Taxonomy & naming
Coptodon nigrans was described in 2010 by Andreas R. Dunz and Ulrich K. Schliewen as Tilapia (Coptodon) nigrans, in a paper in the Munich journal Spixiana that erected three new species from Lake Ejagham and redescribed a fourth, Tilapia deckerti Thys van den Audenaerde, 1967. At that time Coptodon was treated as a subgenus of the sprawling, polyphyletic genus Tilapia. Three years later Dunz and Schliewen (2013), in a molecular phylogeny in Molecular Phylogenetics and Evolution that dismantled the old catch-all Tilapia, raised Coptodon to a full genus and placed it in its own tribe, the Coptodonini; the species has been Coptodon nigrans ever since, the original combination Tilapia nigrans falling into synonymy. Eschmeyer's Catalog of Fishes records the name as valid. Coptodon is a West and Central African lineage of roughly thirty substrate-spawning tilapiines, biparental nest-builders and herbivore-detritivores quite distinct from the mouthbrooding Oreochromis and Sarotherodon with which they were long lumped. Within the Ejagham flock, nigrans belongs to a small monophyletic radiation that also includes Coptodon deckerti, Coptodon ejagham, Coptodon fusiforme and Coptodon pelewensis; genome-scale work (Poelstra et al. 2018) shows the lake was colonised by a single Coptodon ancestor soon after it formed about nine thousand years ago, with the species themselves arising only in the last one to two millennia. The epithet nigrans is a Latin participle meaning 'becoming black' or 'darkly coloured', a reference to the species' generally dusky pigmentation.
Morphology
Coptodon nigrans is a modest-sized cichlid, reaching about 6 in in standard length in the type series — among the larger members of the Ejagham flock, since the diagnosis notes that its smallest reproductively active individuals exceed the largest mature specimen of its close relative Coptodon deckerti. The body is fusiform and laterally compressed, the typical 'normal' tilapiine shape. Fin counts run to 14–16 dorsal spines with 11–13 soft rays, and three anal spines with 8–9 soft rays. The species is named for its colour: it is generally dark, and the most useful field mark is the pattern on the flanks, where each scale carries a dark, square-shaped basal blotch, either solid or with a pale window at its centre. This distinguishes it from the sympatric Coptodon ejagham, in which the scales instead show a dark margin around a light centre, the reverse arrangement. Dunz and Schliewen separated nigrans from the rest of the flock and from other Coptodon on a combination of dental and proportional characters: it bears quadricuspid to pentacuspid posterior teeth on the lower pharyngeal jaw (a feature shared with only a handful of congeners), and it has a notably long lower jaw (13.9–17.3% of standard length) and a long snout (15.8–18.2%) compared with several relatives. Like the other Ejagham Coptodon it is essentially sexually monochromatic — the sexes do not differ in a bold, fixed nuptial colour the way many rift-lake cichlids do — which is itself part of what makes the lake's speciation so puzzling. Breeding individuals do develop reproductive coloration, but it is subtle, and there is no strong, persistent external dimorphism between males and females.
Habitat
The entire world range of Coptodon nigrans is Lake Ejagham, a small oval lake roughly 1,050 by 700 metres set in lowland evergreen rainforest near the village of Eyumojok in southwestern Cameroon, within the Western Equatorial Crater Lakes freshwater ecoregion. Unlike the volcanic crater lakes around it, Ejagham is of non-volcanic origin — most likely a solution basin formed during the last glaciation — and it is young, with sediment cores dating its origin to about nine thousand years ago. It is shallow for a lake that holds a fish radiation: maximum depth is only about 56–59 ft. The basin has a narrow belt of sandy shallows that dips steeply from the shoreline down to a broad, flat floor of fine organic sediment offshore. The water is soft and circumneutral; in-situ measurements give a conductivity of roughly 86–120 µS/cm and a near-neutral pH, with warm tropical temperatures typical of a lowland Cameroonian lake. Crucially, the lake's single small outlet is isolated from the nearby Munaya River by a waterfall that cichlids cannot pass, which has kept the flock penned inside Ejagham for its entire history. Within that small basin, nigrans is the deep-water specialist of the Coptodon flock: while its relatives occupy the lit shallows, nigrans is tied to deeper water for reproduction, a partitioning of the steep depth gradient that is one of the few obvious ecological axes available in so small a lake.
Feeding
Like Coptodon generally, Coptodon nigrans is a low-trophic-level forager rather than a predator. FishBase estimates its trophic level at about 2.6, placing it among the herbivores and detritivores that define the genus and the tilapiines as a whole. The Ejagham Coptodon have not radiated into the dramatic trophic specialists seen in the older, deeper Cameroonian crater lakes — Lake Bermin's flock, for instance, includes a phytoplankton-feeder and a sponge-eater — and the Ejagham species remain comparatively generalised feeders that work over algae, fine plant material and the organic detritus that blankets the lake bottom. Their bicuspid and tricuspid outer jaw teeth and the molariform-to-cuspidate pharyngeal mill of the genus are tools for cropping and grinding soft plant and detrital material rather than for seizing prey. In Ejagham, where ecological divergence between the species appears weak and sexual isolation seems to have done most of the work of speciation, diet differences among the flock members are subtle, and nigrans is best understood as a benthic herbivore-detritivore foraging the deeper margins of its range.
