Taxonomy & naming
Cnesterodon hypselurus was described by Paulo H. F. Lucinda and Julio C. Garavello in 2001, from material collected in the upper rio Paraná system of southern Brazil. It belongs to the genus Cnesterodon, a group of small South American livebearers within the subfamily Poeciliinae of the family Poeciliidae — the New World toothcarps that include the guppy, mollies, swordtails and mosquitofishes. Eschmeyer's Catalog of Fishes treats Cnesterodon hypselurus Lucinda & Garavello, 2001 as the valid name, and it is the authority that governs the spelling and combination used here.
The genus name Cnesterodon derives from the Greek knester (a scraper), a reference to the dentition characteristic of the group; the species epithet hypselurus combines Greek elements meaning roughly "high-tailed," describing the deep caudal region that distinguishes it from its congeners. The best-known member of the genus is Cnesterodon decemmaculatus, the widely distributed ten-spotted livebearer of the La Plata basin; C. hypselurus is by contrast a narrow-range endemic, and much of what can be said about its biology is inferred from these better-studied relatives rather than documented directly for the species.
The vernacular name recorded in databases for this fish is "Cilida toothcarp." Like many recently described Neotropical poeciliids of no commercial interest, it has accumulated little common-name usage, and in the literature it is referred to almost exclusively by its scientific name.
Morphology
Cnesterodon hypselurus is a very small fish, reaching a maximum recorded length of about 1 in in standard length (the measurement from the snout to the base of the tail, excluding the caudal fin). This places it firmly among the miniature poeciliids, comparable in size to a small wild guppy. The species was diagnosed by Lucinda and Garavello on a combination of meristic and morphometric characters, including features of the caudal-peduncle depth that gave rise to the "high-tailed" epithet; the formal diagnosis is contained in the original 2001 description.
As in poeciliids generally, the most reliable external distinction is between the sexes. Males are smaller and bear a gonopodium — a slender intromittent organ formed from modified rays of the anal fin — while females are larger and deeper-bodied. Detailed published descriptions of live colouration are sparse; like its congeners the fish is expected to be modestly pigmented, without the elaborate ornament of domesticated livebearers. Beyond the diagnostic characters in the type description and FishBase's summary figures, species-specific morphological data are limited, and finer points of pattern and proportion are best taken from the original paper.
Habitat
Cnesterodon hypselurus is a freshwater, benthopelagic fish — meaning it lives in the water column near the bottom rather than strictly at the surface or deep below — recorded from tropical streams of the upper rio Paraná drainage in southern Brazil. Its documented range lies in the states of Paraná and Sao Paulo, and includes the rio Paranapanema basin together with headwaters of the alto rio Ivai. The IUCN assessment recognises only about five known locations across this restricted area, marking the species as a narrow-range endemic.
The small streams and headwaters of this part of the upper Paraná are warm, generally clear waters draining the interior plateau of southern Brazil. Precise water-chemistry measurements tied to collection sites for this species are not published in the sources at hand, so temperature, pH and hardness figures for the fish are best treated as the broad tropical-stream envelope typical of the region rather than as measured values. What is documented, and central to its conservation status, is that this habitat is limited in extent and subject to continuing decline in quality — the basis for the species' Endangered listing.
Feeding
No species-specific diet study for Cnesterodon hypselurus appears in the available sources, and FishBase carries no populated feeding record for it. Its trophic level is estimated at roughly 3.0, the value expected for a small omnivorous micro-predator rather than a strict herbivore or a piscivore.
Inference from the genus and from the ecology of small poeciliids generally suggests a diet of algae and detritus together with small aquatic invertebrates — micro-crustaceans, insect larvae and other tiny drifting and benthic items taken from the water column and substrate. Like other small livebearers it is most plausibly an opportunistic feeder, adjusting to whatever small food is locally abundant. These statements are offered as genus-level inference; precise dietary data for this species are sparse and have not been measured.
Mating
Cnesterodon hypselurus is a livebearer, and reproduction in the genus follows the standard poeciliid pattern of internal fertilisation. The male transfers sperm to the female by means of the gonopodium, the modified anal fin that serves as an intromittent organ — the defining reproductive feature of the family. There is no egg-laying and no external spawning; fertilisation takes place inside the female.
Direct observations of courtship and mating behaviour for this particular species have not been published in the sources reviewed. In its well-studied congener Cnesterodon decemmaculatus, as in poeciliids broadly, males pursue females and attempt insemination more or less continuously, and females are larger than males. It is reasonable to expect C. hypselurus to share this general pattern, but the specifics — display behaviour, any sperm storage, mate preference — are not documented for the species and should be regarded as inferred rather than established.
Breeding
As a viviparous member of the Poeciliinae, Cnesterodon hypselurus gives birth to free-swimming young rather than laying eggs. Following internal fertilisation the female carries the developing embryos and releases fully formed fry. Members of the genus are presumed lecithotrophic — the embryos nourished mainly by yolk laid down in the egg, with little or no maternal provisioning during gestation (a matrotrophy index near one, typical of the subfamily) — rather than placental matrotrophs.
No documented gestation length, brood size, or instance of superfetation (the carrying of several broods at different developmental stages simultaneously, which occurs in some poeciliids) has been recorded for this species in the sources at hand. The fish does, however, carry the genus-typical hallmarks of high reproductive resilience: FishBase classes it as having high resilience, with an estimated population doubling time of under fifteen months, consistent with the small body size, early maturity and live-bearing fecundity of the group. Concrete breeding figures for C. hypselurus remain unrecorded — data sparse — and the account above is inference at genus level, not direct observation of this species.
In the aquarium
Cnesterodon hypselurus has essentially no presence in the aquarium hobby. It is a recently described, narrow-range endemic of scientific rather than ornamental interest, and FishBase carries no aquarium or husbandry record for it. It is not a fish encountered in the trade, and there is no established body of keeping experience to draw on.
Were it kept — as its congener C. decemmaculatus occasionally is by livebearer specialists — the expectations would be those of a small, warm-water Neotropical livebearer: a modest planted aquarium, stable clean water in the broad tropical range, fine and varied small foods, and the usual livebearer caveat that adults may predate their own fry, so cover or separation is needed to raise young. These are general livebearer husbandry principles, not species-specific guidance; there is no documented care protocol for C. hypselurus, and its conservation status makes it a fish to study and protect in the wild rather than to seek for the tank.
Conservation
Cnesterodon hypselurus is assessed as Endangered (EN) on the IUCN Red List under criterion B1ab(iii). The assessment was carried out on 7 November 2018 and published in 2022. The listing reflects the species' very small extent of occurrence — it is known from only about five locations within the upper rio Paraná drainage of the Brazilian states of Paraná and Sao Paulo — combined with a continuing decline in the quality of its habitat.
The upper Paraná basin is among the more heavily modified river systems of South America, subject to damming, agricultural runoff, deforestation of riparian zones and other pressures that degrade the small clear-water streams a restricted endemic like this depends on. For such a fish, with a naturally limited range and few populations, even localised habitat loss can represent a meaningful threat to the species as a whole.
There is no indication that the aquarium trade plays any role in its status — the fish is not commercially collected — so its prospects rest entirely on the condition of its native streams. The responsible conservation interest in C. hypselurus is therefore in protecting the small headwater habitats of the upper Paraná that hold its few known populations.