Taxonomy & naming
Cypholebias robustus was described by Albert Günther in 1883 as Cynolebias robustus, from a collection made 'within 10 miles of San Antonio' in the La Plata region of Buenos Aires Province, Argentina; the unique holotype is held at the Natural History Museum, London (BMNH 1879.6.28.12). The parenthesised authority records that the species no longer stands in its original genus.
The name has had an unsettled history. Costa (1995) treated robustus as a synonym of Cynolebias elongatus, and Lazara (2001) as a synonym of Austrolebias bellottii, before Costa (2002) and Calviño (2003) — the latter in a rediscovery account published by the Killi Club Argentino — re-established it as a valid species of Austrolebias. Alonso et al. (2023), in a phylogenetic revision of the Austrolebias group published in the Zoological Journal of the Linnean Society, broke that large genus into several smaller ones and placed robustus in Cypholebias; Eschmeyer's Catalog of Fishes follows this arrangement, recording the current valid name as Cypholebias robustus (Günther, 1883) in the family Rivulidae, subfamily Cynolebiinae.
The Catalog also treats Cynolebias nonoiuliensis Taberner, Fernández-Santos & Castelli, 1974 — described from temporary swamps at Nueve de Julio, Buenos Aires Province — as a synonym of this species, following Koerber et al. (2017) and Alonso et al. (2023), so fish once labelled Austrolebias nonoiuliensis belong here.
The combination Austrolebias robustus, used by the IUCN assessment and much hobby literature, refers to this same fish; Eschmeyer's Catalog of Fishes follows Alonso et al. 2023 in placing it in Cypholebias.
Morphology
FishBase records a maximum size of 3 in standard length for males, with females reaching about 2 in SL — a pronounced size dimorphism, and on the large side for the Pampas annual killifishes. The body follows the deep, laterally compressed plan of the old Austrolebias group, with dorsal and anal fins set far back and a rounded caudal fin; the genus name Cypholebias refers to the convex, humped predorsal profile that develops in large adults.
Species-specific colour documentation in accessible literature is thin; the detailed treatment lives in Günther's original description and in Calviño's (2003) redescription. As throughout the annual Cynolebiinae, males are the larger and more strongly pigmented sex, while females are plainer and cryptically patterned.
Habitat
Cypholebias robustus is endemic to the Pampean region of Buenos Aires Province, Argentina, within the La Plata River basin. It is a seasonal killifish of temporary ponds: the IUCN assessment describes ephemeral pools that typically fill with the autumn rains and dry up in summer, and notes that in some years a pond dries again in winter, squeezing two complete reproductive periods into a single year (Alonso et al. 2016). These ponds sit in the low points of flat agricultural grassland, which makes them the collecting basins for whatever the surrounding land sheds — an ecological position that figures heavily in the species' conservation problems.
The known distribution is small and broken. The 2020 IUCN assessment estimated an area of occupancy of about 1,587 mi² across only a handful of known locations, with the population severely fragmented and declining. No verified water-chemistry measurements for the species' ponds were located for this article; as a temperate-zone annual whose aquatic phase spans the cool Argentine autumn and winter, it should not be assumed to want tropical temperatures.
Feeding
No species-specific diet studies were located for this article. Like the other annual killifishes of the Pampas pools, Cypholebias robustus is a carnivore of small aquatic invertebrates — the insect larvae, microcrustaceans, and worms that bloom in temporary waters — and nothing about its morphology suggests a departure from that guild.
In captivity, keepers of the Austrolebias-group annuals feed live and frozen invertebrate foods: daphnia, bloodworm, white worm, grindal worm, and blackworm are the usual staples, with dry foods at best a supplement for fish accustomed to live prey. Regular, varied feeding maintains condition through the compressed adult lifespan and supports the intensity of male colouration.
Mating
FishBase records the species as a bottom spawner, the reproductive mode shared across the annual Cynolebiinae. In this group, courtship is driven by the male, which displays to females and leads a receptive partner to the soft pond bottom; the pair dive into the substrate together, and eggs are fertilised and buried below the surface, a few per dive, over repeated spawning bouts. Burial is the point: only eggs lodged in the sediment survive the pond's disappearance.
The courtship of C. robustus specifically has not been described in detail in accessible literature, but its membership in this guild — and the IUCN's characterisation of it as a seasonal killifish of drying Pampas ponds — leaves no doubt about the outline.
Breeding
The eggs pass the dry season buried in the pond bed in diapause, hatching when the basin refloods; in nature the adults die as the ponds evaporate. The IUCN assessment notes a wrinkle unusual among annuals: because Buenos Aires ponds sometimes dry in winter as well as in summer, the species can run through two reproductive periods in a single year (Alonso et al. 2016).
In the aquarium, the standard annual method for the group applies: a container of peat or coir receives the spawning pair for some weeks, then the medium is removed, squeezed to damp, and stored; rewetting with cool, soft water triggers hatching, and fry take newly hatched brine shrimp nauplii immediately. Published, species-specific incubation times for C. robustus were not located for this article — the practice of Argentine annual keepers, and exchange through killifish clubs such as the Killi Club Argentino (which documented the species' rediscovery in 2003), are the practical guides. As a temperate Pampas fish it should be kept and bred cool rather than at tropical temperatures.
In the aquarium
This is a fish of the specialist annual-killifish hobby, not the general trade — though the IUCN assessment records that wild fish are occasionally collected for national and international aquarium keeping. Husbandry follows the pattern of the other Pampas annuals: a modest species tank for a pair or trio, a soft spawning substrate (peat or coir, or a dedicated spawning container), gentle sponge filtration, cover for females, and cool water in keeping with its temperate origin. Males of the annual Cynolebiinae are territorial, and the species' short natural lifespan and specific requirements rule out community keeping in any case.
For a Vulnerable species with shrinking wild habitat, a well-run hobby population has real conservation value: diapausing eggs stored in peat travel by mail, and killifish-club exchange has kept other threatened South American annuals in circulation long after their pools were ploughed. Anyone working with C. robustus should keep collection-locality information attached to their stock.
Conservation
The IUCN Red List assesses this species as Vulnerable (VU B2ab(iii,iv); D2), assessed on 17 December 2020 and published in 2022 under the name Austrolebias robustus (Alonso, F. 2022; e.T177559215A177559254). The justification is stark: an area of occupancy under 2,0 mi², a severely fragmented population known from only a handful of locations, and continuing decline in the area, extent, and quality of habitat and in the number of subpopulations. The assessment warns that agricultural expansion — which drains, fills, or ploughs seasonal pools outright — could push the species to Critically Endangered or Extinct in a very short time.
The threat list is the familiar one for Pampas annuals, sharpened by geography: the ponds occupy the lowest ground, so herbicides, pesticides, and nutrient run-off from surrounding cropland drain straight into them. Roads and utility corridors, altered fire regimes, and climate-driven droughts and temperature extremes are also recorded. No part of the population falls within a protected area, no monitoring scheme exists, and the species is not the subject of any formal ex-situ conservation programme — which leaves habitat persistence, protection of the known sites, and the informal captive reservoir of the killifish hobby as the levers that matter.