Taxonomy & naming
Atlantirivulus jurubatibensis was formally described by Wilson J.E.M. Costa in 2008 in Ichthyological Exploration of Freshwaters (volume 19, number 2, pages 135–140) under the original combination Rivulus jurubatibensis. The type locality is Parque Nacional de Jurubatiba, a protected coastal-plain park on the northern coast of Rio de Janeiro state in southeastern Brazil. Eschmeyer's Catalog of Fishes (CAS reference ID 29871) records the publication and confirms the original authority as Costa, 2008.
The name has since been transferred from the broad, polyphyletic Rivulus to Atlantirivulus, a genus erected to accommodate the Atlantic-coastal South American rivuline clade as part of the ongoing taxonomic reorganisation of the family Rivulidae. The valid current combination is Atlantirivulus jurubatibensis (Costa, 2008), with the parenthetical brackets around the authority indicating the original genus assignment has changed. FishBase synonymy records Rivulus jurubatibensis Costa, 2008 as the original combination and lists Atlantirivulus jurubatibensis as the accepted name.
Within Atlantirivulus, the species is placed in the Rivulus santensis species group, a cluster of closely related small-bodied rivulines distributed along the Atlantic coast of southeastern Brazil. The genus Atlantirivulus remains modestly sized and largely restricted to coastal drainages of Brazil.
Morphology
Atlantirivulus jurubatibensis is an exceptionally small fish: FishBase records a maximum standard length of 1 in for males and 1 in for females, making it one of the smallest members of its genus. The body shape is the typical elongate-fusiform of South American rivulines — somewhat laterally compressed, with a rounded head and a subterminal mouth adapted for taking prey at or just below the water surface.
Colouration follows the pattern common to Atlantic-coastal Atlantirivulus species. Males display more intense pigmentation than females, typically showing iridescent greenish or bluish flanks with reddish or brownish spotting arranged in horizontal rows, and orange-tinged unpaired fins. Females are more cryptically coloured — brownish to olive with less-defined patterning — the standard sexual dichromatism of the genus. The finnage is rounded throughout.
The species shows no extraordinary morphological features that set it apart from its closest congeners; separation from sympatric or parapatric relatives relies on meristic counts, colour-pattern details, and molecular data as documented in Costa's 2008 description.
Habitat
The species inhabits freshwater swamps and coastal lagoon margins on the southeastern Brazilian coastal plain. The known locality is centred on Lagoa do Pires near Quissamã in Rio de Janeiro state, within or adjacent to the protected area of Parque Nacional de Jurubatiba — a mosaic of restinga scrub, freshwater lagoons, and associated wetlands along the Atlantic coast. The water bodies are shallow, still or nearly still, and characterised by organic substrates, submerged and emergent aquatic vegetation, and low transparency.
FishBase records the water chemistry at the type locality as acidic, with a measured pH of 5.28 — consistent with the tannin- and humic-acid-rich blackwaters typical of coastal restinga and Atlantic Forest swamps in southeastern Brazil. Water temperatures in these lowland coastal wetlands are warm year-round. FishBase classifies the species as freshwater, benthopelagic, and occurring in permanent water — not seasonal pools subject to dry-season desiccation, which is consistent with its non-annual reproductive strategy.
The restinga wetlands of the Rio de Janeiro coast are a highly threatened habitat type, and Jurubatiba's protected status is significant for the species. The surrounding coastal plain has been substantially modified by agriculture, eucalyptus plantations, and urban development, leaving the national park as a critical refuge for the endemic biota of the system.
Feeding
Atlantirivulus jurubatibensis is a micro-predator in the manner of most small rivulines. In its natural swamp habitat it would forage on small invertebrates, zooplankton, and aquatic insect larvae associated with the vegetation and organic substrates of the lagoon margins. The subterminal mouth and small body size are consistent with a diet of invertebrates taken at or near the water surface and among fine-leaved aquatic plants.
In captivity the species accepts standard small live and frozen foods appropriate to its size. Freshly hatched Artemia nauplii are a staple for both adults and growing fry, according to hobbyist experience documented in the non-annual rivuline literature. Micro-worms, small Daphnia, and finely chopped frozen bloodworm are also suitable. The very small adult size means appropriately scaled foods are essential — standard-sized live or frozen foods intended for larger killies or community fish will be ignored or are physically impossible for the fish to consume.
