Taxonomy & naming
The species was described by the Belgian-British ichthyologist George Albert Boulenger in 1916, in the fourth volume of his monumental Catalogue of the Fresh-water Fishes of Africa, as Pelmatochromis humilis. The type locality is the North Sherbro District of Sierra Leone, and the unique holotype (BMNH 1915.4.13.44) is held at the Natural History Museum in London, with one additional registered specimen (BMNH 1915.5.21.18). For much of the twentieth century the fish sat in the catch-all West African genus Pelmatochromis; when Thys van den Audenaerde erected Pelvicachromis in 1968 around the well-known rainbow krib (Pelvicachromis pulcher), humilis was transferred there, and it is under the combination Wallaceochromis humilis that the species became known to the literature and the aquarium hobby. Hobbyists informally split Pelvicachromis into two groups: the smaller, deep-bodied 'Type 2' kribs (Pelvicachromis pulcher, Pelvicachromis taeniatus, Pelvicachromis subocellatus, Pelvicachromis roloffi) and a larger, longer-snouted 'Type 1' cluster comprising humilis and its allies. Two members of that humilis complex were formally separated as distinct species by Anton Lamboj in 2004 — P. rubrolabiatus and P. signatus, both from Guinea. In 2016 Lamboj, Trummer and Metscher used skeletal and micro-CT evidence to remove this Type 1 group from Pelvicachromis entirely, naming the new chromidotilapiine genus Wallaceochromis with humilis as its type species; the genus name honours the naturalist Alfred Russel Wallace. Eschmeyer's Catalog of Fishes accordingly lists the current valid name as Wallaceochromis humilis, while a great deal of aquarium and even taxonomic usage continues under Wallaceochromis humilis.
Morphology
Wallaceochromis humilis is the largest species of the krib radiation. FishBase gives a maximum total length of about 5 in, with males reaching roughly 4.5 in and females staying noticeably smaller; the largest preserved specimen measured corresponds to about 3.5 in standard length, so most wild and aquarium fish fall in the 3–5 in range. The body is fusiform and distinctly more elongate than that of its rounder congeners, carried on a comparatively long head with a low, straight, sloping dorsal profile and a slightly downturned mouth — features that, together with a single series of conical jaw teeth, set it apart at a glance. Meristics run to 17–18 dorsal spines and 9–11 soft rays, three anal spines and six to eight soft rays. Coloration is strongly form-dependent and mood-dependent. Males are typically brown to tan with a row of about eight vertical bars that flush in and out with mood, sometimes overlaid by a lateral stripe in certain populations; the unpaired fins usually carry red, and the caudal fin is rounded to rhomboid, with older males developing a few elongated rays in its upper lobe. Females are smaller, silvery-grey with a rosy belly that darkens to near-black on the ventral fins when they are ripe, and they flash an iridescent silvery-blue in the upper caudal — the genus-wide pattern in which the female is, if anything, the more conspicuously ornamented sex outside of display. Because the species spans several geographic forms (traded under names such as 'Guinea' and 'Liberia red'), the precise pattern, bar intensity and presence of a lateral stripe vary from river to river, and some of these forms may yet prove to be undescribed species.
Habitat
The species is endemic to the Upper Guinea region of West Africa, ranging from the Kolenté (Great Scarcies) basin in Guinea — including the Forecariah and Kambo drainages — southeast through Sierra Leone (the Waanje, Moa, Sewa and associated coastal streams) into Liberia, where it reaches the middle courses of the St. Paul and St. John rivers and the Lofa. This places it across two adjacent freshwater ecoregions, the Northern Upper Guinea and the Southern Upper Guinea, both short-river coastal systems descending from the Guinean highlands and noted for high fish endemism. Its biotope is the shaded forest stream: small rivers and larger brooks running through closed-canopy forest, where overhanging trees keep the water dim and cool. Museum collection notes repeatedly place it in 'marigots' — small forest creeks and side-channels — and in pools and tributaries of the Moa and Waanje. The water is soft, well-oxygenated and acidic to near-neutral; FishBase records a pH window of about 6.0 to 8.0 and a hardness of roughly 5 to 12 dH, with a thermal range of 75–79 °F, while aquarium-trade and field accounts emphasise the soft, acidic end (pH around 5 to 6.5) of forest blackwater. These are shallow-water fish: they live among leaf litter, roots and submerged wood in the upper metre or so of small streams rather than in deep open channels. In at least part of its range it shares the habitat with other Upper Guinean cichlids such as Pelvicachromis roloffi and the African butterfly cichlid Anomalochromis thomasi.
Feeding
Wallaceochromis humilis is a benthic omnivore that works the stream bed and leaf litter of its forest habitat. FishBase estimates a trophic level near 3.3, consistent with a diet that mixes small invertebrates with a substantial plant and detritus component. In the wild it would take aquatic insect larvae, micro-crustaceans and other small invertebrates picked from the substrate, supplemented by aufwuchs — the algal and detrital film grazed from submerged wood, leaves and stones — and by organic detritus. Keepers consistently describe it as a voracious feeder that needs a diet 'heavy in plant material' to stay in condition, accepting live and frozen foods (daphnia, brine shrimp, glassworm, grindal and white worms, mysis, chopped krill) readily but doing best when those are balanced against vegetable-based flake and pellet. As a bottom-oriented forager in shaded, low-productivity streams, it occupies the generalist invertivore-detritivore niche typical of the krib lineage rather than any specialised feeding role.
