Taxonomy & naming
Atlantirivulus depressus has a complex nomenclatural history that reflects decades of revision within the Neotropical rivuline killifish. The species was described by Wilson J.E.M. Costa in 1991 from Atlantic Forest coastal streams of southeastern Brazil, originally placed in the broad genus Rivulus. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong, CAS Ichthyology) records the current valid name as Atlantirivulus depressus (Costa, 1991), placed in the family Rivulidae, subfamily Rivulinae. The parentheses around the authority indicate the species has been transferred from its original genus assignment since description.
The reclassification into Atlantirivulus was formalised by Costa (2008) in the journal Vertebrate Zoology, where he established Atlantirivulus as a subgenus to accommodate a coherent group of rivulines distributed exclusively across the Atlantic Forest biome of eastern Brazil. Subsequent workers have treated Atlantirivulus at varying ranks — as a subgenus of Rivulus or as a full genus — and some databases continue to list the species under Rivulus or Cynodonichthys during the transitional period. The specific epithet depressus is Latin for 'flattened' or 'depressed', referring to the comparatively smaller head depth that distinguishes this species from closely related congeners, as confirmed by the ETYFish Project database on Cyprinodontiformes.
The genus name Atlantirivulus combines Atlantic — referencing the Atlantic Forest coastal plain of eastern Brazil — with rivulus (Latin: small stream), signalling the group's geographic restriction to the streams and forest pools draining the eastern escarpment toward the Atlantic coast. Within the site's taxonomic framework, this species is treated under the group South American rivulines.
This fish was long listed as Anablepsoides depressus; Eschmeyer's Catalog of Fishes places the species in Atlantirivulus, and the current combination Atlantirivulus depressus is followed here.
Morphology
Atlantirivulus depressus is a slender, subcylindrical killifish reaching a maximum total length of 2.6 in, based on the FishBase record. Like other members of the Atlantic Forest rivuline assemblage, the body is elongate with a slightly dorsoventrally compressed head — the feature referenced in the species name — and a superior or slightly terminal mouth suited to feeding at or near the water surface and in the mid-water column. Scales are cycloid and moderately large, and the fins are rounded in both sexes.
Colouration in males follows the pattern common to Atlantic Forest rivulines: a brownish to olive-tan base on the flanks marked with a series of red or orange spots or short bars arranged in longitudinal rows, intensifying during courtship and territorial displays. The unpaired fins in males typically carry pigmentation in the form of spots, ocelli, or marginal banding. Females are plainer, showing a translucent body with reduced spotting and a stouter profile when gravid. Specific published colour descriptions for this species are sparse in the accessible hobby literature, reflecting its infrequent appearance in the aquarium trade outside of Brazilian specialist circles.
Habitat
Atlantirivulus depressus is native to Atlantic coastal river basins of southeastern Brazil, with its documented range spanning São Paulo state and Rio de Janeiro state based on FishBase distribution records confirmed by the Catalog of Fishes. It inhabits the permanent and near-permanent forest streams, pools, and shallow margins that characterise the Atlantic Forest biome — shallow, shaded waterways with overhanging riparian vegetation, submerged leaf litter, fine-leaved aquatic plants, and exposed root systems that serve both as refuge and as spawning substrate.
As a non-annual rivuline, it is associated with water bodies that persist year-round or through most of the year, distinguishing it from the annual killifish of seasonally desiccating savanna pools. The Atlantic Forest streams of southeastern Brazil are typically soft and slightly acidic to near-neutral, reflecting the high rainfall, dense canopy shading, and organic-rich substrate of the biome. FishBase classifies the species as benthopelagic and tropical freshwater, consistent with a fish that exploits the full depth of shallow forest-stream habitats from the substrate to the surface film.
The broader Atlantic Forest coastal plain receives high annual rainfall distributed across much of the year, so the streams Atlantirivulus depressus occupies are not subject to the complete desiccation that drives annual killifish biology. Water temperatures in these lowland forest streams are warm and relatively stable, consistent with the tropical latitude and the insulating effect of the forest canopy.
Feeding
Atlantirivulus depressus is a generalised micro-predator, taking the small invertebrates available in Atlantic Forest stream margins: chironomid larvae, small crustaceans such as daphnia and copepods, fallen surface insects, small worms, and other soft-bodied invertebrates that accumulate in the leaf litter and root systems of its habitat. The slightly superior mouth position is suited to targeting prey at or just below the water surface, though the species also feeds at mid-water and near the substrate.
In captivity, non-annual rivulines from similar Atlantic Forest habitats readily accept small live and frozen invertebrates — artemia nauplii and adults, daphnia, cyclops, bloodworm, and blackworm. FishBase notes the species as difficult to maintain in aquarium conditions, which may reflect sensitivity to water-quality deviations rather than any unusual dietary requirement. Offering a varied diet of appropriately sized live and frozen foods, supplemented with high-quality prepared micro-pellets, supports long-term condition. The relatively small body size (max 2.6 in TL) limits prey size accordingly.