Mating
As a Coptodon, Coptodon nigrans is a substrate-spawning, pair-bonding cichlid rather than a mouthbrooder, so its reproductive biology turns on the establishment and defence of a nest site by a male-female pair. In Lake Ejagham the flock is sexually monochromatic and disruptive selection on ecology is currently weak, which led researchers to conclude that mate choice — sexual isolation — rather than ecological competition has been the principal engine separating the species. Genomic work by Poelstra and colleagues (2018) found that an introgressed cluster of olfactory-receptor genes, picked up from a riverine relative around the time of the first speciation event, coincided with the divergence; because olfactory signalling underpins mate recognition in cichlids, the implication is that the Ejagham species came to recognise and choose their own kind partly by smell. For nigrans, the decisive mating trait is where it chooses to breed: by pairing and nesting in deep water it is spatially and behaviourally separated from the shallow-nesting members of the flock even though all of them share the same small lake. Detailed courtship sequences for this particular species have not been published, but the genus-typical pattern — a pair forming, jointly clearing and defending a nest, and signalling with developed but understated breeding coloration — applies.
Breeding
The breeding mode is the single most distinctive thing about Coptodon nigrans. Where every other Coptodon in Lake Ejagham nests in the shallows, nigrans breeds exclusively in excavated caves below about five metres of water, while its closest relative Coptodon deckerti always spawns above two metres — a depth segregation that is part of the formal diagnosis separating the two species. Like all members of its genus it is a biparental substrate spawner: the pair excavates and tends the nest, the adhesive eggs are laid on the substrate (here, the wall of the dug cave) and guarded and fanned by the parents, and after hatching the wrigglers and then free-swimming fry are shepherded and defended by both adults — the shared parental-care strategy of the substrate-brooding tilapiines, in contrast to the mouthbrooding of Oreochromis. A precise clutch count for nigrans has not been published; Coptodon nests of comparable small species typically hold on the order of a few hundred to perhaps a thousand eggs, and that genus-typical figure is the best available guide until the species is studied directly. The deep, sheltered cave that nigrans chooses presumably shields its brood from the same egg and fry predators that the shallow-nesters face, and the choice of breeding depth is one of the clearest ecological-cum-behavioural differences distinguishing it within the flock.
In the aquarium
Coptodon nigrans is essentially a scientific and conservation fish rather than an aquarium one. It has no established presence in the ornamental trade, no recognised common name in the hobby, and is known to aquarists mainly through the handful of specimens that researchers and a few specialist keepers (such as Oliver Lucanus, who has photographed lake-collected fish in the aquarium) have maintained. As a Critically Endangered single-lake endemic it is not a fish to seek out for a community tank, and there is no responsible commercial source for it. That said, its requirements can be inferred from its biology and from experience with its better-known West African Coptodon relatives, which are robust, digging, substrate-spawning tilapiines. It would need a long tank rather than a tall one, soft and circumneutral water at warm tropical temperatures (broadly pH around 6.5–7.5 and roughly 75–82 °F, matching its lake), a deep sand substrate it can excavate, and structure — rock, wood, and cave-like recesses — that lets a pair claim and defend a nest, since nigrans is by nature a cave-excavating breeder. Like other Coptodon it would be expected to be territorial and destructive toward plants and tankmates when spawning, so generous space and the option to remove other fish are prudent. Diet would pose no difficulty: a herbivore-detritivore, it would take vegetable-based prepared foods, algae and the usual frozen and live items. None of this should be read as encouragement to keep it; the honest position is that this is a fish best left to the lake and to managed conservation efforts, and its inclusion here is to document, not to promote, its care.
Conservation
The IUCN Red List assesses Coptodon nigrans as Critically Endangered, assessed on 9 March 2023 by C.H. Martin, under criteria B1ab(iii)+2ab(iii) — the categories that flag a species confined to a single, tiny location whose habitat is in decline. The qualification is straightforward: the species exists in just one lake of about half a square kilometre, so any deterioration of that one water body threatens the whole population at once, and the population trend is recorded as unknown. The threats the assessment cites are local but real. Deforestation in the lake's catchment — timber harvest, clearance for oil-palm plantation, and swidden agriculture — drives erosion and sedimentation into a small, shallow basin that cannot absorb much of it, while unsustainable water extraction lowers the lake itself. The most acute biological threat is an invasive predatory catfish: around 2000–2001 a local official introduced large numbers of catfish to Ejagham, and a breeding population of Parauchenoglanis has persisted there ever since, adding predation and competition that the lake's small endemic cichlids never evolved to face. Because the entire Coptodon flock — and the lake's two endemic Sarotherodon as well — is locked behind the waterfall that isolates Ejagham from the Munaya River, none of these species has anywhere else to go. Coptodon nigrans carries no CITES listing. Its long-term survival depends entirely on protecting the single small Cameroonian lake that is its only home.