Taxonomy & naming
Hasemania maxillaris was described by Max M. Ellis in 1911. The valid name as maintained in Eschmeyer's Catalog of Fishes is Hasemania maxillaris Ellis, 1911. The genus Hasemania is a small South American characid group within the diverse family Characidae; species in the genus are small, silvery tetras found in Atlantic drainage rivers of eastern Brazil.
The genus name Hasemania commemorates John D. Haseman, the American ichthyologist and fish collector who worked extensively in South America in the early twentieth century and whose specimens formed the basis of several contemporaneous descriptions. The specific epithet maxillaris refers to the jaw (Latin maxilla), likely noting a feature of the upper jaw that distinguished it in Ellis's original material.
Morphology
Hasemania maxillaris is a very small fish, with a maximum recorded total length of 1 in. Like other members of the genus it is a slender, laterally compressed characid with the family's characteristic adipose fin between the dorsal and caudal. Detailed meristic data for the species are sparse in the accessible literature, consistent with its rarity and limited collection history.
Colouration is not well documented from published accounts in the available sources; as a small Hasemania it is likely silvery to pale with subdued body pigmentation, similar to the plain-bodied appearance typical of the genus. Sexual dimorphism, if present, has not been characterised in the available literature.
Habitat
The species is endemic to the Iguaçu River basin in the state of Paraná, Brazil. The Iguaçu (Iguassu) is biogeographically exceptional among South American river systems: the massive barrier of Iguazu Falls isolates the upper basin from the lower Paraná and has driven high endemism in the fish fauna confined above the falls. FishBase records the habitat as freshwater, pelagic, subtropical.
Beyond its basin-level distribution, the precise micro-habitats occupied by H. maxillaris within the Iguaçu system are not well characterised in the available sources. Research documenting the ichthyofauna of the basin from its headwaters to the falls — including a specimen record under FMNH 54303 — confirms the species as autochthonous (native) to the basin, but the specific stream types, substrates, or vegetation associations it prefers remain poorly studied. The combination of extreme endemism and data sparsity is itself a conservation concern.
Feeding
No detailed dietary studies for Hasemania maxillaris appear in the available literature. As a small characid in a subtropical South American stream system it is likely an opportunistic micro-predator or omnivore, consuming small invertebrates, microcrustaceans, and organic particulates in the manner typical of small-bodied members of Characidae.
Data are too sparse to characterise its trophic ecology with confidence. Any captive husbandry accounts would need to draw on analogies with related small characids rather than published observations of this species specifically.
Mating
No direct observations of mating behaviour in Hasemania maxillaris appear in the available literature. By analogy with other small characids in the subfamily — which are generally egg-scattering, open-water or plant-associated spawners with no parental care — the species is presumed to follow a similar reproductive strategy.
A displaying male in this group would likely engage in brief chasing and lateral display alongside a gravid female before a pair or small group releases adhesive or semi-adhesive eggs among fine-leaved vegetation or over the substrate. The absence of parental care is consistent with all Hasemania congeners examined to date, but specific behavioural documentation for H. maxillaris is lacking.
Breeding
Breeding in captivity has not been documented in the accessible literature for Hasemania maxillaris. Given its Critically Endangered status and extreme rarity, the species does not appear in the ornamental trade and captive-breeding accounts are absent from available sources.
By inference from related small Hasemania species and the broader small-characid pattern, reproduction would involve egg scattering over fine-leaved plants or open substrate, with adhesive eggs incubating without parental attendance. Fry would be expected to be very small at hatching, requiring fine first foods. These details remain unconfirmed for this species specifically.
In the aquarium
Hasemania maxillaris is not established in the ornamental-fish trade and is essentially unknown in the aquarium hobby. Its tiny size, Critically Endangered status, and extremely restricted wild range make it unlikely to appear in commercial fisheries, and no husbandry accounts are available in hobby literature.
Should the species ever become available through conservation breeding programmes, the expectation — based on habitat data and family-level norms — would be a subtropical, soft-to-moderately-hard freshwater setup reflecting the Iguaçu basin's conditions, with gentle flow, fine-leaved planting, and small live or frozen foods. For practical purposes this species is best regarded as a conservation subject rather than an aquarium fish.
Conservation
Hasemania maxillaris is assessed as Critically Endangered (CR) on the IUCN Red List, with the assessment carried out on 7 November 2018 under criterion D — meaning the species qualifies based on a very restricted area of occupancy or occurrence, placing the entire population at risk from even localised threats. The species is also listed as a conservation priority by SHOAL (Saving Habitats, One Aquatic Life), an initiative that targets freshwater fish species at acute risk of extinction.
The threats driving the listing are consistent with those facing other endemic species in the Iguaçu basin: habitat degradation within the catchment, pressure from non-native fish introductions that have progressively increased in the basin over recent decades, and the inherent vulnerability of a species whose entire range falls within a single river system. The isolating effect of Iguazu Falls, which created the basin's extraordinary endemism, also means there is no immigration from outside populations to buffer local declines. Increased inventory and population survey work — including studies of the species' current distribution and biological requirements — has been identified as a priority by conservation assessors. Without intervention and protection of the Iguaçu basin's native ichthyofauna, species such as H. maxillaris face a high risk of extinction.