Taxonomy & naming
Hypsolebias guararug was described in 2026 by Yuri G. Abrantes, Waldir M. Berbel-Filho, Ranulfo A. Carvalho, Telton P. A. Ramos and Sergio M. Q. Lima, in Neotropical Ichthyology (volume 24, issue 1, article e250044), alongside a second new species from the same river basin, H. negobispoi. It is placed in the genus Hypsolebias within the subfamily Cynolebiinae of the family Rivulidae, and specifically within the Hypsolebias antenori species group (HAG) — a set of Caatinga coastal-basin killifishes diagnosed by a subdistal orange-red band and distal black band on the anal fin, a light blue or metallic band on the caudal fin, and blue or white dots on the flank and fins of males. Before this description the HAG comprised three species: H. gongobira from the Pacoti River basin, H. antenori from the Jaguaribe River basin (both Ceará State), and H. bonita from the Apodi-Mossoró River basin in Rio Grande do Norte.
The holotype is a 1.35 in SL male (MNRJ 56075) from a temporary pool near the Piranhas-Açu River in Pendências Municipality, Rio Grande do Norte, with paratypes held at MNRJ and UFRN. Eschmeyer's Catalog of Fishes treats Hypsolebias guararug Abrantes, Berbel-Filho, Carvalho, Ramos & Lima, 2026 as a valid, currently accepted name. A time-calibrated molecular phylogeny built from the mitochondrial cox1 gene placed H. guararug as more closely related to H. bonita and H. gongobira than to the geographically nearer H. negobispoi, despite the two new species' type localities lying only about 6 mi apart — evidence of at least two independent waves of HAG colonisation into the Piranhas-Açu basin, rather than a single local speciation event. Genetic (cox1) distances between H. guararug and its closest relatives (H. bonita, H. gongobira) were as low as 1.4–1.5%, while the distance to H. negobispoi reached 3.2%.
The species is diagnosed from all three other HAG species by polymorphic males bearing one to three black spots on the anterocentral flank and caudal peduncle — a trait absent in polymorphic males of its congeners. It differs from H. gongobira in pelvic-fin ray count (6 vs. 5) and a lower longitudinal scale-series count (27–28 vs. 33); from H. antenori in lower peduncle scale-series and infraorbital-neuromast counts and a shallower male head; and from H. bonita in the orientation of a dark bar through the eye, a higher lateral mandibular neuromast count, and fewer vertebrae.
Morphology
Hypsolebias guararug is a small annual killifish; the largest male examined reached 1.34 in SL and the largest female 1.04 in SL. The body is laterally compressed and relatively deep, the head narrow and elliptical in lateral view, the snout truncated, with numerous small conical teeth and no vomerine teeth. Fins are pointed in males and rounded in females: males carry one to three filaments on the anal fin, absent in females, and the caudal fin is subtruncate in males but rounded in females. Cephalic neuromast counts (used as taxonomic characters throughout the genus) include 17–19 supraorbital, 15–16 infraorbital, and 10–13 lateral mandibular series, with no contact organs on the unpaired fins.
Colour in life differs sharply between the sexes in the typical pattern: males show a light grey flank shading to grayish purple posteriorly, with bluish-white iridescent spots on the rear flank and caudal peduncle, a pale orange venter, a yellowish-green dorsal fin with large white iridescent spots, one to three short black filaments at the tips of the dorsal and anal fins, and an anal fin with a distal black stripe. Females are plainer, with light brown flanks marked by one to fourteen black spots on the anterocentral portion and hyaline paired fins.
The species' most notable feature, however, is a colour polymorphism never before documented in Hypsolebias. Of 40 males examined, 17 (42.5%) showed one to two black spots on the flank and caudal peduncle — the pattern normally restricted to females — while females themselves proved highly variable, with 16 of 20 (80%) showing six to fourteen spots. Molecular analysis confirmed these atypical males share identical cox1 haplotypes with typical males of the same population, ruling out a second cryptic species as the explanation, and the atypical pattern is present from juvenile through adult stages rather than being a transient developmental stage.
