observation. What is consistent across Jenynsia is the combination of internal fertilisation and the one-sided mating constraint.
Breeding
Like other members of its genus, Jenynsia sanctaecatarinae is viviparous, giving birth to live, free-swimming young rather than laying eggs. A distinctive feature of Jenynsia reproduction is matrotrophy: the developing embryos are nourished by the mother during gestation through placenta-like provisioning (trophotaeniae and ovarian nourishment), rather than relying solely on yolk reserves as in many egg-laying or lecithotrophic livebearers. This maternal investment means the embryos draw on the mother throughout development.
Given the species' small body size, broods are expected to be correspondingly modest, though precise brood counts for J. sanctaecatarinae are not recorded in the available sources — data are sparse at the species level. One point worth stating plainly: superfetation, the carrying of overlapping broods at different developmental stages found in some poeciliids and goodeids, is not characteristic of Jenynsia, so this species should not be assumed to display it. The reproductive picture here is reconstructed from genus-level biology and should be read as such.
In the aquarium
Jenynsia sanctaecatarinae is essentially not an aquarium-trade fish: it is a small, range-restricted, little-known species rather than a hobby staple, and reliable husbandry accounts specific to it are not available. Any keeping notes therefore have to be reasoned from its biology — a small subtropical stream livebearer of roughly 1.5 in — rather than quoted from documented practice.
On that basis, a keeper would aim for clean, well-oxygenated water in the warm-temperate to subtropical range (around the mid-20s Celsius indicated for the species), a group of fish with females outnumbering males to spread out mating activity, and the usual small-livebearer diet of varied flake, micro-pellets and small live and frozen foods. The honest position is that this fish belongs to conservation and ichthyology far more than to the aquarium, and that anyone encountering it should treat husbandry details as provisional.
Conservation
Jenynsia sanctaecatarinae is assessed as Endangered (EN) on the IUCN Red List under criteria B1ab(iii), in an assessment published on 7 November 2018 and carried out by Brazil's Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio). The B1 criterion reflects a small extent of occurrence, and the ab(iii) qualifiers indicate a severely restricted, fragmented distribution together with a continuing decline in the quality of the species' habitat.
In plain terms, the threat is geographic: a fish confined to a small patch of upland streams in southern Brazil has little buffer against local pressures such as water-quality decline, habitat alteration and the loss or degradation of the particular streams it depends on. There is no indication that aquarium collection is a factor here; the concern is the integrity of the species' narrow native range. For a fish this localised, conservation rests on protecting the specific southern-Brazilian drainages it occupies rather than on any captive or trade-based measure.