Taxonomy & naming
Hylopanchax thysi was described by Bragança, van der Zee, Sonnenberg, and Vreven in 2020, with the formal account published in the Journal of Fish Biology (volume 98, issue 3, pages 655–667, 2021; doi:10.1111/jfb.14606). The paper introduced two new miniature Hylopanchax species from the Ivindo River basin of northeastern Gabon and provided an updated generic diagnosis based on morphology, colouration, and osteology. The holotype is a male collected 7 mi south of Makokou on 19 August 2012 by L. Chirio; additional material was obtained from a second locality 97 mi southeast of Makokou in July 2006.
The genus Hylopanchax was established by Poll and Lambert in 1965 and belongs to the family Procatopodidae — the African lampeyes — within the order Cyprinodontiformes. Eschmeyer's Catalog of Fishes is the nomenclatural authority for the valid combination Hylopanchax thysi Bragança, van der Zee, Sonnenberg & Vreven, 2020. The specific epithet thysi is a patronym, honouring a person whose contributions to ichthyology the authors wished to recognise; the dedication is recorded in the original description.
H. thysi is distinguished from most other Hylopanchax species by its vertebral count of 30, which it shares only with H. paucisquamatus (which has 31–33 vertebrae), and by the combination of a hyaline male urogenital papilla with black spots and a dark-brown reticulate flank pattern. The revised generic diagnosis based on the 2020 study incorporates osteological characters alongside the colour and squamation criteria used historically.
Morphology
With a maximum recorded standard length of 1 in, Hylopanchax thysi ranks among the smallest lampeyes in a genus of already diminutive fishes. The body plan is typical of the procatopodid lampeyes: slender, laterally compressed, with the dorsal fin positioned posteriorly and the anal fin moderately long, as in most killifishes of the African clade.
The most readily visible diagnostic character in males is the dark-brown reticulate patterning on the flanks — a net-like overlay of darker pigment on a lighter base that distinguishes H. thysi from congeners with plain or striped flanks. The male urogenital papilla is hyaline (translucent) and carries black spots, a feature cited in the original description as diagnostic. The supraorbital iridescent patch — the light organ above the eye that gives lampeyes their common name — is present as in all members of the group, appearing blue-green to silvery under reflected light.
Females are presumably plainer in colouration, as is typical in Hylopanchax and the broader Procatopodidae, with reduced or absent reticulation; detailed published description of female colouration in this species is limited given the recency of its description and the absence of aquarium documentation.
Habitat
Hylopanchax thysi is known from two localities in the Ivindo River drainage, a right-bank tributary system of the Ogowe (Ogooué) River basin in northeastern Gabon. The Ivindo drains dense Central African lowland rainforest, and the smaller streams and affluents within this system — the likely habitat of a miniature procatopodid — are typically clear to lightly tannin-stained, well-oxygenated, and flowing over sand and leaf-litter substrates under dense forest canopy.
Procatopodid lampeyes in this region generally inhabit permanent water: forest streams, stream margins, swampy forest pools, and the quiet backwaters of larger watercourses. They are not seasonal-pond species and do not require desiccation to complete their life cycle, in contrast to the annual killifish of drier regions. The absence of published water chemistry data for this specific locality means that precise temperature and pH ranges are inferred from the broader Ivindo basin context rather than direct measurement, but Central Gabonese forest streams are typically soft, slightly acidic to neutral, and warm (approximately 72–79 °F) year-round.
The Ogowe basin is one of the most biodiverse freshwater systems in West-Central Africa, and the Ivindo tributary in particular is noted for endemic and range-restricted ichthyofauna. The small size and presumed restricted range of H. thysi make its habitat characteristics ecologically significant.
Feeding
No specific dietary data have been published for Hylopanchax thysi. Based on the feeding biology of the genus and family more broadly, lampeyes of the Procatopodidae are primarily micro-predators and planktivores. They feed on small invertebrates — copepods, ostracods, chironomid larvae and pupae, water fleas, and similar crustaceans — as well as terrestrial invertebrates that fall to the water surface beneath the forest canopy.
At 1 in maximum standard length, H. thysi is a genuinely tiny fish, and its prey items in nature will be correspondingly small — microcrustaceans and invertebrate larvae are the most plausible dietary staples. In aquarium conditions, similarly sized lampeyes are typically maintained on live or frozen Daphnia, Artemia nauplii, micro-worms, Cyclops, and other fine invertebrate foods, supplemented with high-quality micro-pellet or crushed flake. A diet with substantial live or frozen invertebrate content supports normal colouration and condition in these fish.