Mating
Like all non-annual South American rivulines, Atlantirivulus jurubatibensis is a plant-spawner rather than a soil-burying annual. Pairs spawn over fine-leaved plants, moss, or floating spawning mops, depositing adhesive eggs one at a time or in small groups over the course of days or weeks rather than in a single seasonal mass-spawning event. There is no diapause in the eggs — they develop continuously in water and do not require a dry period to complete incubation.
The mating behaviour typical of the group involves the male courting the female with fin displays and lateral posturing, followed by the pair briefly aligning side by side at the spawning substrate as one or a few eggs are fertilised per embrace. This pattern is repeated repeatedly across an extended spawning period. According to hobbyist accounts, pairs can produce 5–15 eggs per day on floating spawning mops under aquarium conditions, with eggs approximately 0.5 in in diameter and amber-coloured.
The sex ratio in bred populations can be influenced by incubation conditions: lower pH during egg incubation increases the proportion of females among the hatching fry, a phenomenon observed in aquarium practice for this species.
Breeding
Breeding Atlantirivulus jurubatibensis follows the straightforward protocol of non-annual plant-spawning rivulines. Hobbyist accounts describe successful breeding in small aquaria of 2.5 US gal or more, furnished with peat moss or dark gravel substrate, a small sponge filter, and diffuse or subdued lighting. Floating spawning mops are provided as the egg-deposition substrate. Eggs are amber-coloured, roughly 0.5 in in diameter, and incubate in water at room temperature for approximately 14 days before hatching — no peat-pack storage or drying period is required or appropriate.
Newly hatched fry are small but immediately free-swimming and capable of taking freshly hatched Artemia nauplii (baby brine shrimp) as a first food. The fry grow steadily and reach sexual maturity at approximately 4–5 months of age. Adult captive lifespan is documented at up to 3 years, consistent with the longer lives of non-annual killies compared with annual species.
Multiple males and females can be housed together if space is sufficient; however, males can be aggressive toward one another and toward females outside of spawning activity, so providing adequate space and sight-line breaks is important. The species is described as not yet widely established in the hobby as of available records, making it a species of interest to specialist rivuline keepers with access to captive-bred stock.
In the aquarium
Atlantirivulus jurubatibensis is suited to a nano or species aquarium given its adult size of under 1 in. A 10-litre tank is sufficient for a breeding pair or small group, though larger volumes ease water quality management and reduce aggression. The tank must be tightly covered: like all rivulines, this species is an accomplished jumper and will escape through any gap.
Water chemistry should reflect the species' natural habitat: soft, acidic water in the pH 5.0–6.5 range, low hardness, and warm temperatures consistent with the subtropical to tropical climate of coastal Rio de Janeiro. A peat substrate or peat-filtered water aids in replicating the tannin-rich blackwater conditions of the restinga swamps. Diffuse or subdued lighting suits the species' natural habitat and displays the colouration of males to best effect. A small sponge filter provides gentle circulation without creating currents incompatible with the still-water preferences of the fish.
The species is not aggressive toward other species of comparable size, but its tiny adult size makes mixing with anything larger inadvisable. Dedicated species tanks or carefully chosen nano-community setups with other small, non-predatory fish from similar biotopes are the practical options. Excellent jumping ability demands permanent attention to lid security.
Conservation
The IUCN Red List assessed Atlantirivulus jurubatibensis as Least Concern (LC) on 7 November 2018, with the assessment conducted by Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio). The LC designation reflects the species' occurrence within the protected area of Parque Nacional de Jurubatiba, which provides a degree of legal protection to its primary habitat.
Nevertheless, the species has a very small known range centred on a single coastal lagoon system, and coastal restinga wetlands in southeastern Brazil are among the most threatened habitat types in South America. The restinga coastal plain of Rio de Janeiro has been heavily fragmented by agricultural conversion, eucalyptus plantations, oil infrastructure, and urban expansion. Pollution, altered hydrology, and introduced species pose ongoing pressures to coastal lagoon communities. Because the entire known population of A. jurubatibensis is concentrated in a very limited area, any significant impact to the Jurubatiba lagoon system could affect the species disproportionately despite its current LC status.
The species was not recorded as established in the aquarium hobby at the time of available records, meaning captive assurance populations are likely absent or extremely limited. Its long-term security depends substantially on the continued integrity of Parque Nacional de Jurubatiba and the surrounding buffer zone.