Taxonomy & naming
Cnesterodon iguape was described by Paulo H. F. Lucinda in 2005 in Neotropical Ichthyology, in the paper "Systematics of the genus Cnesterodon Garman, 1895" (Neotrop. Ichthyol. 3(2):259–270). The species was erected on a small type series collected from a single creek: the holotype, a male catalogued as MZUSP 79672, came from a creek in the headwaters of the rio Iporanga inside the Mineradora Oxical property at Apiaí, São Paulo, Brazil (24°24′42″S, 48°39′25″W), collected on 4 April 1999, with paratypes catalogued as MZUSP 54978. Eschmeyer's Catalog of Fishes treats Cnesterodon iguape Lucinda, 2005 as the valid name, placing it in the genus Cnesterodon within the subfamily Poeciliinae of the family Poeciliidae — the New World livebearers.
The genus Cnesterodon Garman, 1895 is a small assemblage of diminutive South American toothcarps centred on the La Plata and adjacent Atlantic drainages; Lucinda's 2005 revision was the work that sorted the group and named several new species, of which iguape was one. The genus name derives from the Greek knester ("scraper"), a reference to the dentition characteristic of these fishes. The specific epithet iguape is geographic: it honours the rio Ribeira de Iguape, the larger river system in whose headwaters the type locality lies. The common name reported in the database literature, "Iporanga tooth carp," likewise refers to the rio Iporanga where the species was found.
Morphology
Cnesterodon iguape is a very small fish. The type series spans roughly 0.5–1 in standard length in females and 0.5–1 in in males, with FishBase listing maxima of about 1 in SL for males and 1 in SL for females — placing it firmly among the miniature poeciliids. The body is compressed, the mouth is superior and set almost level with the upper border of the pupil, and the dorsal fin is positioned behind the middle of the body. Meristic counts from the original description give dorsal-fin rays mostly 8 (range 7–9), pectoral rays 10–12, caudal rays 26–28, longitudinal scale series 27–33, and 32–33 vertebrae; the anal-fin soft-ray count is reported as 10.
The colour pattern, as preserved in alcohol, is a cream-yellow ground overlaid with a reticulate brown network. Along the sides run seven to nine dark brown vertical bars confined to the midline, with median dark brown lines along the predorsal and preanal surfaces. The diagnostic feature that sets adult males apart — the autapomorphy Lucinda used to distinguish the species — is a large dark blotch on the ventral profile just behind the gonopodium. The male gonopodium itself is symmetrical and built of eight gonopodial rays, with the gonactinosts of segments 2-3-4 fused and a bony style ending in a terminal filament, the kind of fine structural detail that underpins poeciliid systematics.
Habitat
The species is a freshwater, benthopelagic, tropical fish known only from the upper rio Iporanga, a headwater stream in the rio Ribeira de Iguape system in Apiaí, São Paulo state, southeastern Brazil. The type locality is a small creek rising within the Mineradora Oxical mining property — limestone (oxical, or quicklime) country in the karst-rich Ribeira valley, a region of caves, springs and clear calcareous streams. Everything reliably documented about the fish comes from this one narrow-range locality, making Cnesterodon iguape a textbook microendemic.
Beyond the type-locality coordinates and the general benthopelagic, tropical-freshwater characterization carried in the database literature, detailed in-situ measurements of temperature, pH and flow for this exact creek are sparse in the published record. The broader Ribeira de Iguape headwaters are warm, well-oxygenated, often hard and alkaline limestone-influenced waters, which is consistent with the calcareous setting of the type locality, but specific habitat parameters for C. iguape itself should be regarded as data-sparse rather than firmly established.
Feeding
No dedicated dietary study has been published for Cnesterodon iguape. FishBase places the species at a trophic level of about 3.0, the value typical of small omnivorous and micro-carnivorous poeciliids. By analogy with its congeners and other diminutive livebearers of similar size and mouth structure, it is expected to feed on small aquatic invertebrates, microcrustaceans, insect larvae, algae, biofilm and organic detritus gleaned from the substrate and water column of its headwater creek.