Mating
Hylopanchax thysi, as a non-annual procatopodid lampeye, reproduces as a continuous or opportunistic egg-layer in permanent water — not as a seasonal annual spawner laying diapausing eggs in drying substrate. The mating system typical of this group involves extended spawning over weeks or months, with pairs or small groups depositing small numbers of adhesive or semi-adhesive eggs at each spawning act among fine-leaved plants, roots, and submerged vegetation or detritus.
Male lampeyes typically display the iridescent supraorbital patch and body colouration to females and rival males, with courtship involving lateral displays and close following behaviour. In the small-bodied Hylopanchax species generally, no nest is built and no parental care is rendered — eggs are deposited and abandoned. The reticulate flank pattern and the distinctive urogenital papilla in male H. thysi may function as species-recognition signals in sympatry with other lampeyes, a common selective pressure in diverse West-Central African killifish assemblages where multiple congeners may share the same stream reach.
Breeding
Like other procatopodid lampeyes, Hylopanchax thysi is presumed to deposit small batches of adhesive eggs among fine plant material or root tangles over an extended period, rather than producing a large synchronised clutch as annual killifish do. The eggs are typically small and develop over one to several weeks depending on water temperature, hatching to produce miniature fry capable of feeding on infusoria and micro-prey almost immediately.
No aquarium breeding accounts specific to H. thysi appear to exist in the hobby literature as of 2020; the species is a very recently described research specimen and is not known to be in general circulation in the killifish hobby. Analogous Hylopanchax species have been kept and bred by specialist killifish hobbyists, and the general Procatopodidae care protocol — soft, slightly acidic to neutral warm water, dense fine-leaved planting, removal of adults or eggs to a separate rearing vessel — would apply. Because the fish is extremely small, fry will require the finest first foods: infusoria, rotifers, or commercially prepared fry powders before graduating to micro-worms and Artemia nauplii.
No diapause is required and no peat conditioning of eggs is appropriate — this is not an annual species, and treating it as one would prevent hatching.
In the aquarium
Hylopanchax thysi is, as of the mid-2020s, a research specimen rather than an established aquarium fish. Its extremely small size — 1 in maximum — and the limited collecting localities in northeastern Gabon mean that material in the hobby is essentially absent, and no established aquarium husbandry accounts exist for the species specifically.
Should specimens become available to specialist killifish keepers, the appropriate setup would follow the pattern used for other small West-Central African lampeyes: a small, well-planted aquarium with gentle filtration, soft to moderately soft and slightly acidic to neutral water at 72–79 °F, and a diet of appropriately scaled live and frozen invertebrate foods. Because of the fish's tiny size, tankmates must be chosen with great care — even ostensibly peaceful small fish may predate on a 1 in lampeye — and a species-only or closely managed community with similarly diminutive, non-aggressive fish would be safest.
The genus Hylopanchax is occasionally of interest to specialist West African killifish enthusiasts, and H. thysi's distinctive reticulate male colouration and the rarity of documented miniature procatopodids from the Ivindo drainage give it scientific and hobbyist interest beyond its modest size.
Conservation
Hylopanchax thysi has not been assessed by the IUCN Red List as of the time of writing; the species was described only in 2020 and is known from two localities within the Ivindo River drainage of northeastern Gabon. It has Not Evaluated (NE) status by default.
The Ivindo River basin lies within a region of Gabon that retains substantial forest cover relative to other parts of Central Africa, and the basin's headwaters include areas associated with national parks and protected reserves. However, the extremely limited known range of H. thysi — two collecting points in a single tributary system — means that any localised disturbance to those stream reaches could pose a disproportionate threat to the species. Deforestation for agriculture and logging, watershed disturbance affecting water clarity and flow regimes, and the introduction of non-native fishes are the primary categories of risk for range-restricted forest-stream endemics in this region.
A formal IUCN assessment, which would require adequate survey data on population size and distribution, is a priority for this and the other newly described Hylopanchax species from the Ivindo. Until such an assessment is completed, the conservation status of H. thysi remains unknown, and precautionary protection of its known stream habitats is warranted given the biological vulnerability inherent in a tiny, range-restricted endemic.