Taxonomy & naming
The French ichthyologist Jacques Pellegrin described this fish in 1900 as Tilapia bilineata, on material from Ndélé on the Alima River in what was then the French Congo. For more than a century it sat among the vast, taxonomically unwieldy assemblage of African cichlids lumped into the genus Tilapia, a wastebasket that successive authors knew to be unnatural but struggled to dismantle. The modern resolution came from the Munich school of cichlid systematics: Andreas Dunz and Ulrich Schliewen, working through the substrate-brooding "Tilapia" with molecular and morphological data, recognised that bilineata and its close relatives formed a distinct Congo-basin lineage. In a 2012 paper in Ichthyological Exploration of Freshwaters (23: 155–179), Dunz, Emmanuel Vreven and Schliewen erected the new genus Congolapia to hold them, designating Tilapia bilineata as its type species and at the same time describing a third member, Congolapia louna, from the Louna River. The genus today contains three species — Congolapia bilineata, Congolapia crassa (Pellegrin, 1903) and Congolapia louna — all confined to the central Congo. The same authors fixed a lectotype for bilineata (MNHN 1886-0455, with one paralectotype) to stabilise the name, since Pellegrin's original series had never been formally restricted. Congolapia is placed in the tribe Tilapiini of the African subfamily Pseudocrenilabrinae; the broader reshuffling of the old Tilapia into modern genera such as Coptodon, Pelmatolapia, Heterotilapia and Congolapia was set out in Dunz and Schliewen's 2013 revision of the haplotilapiine cichlids.
Morphology
Congolapia bilineata is a small, short-bodied and laterally compressed cichlid, reaching about 7 in in standard length — modest by the standards of the food-fish tilapias it was once grouped with. The fin counts are typically tilapiine: 16–17 dorsal spines followed by 9–11 soft rays, and 3 anal spines with 8–9 soft rays. All scales are cycloid, with about 26–27 scales along the horizontal line of the body. The features that separate it from its two congeners are squarely scale- and proportion-based, as set out in the 2012 revision. It differs from both Congolapia crassa and Congolapia louna in having one complete scale row plus an additional row of smaller, dorsoventrally compressed scales between the upper lateral line and the base of the last dorsal spine, where the others carry two complete rows. It is a shallower-bodied fish than the appropriately named Congolapia crassa (body depth 32–40% of standard length, versus 40–48% in crassa), and it has a shorter last dorsal-fin spine than Congolapia louna. The common-name reference to two lines comes from the dark stripes that follow the upper and lower lateral lines along the flanks. Sexual dimorphism is not strongly marked in the literature; as in related substrate-spawning tilapiines, ripe females become noticeably fuller-bodied, while breeding adults of both sexes intensify their pattern, but no dramatic difference in colour or finnage between the sexes has been documented for this species.
Habitat
The species is a Congo-basin endemic, restricted to the western and central parts of the Cuvette Centrale — the vast, flat, swamp-forested saucer through which the middle Congo flows. It has been recorded from the Alima, Lefini, lower Sangha, Ruki, Maringa and related rivers, spanning the Republic of the Congo and the Democratic Republic of the Congo. (Eschmeyer's Catalog of Fishes also lists the Central African Republic for the genus, but the firmly documented range lies in the two Congos.) Within this region it is a fish of the slow, tannin-stained backwaters rather than the main river channel: it is found in swampy forested zones, in both grass-bank and forest-bank habitats, and is taken in the dry season as well as the wet. These are warm, soft, acidic blackwaters typical of the Cuvette Centrale, where forested tributaries drain peat swamps and the main channel itself often runs deep brown. FishBase classes the species as demersal and freshwater; it is a shallow-water, near-bottom fish of the floodplain margins, but no precise depth band has been published, so the depth here is given as the documented shallow floodplain habitat (roughly 0–10 ft) rather than a measured range. No in-situ chemistry has been formally tabulated for the species, but the wider ecoregion is consistently characterised by warm temperatures, low conductivity and acidic to near-neutral pH.
