Taxonomy & naming
Jenynsia sulfurica was described in 2019 by Aguilera, Terán, Mirande, Alonso, Rometsch, Meyer and Torres-Dowdall in a paper in PLoS ONE titled "Molecular and morphological convergence to sulfide-tolerant fishes in a new species of Jenynsia (Cyprinodontiformes: Anablepidae), the first extremophile member of the family." The species belongs to the order Cyprinodontiformes and the family Anablepidae — the four-eyed fishes and one-sided livebearers — within the subfamily Anablepinae. Eschmeyer's Catalog of Fishes treats Jenynsia sulfurica as the valid name.
The genus Jenynsia commemorates the Reverend Leonard Jenyns (1800–1893), the English clergyman and naturalist who described the fishes from Charles Darwin's Beagle voyage. The specific epithet sulfurica is a Spanish-derived adjective meaning "related to, or from, sulfur," coined for the sulfur-rich spring that is the species' only known home. Phylogenetic analysis places J. sulfurica as the sister species of J. alternimaculata, which lives in nearby non-sulfide waters that are directly connected to the spring; the authors infer that the toxic, hydrogen-sulfide-rich water itself acted as a barrier to gene flow, driving the two lineages apart even in the absence of any geographic separation.
Morphology
Jenynsia sulfurica is a small fish, reaching only about 1.5 in in standard length. Its colouration is understated: a series of 8 to 11 irregular blotches of dark-brown chromatophores runs along the mid-lateral line of the body, varying from rounded spots to short vertical bars up to about three scales deep. The ventral surface is distinctive within the genus — it either lacks scales entirely or bears only a single, sometimes interrupted, row of scales running from the isthmus to the bases of the pelvic fins, rather than being fully scaled like its congeners.
The species' most telling features are those tied to life in oxygen-starved water. It has an enlarged head and an expanded postorbital area, together with a prognathous (forward-projecting) lower jaw bearing a hypertrophied, fleshy lip. These same structures have evolved independently in the sulfide-spring poeciliids of Central America, where they are understood to aid aquatic surface respiration — skimming the thin, comparatively oxygen-rich film of water at the air–water interface, a critical behaviour for a fish living in chronically hypoxic conditions. In Jenynsia sulfurica these traits are a striking case of morphological convergence on a distant family confronted with the same chemical challenge.
Habitat
Jenynsia sulfurica is endemic to Argentina and, as far as is known, to a single locality: Laguna La Quinta in Jujuy Province, in the north-west of the country. The type locality is a sulfide (sulphur) spring, and the fish is restricted to the lagoon and the small ponds associated with it. This is an extreme environment by any measure. The surrounding water carries elevated temperatures — reported in the range of roughly 102–122 °F — together with very low dissolved oxygen, from about 0.25 to 5 mg/L, and high sulfate concentrations of roughly 990 to 1,015 mg/L, reflecting the hydrogen-sulfide chemistry of the spring.
Within this system the fish distributes itself by life stage. Adults are found mostly in the deepest ponds, while juveniles school in the small, shallow ponds at the margins. The species is a sulfide-tolerant extremophile — a fish that has not merely strayed into a harsh habitat but is physiologically built for it. FishBase classifies it as a freshwater, pelagic, tropical fish of high resilience, but that resilience is measured against the chemistry of its home water, not against the loss of the single, small system that contains it.
Feeding
Detailed dietary data for Jenynsia sulfurica are sparse. FishBase places it at an estimated trophic level of about 3.0, consistent with a small omnivore or micro-predator taking a mix of animal and plant matter rather than feeding high on the food chain. In its sulfide-spring habitat, productivity is dominated by sulfur-oxidising bacterial mats and the invertebrates that exploit them, and small livebearers in comparable extreme springs elsewhere subsist largely on these bacterial films, detritus and the few invertebrates able to tolerate the water.
The species' enlarged head and prognathous, thick-lipped lower jaw are primarily respiratory adaptations for skimming the oxygenated surface film, but a robust mouth positioned to work at and just below the surface is also well suited to grazing biofilm and gathering small items there. In the absence of a published stomach-content study for this particular species, any finer description of its diet would be speculation; what can be said is that it occupies the modest omnivore niche typical of small anablepid and poeciliid livebearers.