Mating
Atlantirivulus depressus is a non-annual killifish and follows the plant-spawning reproductive strategy characteristic of rivulines from permanent-water habitats. Males court females through lateral displays in which the dorsal and caudal fins are erected to exhibit colour pattern, and the male positions himself alongside or slightly behind the female to initiate an embrace. Courtship in Atlantirivulus is iterative: pairs spawn repeatedly across days to weeks, depositing small numbers of adhesive eggs per spawning act rather than releasing eggs in a single mass event.
Eggs are adhesive and are attached individually or in small clusters to fine-leaved aquatic plants, exposed root masses, floating Java moss, or artificial spawning mops positioned near the water surface or among submerged structure. Incubation proceeds entirely in water — no diapause, no peat burial, and no drying period is required, which sharply distinguishes non-annual plant-spawning rivulines from the annual killifish that deposit diapausing eggs in substrate. This aquatic incubation mode is tied to the permanent-water environment: the species evolved in streams that persist year-round, so the embryo-dormancy adaptations of annual killies conferred no advantage.
Breeding
In the wild, breeding likely occurs across an extended season that tracks the availability of surface vegetation and food in Atlantic Forest streams, rather than being tightly coupled to a single drying event as in annual species. Under aquarium conditions, non-annual rivulines of the Atlantic Forest group are typically conditioned with a varied diet of live and frozen invertebrates and spawned in small species tanks with a floating spawning mop or a clump of fine-leaved plants such as Java moss.
Eggs are removed from the spawning medium every few days and incubated separately in shallow water, where they hatch in approximately 14–21 days at tropical temperatures — timescales consistent with non-annual rivulines in similar genera. Fry are small at hatching and require appropriately fine first foods: infusoria, micro-worm, or vinegar eels for the first days, graduating to artemia nauplii and finely crushed prepared food as they grow. FishBase notes the species as difficult to maintain, suggesting that water quality and stable, soft, slightly acidic conditions merit careful attention both for conditioning adults and for rearing fry.
Because the Atlantic Forest biome supports a high diversity of rivuline species with overlapping captive requirements, Atlantirivulus depressus has occasionally been kept alongside other Atlantic Forest rivulines by Brazilian specialists, but it remains uncommon in wider international killifish collections.
In the aquarium
Atlantirivulus depressus is a challenging species for the general fishkeeper and is best suited to dedicated killifish enthusiasts with experience maintaining Atlantic Forest rivulines. FishBase explicitly notes it as difficult in captivity, which places it among the more demanding non-annual rivulines. A small, densely planted species tank — 11–16 US gal is adequate for a pair or small group — with a tightly fitting lid (killies are skilled jumpers), soft slightly acidic water, dim lighting, and abundant fine-leaved plants or floating mops replicates key features of its natural habitat.
Water conditions should reflect the soft, shaded streams of the Atlantic Forest coastal plain: temperatures in the range of 72–79 °F, pH around 6.0–7.0, and low to moderate hardness. Efficient but gentle filtration is preferable to strong currents. The species is reported as benthopelagic, meaning it uses the full depth of shallow water, so some substrate cover — fine gravel or sand with leaf litter — adds both visual security and foraging surface. Tankmates, if any, should be small, peaceful species with identical water-chemistry requirements; the 2.6 in maximum size places it at risk from larger or aggressive companions.
Because Atlantirivulus depressus is a non-annual killifish with an extended lifespan compared to annual species, it rewards the patience of specialist keepers. The combination of soft water requirements, reported husbandry difficulty, and rarity in trade means it is primarily encountered in the collections of killifish societies in Brazil and specialist hobbyists in Europe who work systematically with Atlantic Forest rivulines.
Conservation
The IUCN Red List assessed Atlantirivulus depressus as Least Concern in November 2018 (species ID 187184, assessment ID 1824379), citing high resilience based on population doubling time estimated at less than 15 months and low fishing vulnerability. Despite the LC assessment, the species' context within the Atlantic Forest biome warrants attention: the Atlantic Forest of southeastern Brazil is one of the world's most threatened tropical forest ecosystems, with estimates suggesting that less than 12–15% of the original forest cover remains, fragmented across a heavily populated and industrialised coastal plain.
The permanent forest streams and coastal river basins of São Paulo and Rio de Janeiro states that Atlantirivulus depressus inhabits are subject to ongoing deforestation, agricultural conversion, urban expansion, and water extraction. The removal of riparian forest cover — which shades streams, maintains water temperature stability, supplies leaf-litter invertebrates, and prevents sedimentation — directly degrades the soft, cool, organic-rich habitat the species requires. Sedimentation and water chemistry changes following deforestation can render previously suitable streams inhospitable to sensitive rivulines.
The Least Concern assessment reflects the species' current range breadth across two states and the absence of documented population declines at a level that would trigger a threatened listing. Given the pace of habitat change in the Atlantic Forest region, periodic reassessment remains important, and the conservation of intact riparian corridors in the coastal river basins of southeastern Brazil is the primary long-term protection measure for this and the many other Atlantic Forest rivuline specialists.