Taxonomy & naming
The blackchin tilapia was described by the German naturalist-explorer Eduard Rüppell in 1852, in his survey of the fishes and skeletons collected on his African travels, under the name he gave it: melanotheron. The type material came from the Guinea coast; the Senckenberg Museum in Frankfurt holds the lectotype (SMF 1316) and two paralectotypes (SMF 5411-12). Over the following century the species was shuffled among the broad tilapiine genera, appearing in the literature as Tilapia melanotheron and, through its junior synonyms, as Chromis macrocephalus, Tilapia heudelotii (Duméril 1861) and Tilapia leonensis. Its modern placement follows Ethelwynn Trewavas's landmark 1983 monograph Tilapiine Fishes of the Genera Sarotherodon, Oreochromis and Danakilia, which split the old catch-all Tilapia into three biologically coherent genera defined largely by how they breed: substrate-spawning Tilapia, maternally mouthbrooding Oreochromis, and Sarotherodon, the mouthbrooders in which the male, or both parents, carry the brood. Within Sarotherodon the species is notably variable across its long coastline, and three subspecies were long recognised — the nominate S. m. melanotheron, the western S. m. heudelotii, and S. m. leonensis of the Sierra Leone region — corresponding to genetically and morphologically distinguishable regional stocks. The genus sits in the African subfamily Pseudocrenilabrinae of the Cichlidae.
Morphology
Sarotherodon melanotheron is a short, deep-bodied, laterally compressed cichlid of modest size. FishBase records a maximum of about 11 in standard length, but most wild and cultured fish are far smaller — a common length around 7 in — and in hypersaline water the species famously stunts, maturing at well under 6 in. The fins carry 15-17 dorsal spines over 10-12 soft rays and three anal spines over 8-10 soft rays, with 26-28 vertebrae. The ground colour is a plain pearly silver to brassy grey, and the diagnostic markings are the dark ones that give the fish its name: melanic (black) patches that in adults typically wash over the lower part of the head and the underside of the jaw — the "black chin" — together with a dark blotch on the shoulder (cleithrum) and dusky tips to the soft dorsal and caudal fins. A median spot or transverse bar across the nape is fairly constant, while irregular, often asymmetrical dark spots may scatter along the flanks, hinting at the vertical bars of related tilapias. Sexual dimorphism is minimal: the sexes are closely alike in size and colour, which is consistent with the species' unusual, female-driven courtship, and reliable sexing usually depends on examining the genital papilla. Juveniles show the throat melanism most strongly, the dark chin fading somewhat with age in some populations.
Habitat
This is a coastal fish of the brackish-water interface. Across its native West African range — coastal drainages, lagoons and estuaries from Mauritania and Senegal south through the Gulf of Guinea to northern Angola — it is most abundant in the mangrove zone, where it forms schools over muddy, detritus-rich bottoms. FishBase places it in shallow water, 0–10 ft deep, demersal and non-migratory, in conditions of about pH 7.0-8.0 and moderate to fairly hard water. What sets the species apart is its salinity range. It is one of the most euryhaline of all teleosts, documented from pure freshwater in the lower reaches of rivers all the way to about 130 parts per thousand — roughly four times the salinity of seawater — in evaporative lagoons. In the inverse (hypersaline) estuaries of Senegal, such as the Saloum and Sine-Saloum, where evaporation drives salinity above that of the open sea, it not only survives but remains the dominant fish, reproducing at 60-70 psu while limiting its growth and reaching maturity at a smaller size as a survival strategy. It also tolerates warm water well, with field temperatures commonly in the high 20s to low 30s °C. This combination of mangrove-detritus habit and extreme salinity tolerance is exactly what has made it such a successful colonist of disturbed coastal waters far from home.
Feeding
The blackchin tilapia is a low-trophic-level micro-grazer and detritivore — FishBase estimates a trophic level near 2.2, about as herbivorous as a fish gets. It works the bottom and the biofilm, picking up and swallowing mouthfuls of material; lacking the filtering gill-raker apparatus of some tilapias, it feeds by gathering "bites" of aufwuchs (the algal-and-microbe film on surfaces) and soft sediment rather than by fine straining. The bulk of the diet is phytoplankton and benthic algae — cyanobacteria, diatoms and green algae — together with organic detritus, supplemented opportunistically by zooplankton, and in some populations small bivalves. The proportions shift with locality: studies in West African coastal basins found the balance between detritus, phytoplankton and zooplankton varying between riverine and lacustrine sites. The fish is mainly nocturnal, with intermittent daytime feeding. As an abundant primary consumer in mangrove and lagoon systems it is an important link converting algae and detritus into fish biomass, which is part of why it supports such substantial artisanal fisheries.