Mating
Like all members of the genus, Jenynsia sulfurica is a viviparous, internally fertilising livebearer, and it shares the genus's most unusual quirk: it is a "one-sided" livebearer. In Jenynsia the male's intromittent organ and the female's genital pore are lateralised — set off to the left or the right — so that a given male can only mate with a female of the complementary, opposite-handed orientation. This dextral/sinistral asymmetry is the structural counterpart of the gonopodium of the poeciliid livebearers (guppies, mollies, mosquitofishes), in which the intromittent organ is formed from modified anal-fin rays; in the anablepids the analogous organ and the female plumbing are themselves handed.
Species-level details of courtship, mate choice and the mechanics of insemination have not been documented for J. sulfurica, and the original description does not report them. What the work does emphasise is the reproductive consequence of the habitat: the toxic sulfide water surrounding the spring appears to function as a barrier to gene flow with the sister species J. alternimaculata in adjacent non-sulfide habitats, so that mating is effectively confined within the extremophile population. Reproductive isolation here is driven not by geography but by the chemistry that only J. sulfurica can tolerate.
Breeding
Jenynsia sulfurica reproduces by internal fertilisation and gives birth to live young, as is universal in the genus and across the livebearing Anablepidae. Beyond this, the specifics of its reproduction — gestation length, brood size, the frequency of broods, and whether it shows superfetation (the carrying of two or more broods at different developmental stages simultaneously, as occurs in some livebearers) — are not reported in the cached material and are best treated as data sparse for this species.
What the habitat suggests is a reproductive strategy tuned to an extreme, patchy environment. The observed segregation of adults into the deepest ponds and juveniles into the shallow marginal ponds implies that the young are dropped into or move into nursery microhabitats distinct from the adult range — a sensible arrangement in water where oxygen, temperature and sulfide all vary sharply over short distances. Producing well-developed, free-swimming young rather than vulnerable eggs is itself an advantage in such conditions, but any quantitative account of brood size or gestation for J. sulfurica would require direct study that has not yet been published.
In the aquarium
Jenynsia sulfurica is not an aquarium fish in any practical sense, and it should not be regarded as a candidate for the hobby. It is a Critically Endangered extremophile restricted to a single sulfide spring, and its biology is bound up with water conditions — temperatures reported as high as the 40s Celsius, dissolved oxygen near zero, and sulfate concentrations around 1,000 mg/L — that are both lethal to ordinary fishes and effectively impossible to reproduce safely in a home aquarium.
For the aquarist the species is best appreciated as a scientific curiosity rather than a fish to keep: a vivid illustration of how livebearing toothcarps adapt to chemically hostile water, and a reminder that some of the most remarkable fishes are also the most fragile. Its congeners in the genus Jenynsia include hardier, more widespread species that occasionally appear in specialist livebearer circles, but J. sulfurica itself belongs in the field and in conservation programmes, not in the trade. Collecting from its one known locality would be both ethically and ecologically indefensible given the species' status.
Conservation
Jenynsia sulfurica is assessed as Critically Endangered (CR) on the IUCN Red List, under criteria B1ab(iii)+2ab(iii). The listing reflects the species' extraordinarily small and restricted range: it is known from a single sulfide-spring system, Laguna La Quinta in Jujuy Province, Argentina, and from nowhere else. A species confined to one tiny, hydrologically peculiar locality is acutely vulnerable to any change in that one place — alteration of the spring's flow or chemistry, pollution, water extraction, or disturbance of the lagoon could remove the entire global population at a stroke.
The same traits that make Jenynsia sulfurica scientifically fascinating also make it a conservation priority. Its dependence on a unique sulfide habitat means it cannot simply relocate if that habitat is degraded, and its evolutionary distinctiveness — a recently diverged, extremophile lineage convergent with sulfide-spring fishes on another continent — gives it value out of all proportion to its small size. Protecting the species means protecting the integrity of the Laguna La Quinta spring system itself: safeguarding the single body of water is, in effect, the whole of the conservation task.