Taxonomy & naming
Sarotherodon knauerae was described in 2011 by Dirk Neumann, Melanie L. J. Stiassny and Ulrich K. Schliewen in the journal Zootaxa (2765: 1–20), in a paper that simultaneously named two new sympatric Sarotherodon endemic to Lake Ejagham and re-examined the wider Sarotherodon galilaeus species complex. Its sister species in that paper, Sarotherodon lamprechti, shares the lake with it; the two are believed to have evolved from a single galilaeus-like riverine ancestor that colonized the lake and then split in situ. The holotype (ZSM 29924), a mature male of 3 in standard length, was collected by Schliewen in February–March 1993 and is housed in the Zoologische Staatssammlung München, with paratype series distributed to the AMNH (New York), ANSP (Philadelphia) and the Royal Museum for Central Africa (Tervuren). Within the African mouthbrooding tilapias, Sarotherodon belongs to the tribe Tilapiini of subfamily Pseudocrenilabrinae. The species is diagnosed from every congener by a single unusual feature — an inflated, vacuolated second pharyngobranchial element in the upper pharyngeal jaw — together with its very small adult size. It is part of one of the most-studied micro-radiations in African ichthyology: the Ejagham cichlid flock, which alongside the lake's Sarotherodon pair includes a separate radiation of small Coptodon (formerly Tilapia) species described by Dunz and Schliewen in 2010.
Morphology
This is a small, deep-bodied cichlid: the largest specimen measured by the describers was the 3 in SL holotype, and FishBase records a maximum of 3 in SL, making Sarotherodon knauerae one of the smallest known Sarotherodon. The snout is acute and the mouth terminal, with lips that are thickened but not fleshy; the head profile is straight and rises sharply behind the nape to the dorsal fin. Greatest body depth falls between the dorsal and pelvic-fin insertions, and the caudal peduncle is deeper than long. The dorsal fin carries 15–16 spines and 13–14 branched rays, the anal fin 3 spines and 10–12 rays; gill-raker counts are high, 24–30 (mode 28), one of the traits separating it from its lake-mate Sarotherodon lamprechti. The pectoral and pelvic fins are short, both ending well in front of the anus, and the pectoral-fin base is scaled. In life the sexes differ clearly. Males are golden, often with a metallic golden-green sheen strongest on the upper lip and cheek; nuptial males turn jet black across the lower jaw, throat, chest and belly back to the anal fin, with an almost black ventral caudal lobe. Females are paler — light grey with a bluish-golden iridescence on the flanks and head, a metallic blue upper lip and a white-to-grey belly. Faint vertical bars and a diffuse "tilapia spot" at the base of the last dorsal spine may show in both sexes. The bicuspid outer-row jaw teeth are slender-necked and set in two inner rows of tricuspid teeth, the brush-like dentition that gives the genus its name.
Habitat
Sarotherodon knauerae is endemic to Lake Ejagham, a small isolated lake in the South-West Region of Cameroon, near the village of the same name in the Manyu Subdivision at roughly 5°45′N, 8°59′E. The lake is tiny — about 0 mi² (49 ha) in surface area — with a maximum depth around 59 ft, and it drains via a short outlet into the Cross River basin. Radiocarbon and sediment-core work by Stager and colleagues (2018) suggests the basin formed only about 9,000 years ago; rather than a volcanic or chemical origin its near-circular morphometry and shocked-quartz signatures point, tentatively, to a meteorite impact, which would make Ejagham one of very few documented impact-crater lakes to host an endemic fish radiation. The water is soft, weakly mineralized and slightly acidic to near-neutral, with low conductivity and warm tropical temperatures year-round; the lake is shallow enough to be only weakly stratified. Within this single small water body Sarotherodon knauerae is a habitat generalist: the describers found it gregarious and present in all habitats and at all depths, but most abundant inshore in shallow water to about 10 ft, thinning out in the deeper zones. Because the entire lake is its range, the species' fate is bound up completely with the condition of this one basin and its forested catchment.
Feeding
Sarotherodon knauerae is a detritivore. The original description reports it taking detritus from both soft and hard substrates and from the water surface, and FishBase places it at a low trophic level near 2.5 — the bottom of the food web, typical of the algae- and detritus-grazing tilapias. Its dentition fits this role: fine, slender, brush-like teeth in the jaws (the literal meaning of the genus name) for scraping and combing loose organic material, backed by a lower pharyngeal jaw whose dentigerous area carries crowded, brush-like unicuspid teeth for processing soft food. The diagnostic inflated second pharyngobranchial element in the upper pharyngeal jaw is a structural peculiarity of this species, though its functional consequence is not fully understood. Within the closed Ejagham community Sarotherodon knauerae and the other endemic cichlids partition a limited set of resources; the long history of evolutionary-biology research on this lake has focused in part on how the sympatric forms diverge in diet, body size and morphology even within so small a basin, with later work (Martin and colleagues) cautioning that the ecological divergence here is real but more modest and complex than the classic sympatric-speciation story first implied.