Mating
Like all kribs, Wallaceochromis humilis forms monogamous pairs and is a territorial cave spawner, but it carries that template at a larger, more pugnacious scale than its dwarf relatives. It is shyer and slower to settle than the common rainbow krib, and a newly introduced pair may stay cryptic for weeks before the male asserts himself as the dominant fish of the tank. Pair formation is best left to the fish: keepers raise groups of young together and let a bond form naturally, because arbitrarily pairing a male and female of this aggressive species can end with one killing the other. When a female ripens she becomes heavy-bodied, her rosy flush deepens and her ventral fins blacken, and her behaviour reverses the usual sexual roles — she becomes the active courting party, seeking out the male and displaying with an arched, side-on, shimmying dance, absorbing his initial aggression until he relents. Courtship is a protracted affair, often two weeks or more, during which the pair systematically excavate and test the caves and pits of their territory before concentrating their attention on a single chosen site, which they then defend together with marked ferocity against any intruder.
Breeding
Wallaceochromis humilis is a biparental cave spawner: the female lays her adhesive eggs on the roof or wall of a chosen cave — a rock crevice, an excavated chamber under wood, or in aquaria an upturned flowerpot or coconut shell — and then tends and fans the clutch while the male patrols and defends the surrounding territory. No detailed wild clutch counts are published; in the genus, cave clutches typically run from a few dozen up to one or two hundred eggs, and humilis, as the largest krib, sits at the upper end of that band. The eggs hatch in roughly two to three days, and the larvae become free-swimming about ten to eleven days after spawning, with the parents relocating the brood to one or more freshly dug pits in the substrate during development. The fry are conspicuously large for a krib — on their first free-swimming day they rival the size of Pelvicachromis taeniatus fry that have been swimming for two weeks. Brood care is shared, but one detailed aquarium account (Ted Judy, Cichlid Room Companion) records a striking maternal behaviour not reported for other kribs: each evening the female barricades herself and the fry inside a cave by piling gravel across the entrance from within, then digs her way out the next morning. The young grow slowly; reared on brine shrimp nauplii and finely crushed vegetable foods with frequent large water changes, they reach 0.5 in in about six weeks. A recurring hazard at the end of a cycle is sudden intraspecific violence — once the fry are removed, the male may turn on the female with little warning.
In the aquarium
This is a rewarding but demanding fish, and decidedly not a beginner's krib. Its larger size and stronger temperament mean a single pair wants more room than the dwarf species: a tank of around 47 in length (roughly 30 US gal or more) is a sensible minimum, and more is better. Because the species is shy and easily bullied while settling in, the aquascape should be heavily structured — driftwood, roots, rockwork and several caves or upturned flowerpots, with at least one cave small enough that only the female can enter, which helps her escape an over-zealous male. A sand or fine-gravel substrate suits its habit of excavating pits, and subdued lighting (one end of the tank kept nearly dark) brings out both its colour and its confidence. Water should reflect the soft, warm forest streams it comes from: a temperature around 75–81 °F and a pH on the acidic-to-neutral side, with breeding best attempted softer and more acidic (pH about 5.5 to 6.5). It is more robust than its congeners but still does poorly with very large or boisterous tankmates; good companions are upper- and mid-water dither fish such as larger Alestiid tetras, plus loricariids, the African butterfly fish and Corydoras-type catfish, though bottom-dwellers should be removed before a spawn since they will eat eggs and fry. The cardinal rule is conspecific: Wallaceochromis humilis is aggressive toward its own kind, so keep only a single compatible pair unless the tank is very large, and be ready to divide the pair when removing a brood, as post-spawning aggression can be lethal. Feed generously and variedly — heavy feeding keeps this voracious species in breeding condition — but offset the resulting waste with frequent, substantial water changes, which the fish reward with their best colour and most outgoing behaviour. It remains a rare fish in the trade, prized by specialists in West African cichlids rather than commonly stocked.
Conservation
The IUCN Red List assesses Wallaceochromis humilis as Least Concern (assessed 7 January 2020 by I. Diallo and M. Entsua-Mensah for the Pan-Africa Freshwater assessment). The species is relatively widespread across the coastal river basins of Guinea, Sierra Leone and Liberia and is not considered to face a population-level threat, and it carries no CITES listing. That said, the Upper Guinean forest-stream ecosystems it depends on are under real and increasing pressure: deforestation and the clearing of catchment forest, agricultural expansion, alluvial and gold mining with associated siltation, and the degradation of the small shaded streams that constitute its habitat. The region's freshwater fauna is highly endemic and its forests among the most reduced in West Africa, so while the species itself is currently secure, the long-term outlook is tied to the fate of those forest watersheds. The taxonomic splitting of the old humilis complex into several narrow-range forms is also a reminder that some local populations may be more restricted — and more vulnerable — than the broad species range suggests.