Feeding
No dedicated dietary study of Congolapia bilineata has been published, and the species' trophic level is estimated rather than measured — FishBase places it near 3.3, in the range expected for a small omnivorous cichlid. The substrate-brooding tilapiines from which Congolapia was split are characteristically generalist feeders, browsing on filamentous algae, plant detritus, aufwuchs and the small invertebrates that live among bottom debris and submerged vegetation. In the grass-bank and forest-bank habitats it occupies, that translates to a diet of plant material, organic detritus and small benthic animals gleaned from the leaf litter, drowned grasses and soft sediment of the floodplain. In the swamp-forest community it functions as a small benthic forager rather than a specialised predator; it is harmless to humans and of negligible commercial importance, taken only incidentally in local subsistence fisheries.
Mating
Congolapia bilineata is a pair-bonding cichlid: rather than the lek-style mouthbrooding of the familiar Oreochromis and Sarotherodon tilapias, it forms monogamous male–female pairs that cooperate through the breeding cycle. Detailed courtship for this particular species has not been described in the wild, but the pattern is well established across the substrate-brooding tilapiines to which Congolapia belongs. A pair takes up and defends a territory centred on a chosen spawning surface, the two fish driving off intruders together, and the bond persists through egg-laying and the entire fry-guarding period. This shared territoriality and parental partnership is the defining social feature of the lineage, and it is what most clearly sets Congolapia apart from the maternal mouthbrooders that dominate the tilapia name in the popular imagination.
Breeding
The species is an open-substratum (open substrate) spawner with biparental care: the pair deposits its eggs on an exposed surface in the open rather than hiding them in a cave or carrying them in the mouth, and both parents then guard the clutch and the resulting fry. This breeding mode is documented for the species in the western-African cichlid literature and is the reproductive signature of the substrate-brooding tilapiines generally. Spawning has been recorded across both the dry and wet seasons in the wild, consistent with a fish that breeds when conditions in its floodplain habitat allow. No clutch count specific to Congolapia bilineata has been published; substrate-spawning tilapiines of comparable size typically lay on the order of several hundred to a few thousand adhesive eggs in a single spawning, and this genus-typical figure is the best available guide pending direct observation. After hatching, the free-swimming fry are shepherded and defended by both parents — the hallmark guarding behaviour that, in many related species, continues until the young are well grown. The species is not established in the aquarium hobby, and a documented captive spawning has not been located, so the fine detail of egg number, incubation time and fry development for this fish remains genuinely undescribed.
In the aquarium
Congolapia bilineata is essentially absent from the aquarium trade. It is a localised Congo-basin fish of little commercial interest, not bred for ornamental export, and it does not appear in the standard hobby price lists; the Cichlid Room Companion and the few hobby references that mention the genus treat it as a wild-collected rarity rather than a kept species. As such, there is no body of first-hand husbandry experience to draw on, and any keeping advice has to be inferred from its biology and from how its substrate-spawning tilapiine relatives are maintained — which should be stated plainly rather than dressed up as proven practice.
From what its habitat demands, a keeper would aim to reproduce warm, soft, acidic blackwater: a temperature around 75–82 °F and a pH on the soft, acidic side of neutral (roughly 6.0–7.5), over fine sand with leaf litter, driftwood and tangles of roots to recreate the forest- and grass-bank margins it lives among. At under 7 in it is not a large fish, but as a territorial, pair-bonding substrate spawner it would need room for a pair to hold a breeding site away from tankmates, so a long tank rather than a tall one, with broken sightlines, would suit it. The realistic expectation, however, is that this species will only ever be encountered by specialists working with wild Congo imports; it is not a fish to recommend to general hobbyists, and the honest position is that its captive requirements have not been tested.
Conservation
The IUCN Red List assesses Congolapia bilineata as Least Concern, assessed on 16 February 2009 (published 2010) by T. Moelants. The assessment describes it as a demersal species of wetlands with no known threats, recorded from the Republic of the Congo and the Democratic Republic of the Congo, and notes its population trend as unknown. It carries no CITES listing. The species' security rests on the sheer scale and remoteness of its habitat: the Cuvette Centrale is the largest tract of lowland rainforest and swamp in Africa, sparsely populated and only poorly explored ichthyologically, so direct pressure on this small, non-commercial cichlid is currently slight. The wider backdrop is less reassuring — the Congo basin's peat-swamp and floodplain ecosystems face mounting pressure from deforestation, road and dam development, and the prospect of peatland disturbance and climate change in a region whose freshwater fauna is still incompletely catalogued. None of these is documented as a present danger to this particular species, but they are the context in which its long-term status should be read, especially given how little is known about its population and ecology.