Habitat
Hypsolebias guararug is known from only two localities in the lower Piranhas-Açu River basin, in the municipality of Pendências, Rio Grande do Norte, Brazil. The type locality is a long seasonal pool along the margin of road RN-18, roughly 12 mi from the Atlantic coast, at 20 ft elevation; it covers about 11,368 m2, holds dark, tannin-stained water 2–3 ft deep over sandy and rocky substrate, and is surrounded by carnaúba palms (Copernicia prunifera). A second, smaller pool (487 m2, 1–3 ft deep, muddy substrate) lies four kilometres south, at 52 ft elevation. Both pools carry surface aquatic vegetation dominated by Echinodorus and Nymphaea.
At the type locality H. guararug was found in syntopy with Astyanax aff. bimaculatus, the livebearer Poecilia vivipara, and the introduced platy Xiphophorus maculatus. The area sits within an active onshore oil exploration field of the Potiguar basin, one of Brazil's economically important petroleum and wind-energy production zones in the semiarid Caatinga. Chemical analysis of water from both pools detected total petroleum hydrocarbon (TPH) contamination, ranging from 284 μg/L at the type locality to 447.9 μg/L at the second site.
Feeding
No dietary study has been published for Hypsolebias guararug specifically. As general context for the genus and tribe rather than a documented finding for this species: Hypsolebias and other Caatinga Cynolebiinae are microcarnivores of seasonal pools, taking small aquatic invertebrates, insect larvae, micro-crustaceans and worms, along with terrestrial invertebrates that fall onto the water surface — a diet suited to the short, high-energy growing window of an annual fish.
Mating
No courtship behaviour has been directly documented for Hypsolebias guararug. As tribe-level context: annual Cynolebiinae, including Hypsolebias, spawn into the soft substrate at the bottom of the pool rather than among plants, with the male displaying to a receptive female before the pair dive together into the mud or detritus, releasing eggs and sperm with each dive; the eggs are then buried rather than left exposed. This description reflects general annual-killifish biology rather than an observation specific to H. guararug.
Breeding
As with mating, no captive or field breeding account exists for Hypsolebias guararug specifically. By tribe-level analogy: eggs of annual Cynolebiinae are buried in the substrate and enter embryonic diapause, a developmental arrest that lets them survive the complete drying of the pool through the long Caatinga dry season, hatching only when renewed rains saturate the substrate. Killifish hobbyists who keep annual Rivulidae typically collect eggs on peat or coir, store the substrate through diapause, and re-wet it to trigger hatching, though this method has not been demonstrated for H. guararug, which is not established in the hobby.
In the aquarium
Hypsolebias guararug is not an aquarium-trade species; it was described only in 2026 and no captive stock is known to us. Any attempt to keep it would, by analogy with congeners and the annual-killifish hobby generally, require a small tank with a soft peat or sand spawning layer, dim cover, and water conditions echoing the neutral, tannin-stained Caatinga pools it comes from — but none of this is documented husbandry, and the species' Endangered conservation profile (see below) makes any wild collection for the trade a serious concern rather than an opportunity.
Conservation
Hypsolebias guararug has not yet received a formal IUCN Red List assessment, as is typical for a species described in 2026. The describing authors, however, explicitly recommended Endangered (EN) status under IUCN criteria B1ab(iii)+2ab(iii), based on an estimated extent of occurrence under 5,000 km2 and an area of occupancy below 10 km2, combined with ongoing habitat decline — a recommendation from the primary literature rather than a completed Red List listing.
Both known pools sit within an active onshore oil exploration field, and water sampled from them carried total petroleum hydrocarbon concentrations of 284–448 μg/L — a level the authors note is not explicitly regulated under Brazil's CONAMA 357/05 freshwater standard, even though comparably toxic petrochemical compounds are. The authors propose, cautiously, that the unusual colour polymorphism documented in this species and in H. negobispoi from the same basin may be a physiological response to this contamination, though they stop short of proving a causal link and call for further study. A second threat, wind-farm construction roughly five kilometres south of both localities, is clearing Caatinga shrub vegetation and degrading riparian habitat. The Piranhas-Açu basin is additionally a receiving basin for the rio São Francisco inter-basin water transfer project, which risks altering the seasonal hydrology the species depends on. The authors recommend ex-situ conservation studies and inclusion of killifish habitat in environmental licensing policy as priorities for the species' survival.