Taxonomy & naming
Hypsolebias negobispoi 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 the same Neotropical Ichthyology paper (volume 24, issue 1, article e250044) that described its Piranhas-Açu basin neighbour, H. guararug. It belongs to the genus Hypsolebias, subfamily Cynolebiinae, family Rivulidae, and specifically to the Hypsolebias antenori species group (HAG) — a Caatinga coastal-basin lineage diagnosed by a subdistal orange-red band and distal black band on the anal fin, a light blue or metallic caudal-fin band, and blue or white dots on the flank and fins of males. Before this description the HAG comprised three species (H. gongobira, H. antenori, and H. bonita); H. negobispoi and H. guararug bring the group to five.
The holotype is a 1.28 in SL male (MNRJ 56076) from a temporary pool near the Cabuji River, Macau Municipality, Rio Grande do Norte, with paratypes at MNRJ and UFRN. Eschmeyer's Catalog of Fishes treats Hypsolebias negobispoi Abrantes, Berbel-Filho, Carvalho, Ramos & Lima, 2026 as a valid, currently accepted name. Despite the two new Piranhas-Açu species' type localities lying only about 6 mi apart, a time-calibrated cox1 phylogeny placed H. negobispoi as sister to the remaining four HAG species (posterior probability = 1), diverging roughly 1.2 million years ago in the mid-Pleistocene — well before H. guararug's more recent split from H. antenori (~0.7 Mya) and from the H. bonita + H. gongobira clade (~0.6 Mya). Genetic distances from H. negobispoi to its relatives were the largest measured within the group, ranging from 3.2% (to H. guararug) to 3.9% (to H. antenori), indicating two separate waves of HAG colonisation into the basin rather than local, in-place speciation.
The species is diagnosed from all other HAG members by a distal black stripe on the dorsal fin of adult males, absent in its congeners. It differs from H. gongobira in scale and neuromast counts and dorsal-fin origin position; from H. antenori in filament number and infraorbital-neuromast count and in flank colour (light orange vs. light grey); from H. bonita in neuromast counts and dorsal-fin base length; and from H. guararug, its closest geographic neighbour, in ventral opercular and preopercular neuromast counts and caudal-fin length.
Morphology
Hypsolebias negobispoi is a small annual killifish; the largest male examined reached 1.35 in SL and, unusually for the genus, the largest female was larger still at 1.55 in SL. The body is relatively deep and laterally compressed, deepest at the pelvic-fin base; the head is narrow and elliptical in lateral view with a truncated snout and numerous small conical teeth, no vomerine teeth. In males the urogenital papilla is cylindrical and exposed, while females have a pocket-shaped urogenital opening. Dorsal and anal fins carry a single short filamentous ray in males, absent in females; the caudal fin is rounded in males and subtruncate in females. Cephalic neuromast counts include 13–16 supraorbital, 16 infraorbital, and 11 lateral mandibular series, with no contact organs on the fins.
Colour in life again separates the sexes sharply in the typical pattern: males show a light orange flank with small bluish-white dots, a light orange venter, a yellow-orange dorsal fin with bluish-white dots and a short black filament at the tip, and a caudal fin with a metallic blue distal margin. Females are plainer, with light grey flanks bearing one or two black spots on the anterocentral portion and one to four spots on the caudal peduncle.
As in H. guararug from the same basin, H. negobispoi shows a colour polymorphism not previously recorded in Hypsolebias: four of ten males examined (40%) carried the female-typical pattern of one to two black spots on the flank and caudal peduncle, rather than the typical male colouration. Molecular data confirmed these atypical males share cox1 haplotypes with typical males of the same population, and the pattern appears consistently from juvenile through adult stages.
Habitat
Hypsolebias negobispoi is known only from its type locality, a temporary pool near the Cabuji River, a tributary of the Piranhas-Açu River, in the municipality of Macau, Rio Grande do Norte, Brazil, at 49 ft elevation. The pool covers about 244 m2, with turbid water 2–3 ft deep over a muddy substrate; surface vegetation is dominated by Echinodorus and Nymphaea, while the marginal vegetation consists of jurema-preta (Mimosa tenuiflora) and faveleira (Cnidoscolus quercifolius) trees typical of Caatinga scrub. During fieldwork, researchers observed a great egret (Ardea alba) foraging at the pool.
The pool sits within an active oil extraction unit, and an oil pumpjack operates less than 33 ft from the water's edge. Chemical analysis detected total petroleum hydrocarbon (TPH) contamination at 291.7 μg/L, comparable to levels measured at the two H. guararug localities in the same basin. No second population of H. negobispoi is currently known, making it a true single-locality endemic.
Feeding
No dietary study has been published for Hypsolebias negobispoi 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 and worms, along with terrestrial invertebrates that fall onto the water surface.
Mating
No courtship behaviour has been directly documented for Hypsolebias negobispoi. As tribe-level context: annual Cynolebiinae, including Hypsolebias, spawn into the soft substrate at the bottom of the pool, 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 and burying them rather than leaving them exposed. This reflects general annual-killifish biology rather than an observation specific to H. negobispoi.
Breeding
No captive or field breeding account exists for Hypsolebias negobispoi specifically. By tribe-level analogy: eggs of annual Cynolebiinae are buried in the substrate and enter embryonic diapause, allowing them to survive the pool's complete desiccation through the Caatinga dry season and hatch once 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. negobispoi, which is not established in the hobby.
In the aquarium
Hypsolebias negobispoi is not an aquarium-trade species; it was described only in 2026 from a single wild population, 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 substrate for spawning, dim cover, and water conditions echoing its turbid Caatinga pool — but this is inference from relatives, not documented husbandry. Given the species' extremely narrow range and Critically Endangered profile (see below), wild collection for the trade would be a serious conservation concern rather than a hobby opportunity.
Conservation
Hypsolebias negobispoi 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 Critically Endangered (CR) status under IUCN criterion B2ab(iii), citing an area of occupancy below 10 km2, a distribution restricted to a single known location, and ongoing habitat decline from oil contamination and deforestation — a recommendation from the primary literature rather than a completed Red List listing.
The species' single known pool sits within an active oil extraction unit, with a pumpjack operating less than 33 ft away; water sampled there carried a total petroleum hydrocarbon concentration of 291.7 μg/L, a level the describing authors note falls outside Brazil's CONAMA 357/05 freshwater regulatory framework even though comparably toxic petrochemical compounds are covered. The authors cautiously propose that the documented colour polymorphism in this species and in H. guararug from the same basin may reflect a physiological response to this contamination, while stopping short of establishing a causal link. Wind-farm construction roughly five kilometres from the locality is additionally clearing Caatinga vegetation important to surrounding wetlands, and the Piranhas-Açu basin receives water from the rio São Francisco inter-basin transfer project, which risks disrupting the seasonal pool hydrology the species depends on entirely. With no second population known anywhere, a single adverse event at this one pool could be catastrophic for the species; the authors call for ex-situ conservation studies and stronger environmental licensing protection for Caatinga killifish habitat.