The superior, upturned mouth — aligned with the upper edge of the pupil — suggests a fish adapted to taking food from the surface film and upper water column, as is common among small surface-oriented poeciliids that pick at drifting particles and emergent invertebrate prey. In the absence of stomach-content data, any more precise dietary statement would be speculation; what can be said with confidence is only that the species is a small generalist micro-feeder at the lower end of the trophic scale.
Mating
Like all members of the Poeciliidae, Cnesterodon iguape reproduces by internal fertilisation. Males possess a gonopodium — an intromittent organ formed from modified anal-fin rays — through which sperm is transferred to the female; in this species the gonopodium is symmetrical and built of eight rays, with a bony style terminating in a filament. The large dark blotch that develops behind the gonopodium in mature males is the species' diagnostic trait and presumably plays a role in sexual signalling or species recognition, as conspicuous male markings commonly do in livebearers, though no behavioural work has tested this directly.
No courtship or mating behaviour has been described for C. iguape specifically, and the original description does not address reproduction. In small Cnesterodon and related poeciliids, males typically pursue females and attempt copulation using the gonopodium, and females of many poeciliid lineages are capable of storing sperm internally to produce successive broods from a single insemination. Whether superfetation — the simultaneous carrying of broods at different developmental stages, known in some poeciliid genera — occurs in Cnesterodon is not documented here and should not be assumed for this species.
Breeding
The Poeciliidae are viviparous (live-bearing): after internal fertilisation the female carries the developing embryos and gives birth to free-swimming young rather than laying eggs. Cnesterodon iguape is therefore a livebearer, but no specific data on its gestation period, brood size or breeding season appear in the original description or in the database record — these reproductive details are genuinely data-sparse for the species.
What can be inferred is constrained by the fish's tiny size. At a maximum of roughly 1 in standard length, females of C. iguape are among the smallest of livebearers, and small-bodied poeciliids characteristically produce small broods — typically a few to a few dozen fully formed fry per cycle in comparable species. FishBase rates the species' resilience as high, with an estimated population doubling time of under fifteen months, which is consistent with the rapid maturation and continuous reproduction typical of small livebearers. Any figure for exact brood size or gestation length, however, would be a guess and is deliberately not asserted here.
In the aquarium
Cnesterodon iguape is not a fish of the aquarium trade. It is a narrow-range microendemic known only from a single creek inside a mining property, it has no record of captive maintenance in the hobby, and its Critically Endangered status makes it an unsuitable and ethically problematic candidate for collection. There is, accordingly, no body of husbandry experience to report.
If the species were ever to be held — most plausibly in a conservation or research setting rather than a home aquarium — its requirements would presumably mirror those of other small Cnesterodon: a well-filtered, stable tank with hard, alkaline, limestone-influenced water reflecting its karst headwater origins, warm tropical temperatures, gentle flow, and the dense fine-leaved cover that small livebearers need so that fry can escape predation by adults. These are reasoned extrapolations, not established care notes; the honest position is that aquarium data for C. iguape do not exist.
Conservation
Cnesterodon iguape is assessed as Critically Endangered (CR) on the IUCN Red List under criteria B2ab(iii), with the assessment dated 7 November 2018 (IUCN species ID 187059). The Critically Endangered listing follows directly from the species' geography: it is known from a single locality — a creek in the headwaters of the rio Iporanga at Apiaí, São Paulo — giving it an extremely small area of occupancy and no documented second population to buffer it against local catastrophe.
The type locality lies inside an active mining property (Mineradora Oxical) in the limestone Ribeira valley, and the central threat to a single-creek microendemic of this kind is degradation or loss of that one habitat — through mining activity, water abstraction, pollution, or alteration of the spring-fed flow on which the population depends. The B2ab(iii) criteria explicitly reflect a restricted, fragmented or single-location range combined with a continuing decline in habitat quality. Securing the species realistically requires protecting the integrity of its headwater creek and ideally confirming whether any additional populations exist elsewhere in the upper Ribeira de Iguape drainage; until then, Cnesterodon iguape remains one of the most precariously placed of all the New World livebearers.