Taxonomy & naming
Prorivulus auriferus was described by Wilson J.E.M. Costa in 2004 in the journal Zootaxa (642: 1–7), making it simultaneously a new genus and a new species. The authority is conventionally cited as (Costa & Leal, 1990) in some databases, but the formal published description attributing the genus and species to Costa 2004 is the reference recorded in the Catalog of Fishes. The holotype is an adult male measuring 1 in standard length, collected at Valença in the state of Bahia on 28 June 2003 by A.O. Lima and Rogério R. Suzart.
The genus Prorivulus is diagnosed by a suite of osteological characters that set it apart from otherwise morphologically similar rivulids: five branchiostegal rays (the bony rods supporting the gill membrane on each side), an unossified interhyal (a small bone in the hyoid arch that remains cartilaginous), and an absent distal process on the second epibranchial (a gill-arch element). Costa placed Prorivulus in a basal position within the family Rivulidae, allied with but distinct from Rivulus and the annual-fish genera, based on these synapomorphies. The American Killifish Association lists the species in its Class 2 Rivulus group.
The Catalog of Fishes treats Prorivulus as a valid monotypic genus with auriferus as its single species. No subspecies or regional variants have been described, and the extremely restricted known range has so far precluded the discovery of additional congeners.
Morphology
Prorivulus auriferus is a slender, small-bodied rivuline reaching a maximum of approximately 1.5 in standard length, with most adults closer to 1 in SL. The body form is typical of the non-annual rivulines: laterally somewhat compressed, elongate, with a slightly upturned mouth suited to surface feeding. Meristic counts from the type description include dorsal fin rays 9–10, anal fin rays 14–15, and approximately 32–33 lateral scales, giving the fish a fairly long, parallel-sided profile.
The species' common name and scientific epithet derive from the distinctive golden spots on the lower lateral body and ventral surface — small, well-defined points of gold or amber pigment set against a brownish to olive background. Males are more intensely coloured than females, developing richer flank markings and often showing reddish or orange tones in the dorsal and caudal fins during display. Females are plainer, with a more rounded abdomen and reduced finnage. The amber eggs, about 0.5 in in diameter, are visible in gravid females held against a light source.
Habitat
The species is known from the coastal lowlands of Valença, Bahia, in the Atlantic Forest biome of northeastern Brazil. This is a region of remnant Atlantic Forest fragments interspersed with cattle pastures, sugar-cane cultivation, and secondary vegetation, running through low-lying ground close to the coast. The natural habitat is permanent or near-permanent freshwater — shallow pools, roadside puddles, and small forest streams — rather than the seasonally drying fields occupied by annual killifish.
As a non-annual species, Prorivulus auriferus does not require a dry-season diapause and is not found in ephemeral habitats. The coastal plain setting suggests soft, possibly slightly acidic lowland water associated with the drainage systems emptying toward the Atlantic, though specific water-chemistry data for the type locality are not published in available sources. Temperatures in the Bahian coastal lowlands are consistently warm, ranging from the high teens at night in the cooler dry season to well above 86 °F in the wet summer months; aquarium experience suggests an optimum in the 72–79 °F range.
Feeding
In nature, small rivulines of this size feed on microcrustaceans, aquatic insects and their larvae, small worms, and whatever invertebrates are available at the water surface or in the substrate litter. Prorivulus auriferus is not documented to have specialised dietary requirements, and its upward-angled mouth suggests a preference for surface or near-surface prey.
In the aquarium the fish is straightforward to feed. It accepts live and frozen Artemia nauplii, Daphnia, small Tubifex, and micro-worms readily, and can be trained onto fine-ground dry foods over time. Frequent small feedings — two or three times daily — suit its small stomach and active metabolism. As with most killies of this size, variety in the diet maintains condition and is especially important for fish kept for breeding.
Mating
Prorivulus auriferus is a non-annual, plant-spawning rivuline. Unlike the annual killifish of the seasonally drying savannas, which bury their eggs in peat and require a dry-season diapause, this species spawns continuously in permanent water, depositing eggs on fine-leaved vegetation, moss, roots, or floating spawning mops over an extended period. There is no concentrated seasonal spawning event — in stable aquarium conditions the pair spawn at low but persistent rates throughout the year.
Males are active and may display with spread fins and lateral posturing, but the species is not notably aggressive. A pair or trio (one male, two females) can be kept in a modest tank without undue stress to females. Courtship involves the male positioning alongside or below the female and nudging toward a chosen spawning site; the brief embrace and egg release follow. Eggs are deposited singly or in very small numbers during each encounter, accumulating slowly rather than in a mass spawn.
Breeding
Prorivulus auriferus spawns 5–15 adhesive eggs per day across its adult life, according to hobbyist accounts. Each egg is approximately 0.5 in in diameter, amber in colour, and attaches lightly to fine plant material or mop fibres. Incubation in water at around 75 °F takes approximately 14 days, after which tiny but fully formed fry emerge; unlike annual killifish eggs, these require no dry storage or peat medium — they develop continuously in water.
A breeding setup of around 2.5 US gal is adequate for a pair or trio. The tank is furnished with dark substrate (fine gravel or peat), a floating spawning mop of Java moss or nylon wool, and subdued lighting. Eggs can be picked out daily or the mop transferred to a separate rearing container. First foods for fry are Artemia nauplii or commercial micro-fry powder; growth is steady and fry reach sexual maturity at around 4–5 months. The fish are comparatively long-lived for killies — individuals have been maintained in good condition for up to three years. A tight-fitting lid is essential, as the fish are capable jumpers.
In the aquarium
Prorivulus auriferus is an undemanding species well suited to the specialist killi keeper. Its small size (under 1.5 in SL) and peaceful temperament make it compatible with other small, quiet fish, though it is best displayed in a species tank where its subtle colouration and spawning behaviour can be observed without distraction. A tank of 10 US gal is comfortable for a colony; heavy planting with fine-leaved species (Ceratopteris, Riccia, Java moss) or floating mops mimics the vegetated margins of its natural habitat and provides spawning sites and fry refuge.
Water chemistry should broadly reflect the soft, warm lowland conditions of the Bahian coastal plain: temperature 72–79 °F, slightly acidic to neutral pH (around 6.0–7.0), and low to moderate hardness. Regular partial water changes maintain quality without the need for intensive filtration. The species is not widely available in the hobby — it has a small but dedicated following among killi enthusiasts — but captive populations circulate through killifish association auctions and specialist breeders, making it accessible to those who seek it out.
Conservation
The IUCN Red List assessed Prorivulus auriferus as Data Deficient (DD) in November 2018, evaluated by the Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio). The DD rating reflects inadequate survey data rather than confirmed population stability: the species is known from a narrow strip of Atlantic Forest coastal lowland in Bahia, a biome that has been reduced to approximately 12–15% of its original extent through centuries of agricultural clearance and is now one of the world's most threatened biodiversity hotspots.
The pressures on the species' habitat are real and ongoing. Coastal Bahia has experienced sustained conversion of remnant forest to sugar-cane cultivation, cattle pasture, and urbanisation around towns such as Valença. The small, shallow freshwater pools and streams that non-annual rivulines depend on are particularly vulnerable to drainage for agriculture, altered hydrology from land clearance, and contamination from agricultural run-off. Because Prorivulus auriferus is the only member of its genus and is known from a restricted area, any significant loss of its coastal lowland habitat could pose a genuine extinction risk. Additional field surveys are needed to establish whether undiscovered populations exist and to quantify the extent and condition of occupied habitat before a more precise threat category can be assigned.