Mating
Like other Sarotherodon, Sarotherodon knauerae is a mouthbrooder rather than a substrate-guarding tilapia, and its mating system reflects that. The describers note that males do not vigorously defend courtship territories and build no bower — a marked contrast to the lek-breeding, pit-digging tilapias of the genus Oreochromis. Instead, breeding appears to centre on brief pairings followed by oral incubation rather than on a defended nest site. Seasonality is inferred from field collections: large numbers of juveniles present in the dry season (January–February) imply that spawning takes place mainly during the rainy season, around August–September. Beyond this, the detailed courtship sequence of Sarotherodon knauerae has not been formally documented in the wild, and much of what can be said about its reproductive behaviour is drawn from genus-typical patterns and from a small number of aquarium observations.
Breeding
In aquaria, Sarotherodon knauerae has been observed to be a maternal mouthbrooder — the female takes the fertilized eggs into her mouth and incubates them there through hatching and the early free-swimming stage, a strategy that frees the fish from any dependence on a safe spawning substrate. This is consistent with the wider genus, in which both maternal and biparental mouthbrooding occur; the original description specifically records maternal mouthbrooding for this species. Clutch size has not been published for Sarotherodon knauerae itself; congeneric Sarotherodon are comparatively low-fecundity mouthbrooders producing on the order of a few dozen to a few hundred large, yolk-rich eggs per spawn (genus-typical figure, not measured for this species), the trade-off that buys each egg the protection of the parent's mouth. The dry-season abundance of juveniles points to a wet-season spawning peak. Detailed data on egg number, incubation period and fry development for this particular species remain undocumented, reflecting how little direct study its reproduction has received outside the type material.
In the aquarium
Sarotherodon knauerae is essentially an aquarium unknown. It has no presence in the ornamental trade, it is not commercially bred, and the only specimens to have lived in tanks are research animals; the photograph on FishBase comes from a small captive group. There is therefore no established hobbyist husbandry for the species, and given its Critically Endangered status, its single-lake endemism and the conservation sensitivity of its only population, wild collection for the hobby would be irresponsible. Any husbandry guidance here can only be inferred from what is known of the fish and from general Sarotherodon care. It is small (under 3 in) and gregarious, so it would suit a modestly sized group rather than a single pair, and it would need warm, soft, gently acidic-to-neutral water mirroring Lake Ejagham — roughly 75–82 °F, pH around 6.5–7.5, low hardness and low conductivity. As a detritivore it would take a vegetable- and detritus-based diet (spirulina-based and other plant-forward foods, with fine sediment to graze). Being a non-territorial mouthbrooder, it would lack the bower-digging aggression of Oreochromis but, like most tilapias kept in groups, would still benefit from space and structure to diffuse social tension. None of this should be read as an invitation to keep the species: it is a fish to study and conserve in place, not to import.
Conservation
The IUCN Red List assesses Sarotherodon knauerae as Critically Endangered under criteria B1ab(iii)+2ab(iii) (assessed 15 October 2014 by D. Geelhand, T. Musschoot and G. Boden of the Royal Museum for Central Africa, published 2023; population trend unknown). The listing reflects the species' extreme rarity of range: it is confined to a single lake of well under one square kilometre, so its entire extent of occurrence and area of occupancy fall below the Red List's critical thresholds, and any decline in the quality of that one habitat threatens the whole species at once. The assessment lists the principal threats as deforestation of the lake's catchment, oil-palm plantation expansion, slash-and-burn agriculture, unsustainable water extraction, and — most insidiously for a tiny endemic flock — competition with and predation by introduced non-native fishes, which have repeatedly destroyed comparable single-lake cichlid radiations elsewhere in Cameroon. Lake Ejagham enjoys no formal protection, and its endemic fauna has no commercial value that would motivate conservation; the lake's scientific importance as one of the clearest cases of sympatric speciation, and possibly a rare impact-crater lake, is the strongest argument for safeguarding it. For Sarotherodon knauerae, conservation is inseparable from the integrity of this one small basin and the forest around it.