Taxonomy & naming
Atlantirivulus riograndensis was described by Wilson J.E.M. Costa in 2002. The Catalog of Fishes (Eschmeyer, Fricke & van der Laan) records the valid combination as Atlantirivulus riograndensis (Costa, 2002), with the authority in parentheses indicating that the species was originally placed in a different genus. The type locality is a shallow pool near Pai João, Rio Grande do Sul, Brazil, characterised by open vegetation at a forest border and orangish, hyaline water stained by organic matter — a habitat typical of the genus.
The genus Atlantirivulus was erected to accommodate a clade of eleven small rivulines distributed along eastern Brazilian coastal drainages from Bahia south to Rio Grande do Sul. The group was formerly included in the broadly defined Rivulus, a catch-all genus that molecular and morphological revision progressively dismembered through the late 1990s and 2000s. Atlantirivulus riograndensis represents the southern limit of the genus' known range, separated from the nearest congeners to the north by the lowland river systems of Santa Catarina and Paraná.
Morphology
Atlantirivulus riograndensis is a very small fish; FishBase records a maximum of 1.5 in standard length for males. This places it firmly within the Atlantirivulus genus size envelope — the genus as a whole spans 1–2 in SL — and makes it one of the smaller species in the clade. Body form follows the generalised rivuline plan: slender and slightly cylindrical, with a subterminal mouth adapted to surface and near-surface feeding.
Colouration in life has not been extensively documented in the hobbyist literature for this species, but males of Atlantirivulus typically display iridescent flanks with faint banding or spotting on a brownish to olive-gold base, and a caudal fin that may show marginal pigmentation. Females are cryptically coloured with little or no iridescence. Sexual dimorphism in the genus is subtle compared with the vivid colour differences seen in African annual killies: males are more intensely pigmented but both sexes share the same basic body plan.
Habitat
This species is endemic to the Laguna dos Patos drainage system and adjacent coastal drainages of Rio Grande do Sul, the southernmost state of Brazil. The Laguna dos Patos basin is the largest coastal lagoon system in South America, draining into the Atlantic between the Serra Gaúcha highlands and the coastal plain. Within this basin, A. riograndensis occupies very shallow standing or slow-moving water at the margins of gallery forest: the type locality records water only 2–4 in deep, with an orangish, hyaline water column characteristic of dissolved humic acids leaching from forest litter.
The species is associated with small, permanent water bodies — pools, drainage runnels, and shallow marshy areas at the boundary between forest and open grass or scrub. Part of its range falls within or adjacent to the Lagoa do Peixe National Park, a Ramsar Wetland of International Importance on the Rio Grande do Sul coastal plain. As a non-annual killifish, it does not depend on ephemeral, seasonally drying pools; permanent or near-permanent water is a requirement, distinguishing it ecologically from the annual rivulines of the Pampa grassland interior.
Feeding
Specific dietary data for Atlantirivulus riograndensis are limited in the published literature, but the genus as a whole occupies a broadly insectivorous/invertivorous trophic niche. The subterminal to terminal mouth position and small size are consistent with surface and near-surface predation on small invertebrates — microcrustaceans, midge larvae, chironomids, and small terrestrial insects that fall onto the water surface at the forest edge.
In captivity, non-annual rivulines of this size are readily maintained on live or frozen Daphnia, Artemia nauplii, micro-worms, and small chironomid larvae, supplemented with finely ground dried foods. The shallow, vegetated habitat of the type locality suggests that the species forages both in the open water column and among plant matter close to the substrate.
Mating
Atlantirivulus riograndensis is a non-annual killifish and a plant spawner — it does not bury eggs in substrate peat or depend on a dry-season diapause. Pairs spawn by pressing adhesive eggs onto fine-leaved aquatic plants, submerged roots, or fibrous spawning mops close to the water surface or among bottom vegetation. Spawning occurs over extended periods, with a few eggs deposited in each encounter rather than in a single mass-spawning event.
Male courtship in non-annual rivulines follows a broadly consistent pattern: the male displays his most intense colouration in lateral postures beside the female, sometimes quivering rapidly, before the pair align side by side and momentarily press together to release eggs and sperm simultaneously. The adhesive eggs cling to the spawning medium and are left without further parental attention. Spawning activity may recur over many days or weeks, and adults coexist peacefully enough for eggs to accumulate on a continuous mop.
Breeding
Eggs of non-annual rivulines like A. riograndensis are incubated in water rather than buried in moist peat. Incubation time is temperature-dependent; at typical tropical water temperatures in the low-to-mid 20s °C, hatching in congeners generally occurs within two to three weeks. Larvae are fully formed miniature fish at hatching and are immediately free-swimming, capable of taking Artemia nauplii from the first day.
Because clutch sizes per encounter are small and eggs are distributed across a spawning mop over time, the practical approach in captivity is to leave a dedicated spawning mop in the tank and rinse it weekly into a shallow hatching container. Eggs can be incubated floating in shallow water or on a damp peat pad; because diapause is absent, there is no need to dry the eggs. Adults are not strongly cannibalistic but will consume eggs they encounter, so removing the mop periodically is prudent. Fry grow comparatively steadily and reach maturity in a few months under good conditions.
In the aquarium
Atlantirivulus riograndensis is a rarely encountered species in the ornamental trade and is maintained primarily by specialist killifish hobbyists, particularly through American Killifish Association (AKA) and affiliated club networks, rather than commercial channels. Its care requirements match those of other small, non-annual South American rivulines: a well-planted, covered tank of modest size (a 10–15 US gal species tank is ample) with gentle filtration, low to moderate lighting, and soft to moderately soft, slightly acidic water reflecting the orangish, humic-stained conditions of the type locality.
Temperature in the mid-20s °C is appropriate. A dark substrate and floating plants that reduce surface light reinforce the forest-pool microhabitat and bring out the best colouration in males. A tight-fitting lid is essential — killies are accomplished jumpers. The species can be maintained as a pair or in a small group with a skewed sex ratio (more females than males) to distribute male attention. It coexists peacefully with similarly sized, non-competitive tankmates if a community setup is preferred, though a species tank simplifies breeding.
Conservation
The IUCN Red List assessment, confirmed as Least Concern (LC) with a 2018 assessment year, reflects the presence of part of this species' range within protected land — notably the Lagoa do Peixe National Park and its surrounding Ramsar wetlands on the Rio Grande do Sul coast. However, the Least Concern listing should not be read as ecological security: research by Lanés et al. (2012) documents that populations are small, geographically restricted, and show a female-biased sex ratio, all of which elevate extinction risk relative to more abundant, wide-ranging species.
The broader Atlantic Forest biome, within which the coastal drainages of Rio Grande do Sul fall, is one of the world's most threatened biodiversity hotspots. Drainage of small coastal pools and marshy areas for agriculture and urban development, degradation of gallery forest that moderates water chemistry and temperature, and the invasion of native waterways by exotic species are documented pressures on endemic freshwater fish across this landscape. As a specialist of shallow, permanent forest pools within a single river basin, A. riograndensis has limited capacity to colonise new habitat should its current sites deteriorate. Ongoing monitoring of population size and water quality at known localities, and the maintenance of intact riparian forest around small coastal wetlands, are the practical conservation priorities for this species.