Mating
Reproduction in Sarotherodon melanotheron is built around an inverted set of sex roles unusual among cichlids and indeed among fishes generally. It is the female that takes the initiative — courting, digging the spawning pit, and driving the encounter — while the male is gradually stimulated into a largely passive partner, eventually forming a pair bond with her. There is no mutual "T-stand" display during fertilisation as seen in many substrate-spawning cichlids. Researchers describe the system as a genuine sex-role reversal: females are the more aggressive and competitive sex, and the mating system runs on male choice rather than female choice. This makes evolutionary sense given who pays the cost of parental care — because the male will carry the brood in his mouth and cannot feed normally while doing so, his reproductive capacity, not the female's, becomes the limiting resource that the sexes compete over. Spawning happens in shallow water close inshore, and in seasonal climates is tied to warming temperatures; in Florida's Indian River Lagoon, for instance, spawning runs from spring into autumn with a peak as water passes 75 °F.
Breeding
The blackchin tilapia is a paternal mouthbrooder — one of relatively few cichlids in which the father, not the mother, incubates the clutch. After the female-led courtship the eggs are laid and fertilised in the pit, whereupon the male takes them into his mouth and broods them there. The eggs are comparatively large and orange. The male churns and oxygenates them for roughly two weeks; they hatch after about five days of incubation, and the larvae then remain sheltered in his mouth through yolk-sac absorption until they reach the free-swimming swim-up stage at around 14-15 days and roughly 0.5 in. Throughout this period the brooding male essentially does not feed. Clutch sizes are moderate: a detailed culture study in the Ebrié Lagoon, Côte d'Ivoire (Legendre & Trébaol 1996) recorded averages of roughly 370-720 eggs incubated per spawn, with individual males re-spawning at intervals of about 12-16 days, so a single male can carry many broods over a season. Annual egg production per pair ran into the thousands. The paternal-care strategy carries a hormonal signature: experiments show that the very presence of eggs in the mouth suppresses circulating androgen and estradiol in the brooding parent, the physiological correlate of switching from courtship to care. This efficient, portable mode of brood protection — combined with the species' salinity tolerance — lets it reproduce successfully in harsh, fluctuating estuarine water where open spawners would struggle.
In the aquarium
Sarotherodon melanotheron has a long history in the aquarium and ornamental trade — it was, at one point, a popular import in the United States, and indeed escapes and releases from that trade and from fish farms helped seed its invasive populations. It is not a delicate fish. For the hobbyist it is best thought of as a hardy, medium-sized, schooling brackish cichlid: undemanding about water chemistry to an almost unique degree, but needing space and producing the heavy bioload typical of a detritus-feeding tilapia. A realistic setup is a long tank of at least 55 US gal for a small group, with robust filtration, a sand or fine-gravel bottom, and water that is hard and alkaline (pH roughly 7-8). The species does not require salt to live — it does perfectly well in freshwater — but it is genuinely euryhaline, so a slightly brackish tank suits it and broadens compatibility with mollies, monos, scats and other brackish tankmates. Temperature should be kept warm, around 75–86 °F; the fish is notably cold-sensitive, and winter cold-kills are a documented limit on its spread in Florida. Diet is easy: a good-quality vegetable-based or spirulina flake or pellet, supplemented with algae and the occasional frozen food, matches its grazing habit. The main caveats are the practical ones — adults are large and messy, they will uproot plants and rearrange substrate, and like most tilapias they can be territorial and boisterous at breeding time. The paternal mouthbrooding is a genuine spectacle if a pair settles in. Because it is an established invasive, it should never be released, and in several jurisdictions keeping or moving live tilapia is regulated.
Conservation
The IUCN Red List assesses Sarotherodon melanotheron as Least Concern (assessed 22 April 2019 by P. Lalèyè). It is a widespread, abundant and adaptable species across a long stretch of the West African coast, heavily exploited yet showing no sign of range-wide decline — its salinity tolerance and high reproductive output buffer it against the pressures that threaten narrower-range fishes. It carries no CITES or CMS listing. The species is in fact more often discussed as a conservation problem than a conservation concern: it is an established invasive in Florida (recorded since 1959, now in many coastal drainages from Tampa Bay to the Indian River Lagoon) and in Hawaii (since the 1960s, spreading through Oahu's streams and reservoirs), where it was introduced via fish farms, bait operations and aquarium releases. In its introduced range it can dominate fish biomass — in one Florida spring it made up some 90% of the total — displacing native sunfishes and other species and acting as a reservoir for fungal disease. Cold winters check its expansion at the northern edge, but as a warm-water generalist it remains a watch-listed pest. In its native lagoons the more salient pressures are local: heavy artisanal and commercial fishing, mangrove loss, and the salinisation and pollution of West African estuaries — none currently severe enough to threaten the species as a whole.