Taxonomy & naming
Fundulopanchax gresensi was described by H. O. Berkenkamp in 2003 in the German aquarium journal Aquarien Aktuell (6/2003), in a paper devoted to the Fundulopanchax mirabilis species complex: "Über den Artenkreis Fundulopanchax mirabilis (Radda, 1970), mit der Beschreibung von Fundulopanchax gresensi sp. nov." The type locality is the Takwai area of West Cameroon (approximately 5°34′N, 9°50′E). Eschmeyer's Catalog of Fishes recognises Fundulopanchax gresensi Berkenkamp, 2003 as the valid name; the authority is not parenthesised because the species was described directly in the genus Fundulopanchax.
Fundulopanchax is a genus of West and Central African killifishes in the family Nothobranchiidae. The genus is divided informally into species groups on the basis of morphology and geography; gresensi belongs to the mirabilis group, a cluster of Lower Guinea species associated with the rainforest zone of Cameroon and adjacent areas. The limits of that group, and the relationships among its members, have not yet been fully resolved by phylogenetic analysis, and additional collecting in Cameroon's highland streams may yet yield closely related undescribed taxa.
Morphology
FishBase records a maximum standard length of 2 in, making Fundulopanchax gresensi one of the smaller members of its genus. Seriously Fish gives a working range of 1.5–2 in SL for mature individuals. The body plan is typical of the mirabilis-group Fundulopanchax: a moderately elongate, laterally compressed fish with a slightly upturned mouth suited to taking prey at or near the water surface, and relatively large scales that reflect the brighter colours of the male in good light.
Detailed colour descriptions from wild-caught Takwai specimens are sparse in the published literature, but males of the mirabilis complex generally display red to orange spotting or banding on the flanks against a blue-green to greenish ground colour, with ornate unpaired fins. Females are smaller, less intensely coloured, and rounder in the belly when gravid. Sexual dimorphism in fin length and body depth is pronounced, as is typical for Fundulopanchax.
Habitat
Fundulopanchax gresensi is known from the Takwai area of West Cameroon, where it occupies shallow, swampy sections of forested streams. The habitat is characterised by dense riparian vegetation, soft substrate, leaf litter, and the tannin-stained, soft, mildly acidic water typical of Cameroonian rainforest drainage. FishBase notes that these streams are seasonal: the water table can fall substantially during the dry season, and pools may partially dry.
Seriously Fish records aquarium water parameters of 72–79 °F and pH 6.0–7.5, with hardness spanning a wide range (18–268 ppm), consistent with soft to moderately hard, mildly acidic West African forest water. The species is therefore adapted to water that cycles between wet-season streams and dry-season remnant pools, a regime that has shaped its reproductive biology without pushing it toward obligate annual diapause.
Feeding
Fundulopanchax gresensi is insectivorous in the wild, taking small aquatic and terrestrial invertebrates — midges, mosquito larvae, small worms, and similar prey that fall onto or develop in the forest stream microhabitat it occupies. The upturned mouth is well suited to surface-feeding, and like most Fundulopanchax it is an active, visually oriented predator.
In captivity the species is easily catered for with live or frozen Drosophila, bloodworm, white mosquito larvae, and small Artemia nauplii. Variety is beneficial for long-term condition. Because of its small size, food items must be appropriately scaled — oversized prey will be ignored or mouthed and rejected.
Mating
Fundulopanchax gresensi is a non-annual killifish. Unlike the annual species of sub-Saharan savannas and the Sudanian zone, it does not require a desiccation period for egg development and does not bury eggs in mud awaiting a dry season that fully eliminates the water column. Instead, it follows a substrate- and plant-spawning strategy in which pairs deposit eggs in clutches on or within the substrate, among fine-leaved vegetation, or on a spawning mop placed in the tank.
Males court females with extended unpaired fins and lateral displays. Spawning is a brief, paired embrace typical of nothobranchid killifishes, producing a small number of eggs per embrace rather than mass-spawning. The non-annual breeding strategy means that egg development can proceed continuously in water, though the eggs are robust enough to tolerate a short period of damp incubation if the spawning medium dries partially — a reflection of the fish's seasonally variable wild habitat.
Breeding
Eggs are deposited singly or in small clutches on or near the substrate, or collected from a fine-leaved plant or peat-fibre mop placed in the aquarium. Incubation can proceed in water at 72–75 °F, with hatching occurring in roughly three to four weeks depending on temperature; alternatively, eggs can be stored in slightly damp peat for a similar period before flooding to stimulate hatching — a technique borrowed from annual killi husbandry that can sometimes synchronise a hatch.
Fry are relatively large at hatching by killifish standards and accept newly hatched Artemia nauplii and micro-worm from the first days of free-swimming. Raising the temperature slightly and performing small, frequent water changes promotes rapid growth. Adults should be separated from the spawning medium after egg collection to prevent predation. In a well-managed colony setup, a pair or trio on a mop or moss clump will produce small numbers of eggs on a near-continuous basis.
In the aquarium
Fundulopanchax gresensi is an attractive and rewarding species for the specialist killifish keeper, though it is infrequently seen in the trade compared with the more flamboyant large Fundulopanchax such as sjoestedti or gardneri. A small aquarium of 10–15 US gal is sufficient for a pair or trio, planted with fine-leaved vegetation or furnished with Java moss and a peat mop. Soft, slightly acidic water in the 72–79 °F range suits the species well; hard or alkaline water should be avoided.
Fundulopanchax gresensi is best kept as a species tank or with very small, peaceful companions — its own small size makes it unsuitable with boisterous or larger fish. Male competition can be an issue in small enclosures; a single male with two or more females typically produces the best results. Regular small water changes and a varied diet of appropriately sized live and frozen invertebrates will keep the fish in breeding condition and highlight the male's colouration. Because the species is a Cameroon endemic with a restricted type locality and limited hobbyist representation, maintaining captive colonies is a conservation contribution as well as a keeper's pleasure.
Conservation
The IUCN Red List assessment of 2009 by T. Moelants placed Fundulopanchax gresensi in the Data Deficient (DD) category. This designation reflects the limited information available about the species' actual distribution, population size, and trend rather than any determination of security. The type locality in the Takwai area of West Cameroon is a geographically restricted area, and if the species is a narrow endemic — as its presently known distribution suggests — it may be genuinely vulnerable to habitat loss.
The pressures facing forest stream killifishes in Cameroon include agricultural expansion and deforestation in the highlands, which degrades riparian vegetation and increases siltation; seasonal stream drying amplified by watershed deforestation; and in some areas the introduction of exotic mosquitofish (Gambusia spp.) that directly prey on eggs and fry of small native killies. Survey work is needed to determine whether Fundulopanchax gresensi is restricted to the Takwai drainage or also occurs in adjacent stream systems, and to assess the condition of its habitat. Until that survey data exists, the DD assessment remains the honest position.