Taxonomy & naming
The species was named by Carl Linnaeus in the tenth edition of Systema Naturae (1758) as Sparus galilaeus, on material from Lake Tiberias — the Sea of Galilee — in what is now Israel; the original types are lost. Linnaeus placed it in the seabream genus Sparus, and the fish travelled through several nineteenth-century combinations (notably under Chromis and the catch-all Tilapia) before Ethelwynn Trewavas's landmark 1983 revision, Tilapiine Fishes of the Genera Sarotherodon, Oreochromis and Danakilia, fixed it in Sarotherodon Rüppell, 1852 — the genus for which it is the type species. Trewavas's revision split the old, sprawling 'Tilapia' into three biologically coherent groups: substrate-spawning Tilapia, maternal mouthbrooding Oreochromis, and the mouthbrooding Sarotherodon, in which incubation is shared by both parents or taken up chiefly by the male. The synonymy is correspondingly long for so old and wide-ranging a fish; junior names placed here include Tilapia lateralis, Tilapia macrocentra and Tilapia pleuromelas (all Duméril, 1861, from Senegal) and Chromis microstomus and C. tiberiadis (both Lortet, 1883, from the Jordan system). The species is conventionally divided into several geographic subspecies — among them the nominate S. g. galilaeus, plus boulengeri (Congo), borkuanus (Saharan oases of Chad), multifasciatus (West Africa) and sanagaensis (Cameroon) — reflecting morphological variation across an enormous range, though the boundaries between them are not sharply drawn.
Morphology
Sarotherodon galilaeus is a robust, deep-bodied and laterally compressed cichlid of typical tilapia build, reaching about 15.5 in total length and roughly 1.6 kg, though most fish taken in fisheries are far smaller. The body is silvery to brassy-grey, often with a greenish or golden sheen on the flanks and a series of faint vertical bars that can come and go with mood; breeding and stressed fish may show darker barring, and there is no conspicuous 'tilapia spot' on the dorsal fin in adults of this species. The defining feature of the genus, and the source of its name, lies in the mouth: the jaw teeth are fine, numerous and brush-like, flexible to the touch, where those of true Tilapia are coarser and more rigid — an adaptation for grazing and filtering fine material rather than tearing tougher plants. Counts run to roughly XV–XVII dorsal spines with 10–14 soft rays, III anal spines with 8–12 rays, and a high number of long, closely set gill rakers (often 13–23 or more on the lower limb of the first arch), the latter forming the fine sieve that lets the fish strain phytoplankton from the water. Sexual dimorphism is modest and far less pronounced than in many cichlids: mature females are heavier-bodied when ripe, and males may grow somewhat larger, but the sexes are most reliably told apart by the genital papilla and by spawning behaviour rather than by colour or fin shape. Subspecies and populations differ in body depth, barring and meristics across the range.
Habitat
Few African freshwater fishes occupy so large a native range. Sarotherodon galilaeus is found through the Jordan system and the coastal rivers of Israel — its type region, the Sea of Galilee, being the best-studied population — across the Nile system including the delta lakes, Lake Albert and Lake Turkana, and throughout West and Central Africa: the Senegal, Gambia, Casamance, Konkouré, Sassandra, Bandama, Comoé, Niger, Volta, Mono, Ouémé, Ogun, Cross, Benue, Logone and Chari rivers, Lake Chad and Lake Bosumtwi, the Sanaga and Nyong of Cameroon, and the Congo (as the subspecies boulengeri). Isolated relict populations persist in Saharan oases — Borku, Ennedi and Tibesti in northern Chad — and in the Draa of Morocco and the Adrar of Mauritania, fossils of a once far wetter Sahara. It is a fish of warm, standing and slow-flowing water: lakes, reservoirs, river backwaters, floodplain pools, lagoons and delta lakes, tolerant of brackish conditions and recorded from fresh water into the lower reaches of estuaries. FishBase reports it from depths of around 16 ft downward in lakes, but it is fundamentally a fish of the warm, productive upper water column rather than the deep bottom. It is strikingly eurythermal and tolerant of a wide chemical envelope: comfortable across roughly 63–91 °F, with a preferred band near 72–82 °F, and recorded across pH from mildly acidic to distinctly alkaline (about 6.5 to 9.0), conditions that span the soft floodplain waters of West Africa and the hard, alkaline, sometimes saline lakes of the Rift and the Levant.
Feeding
Sarotherodon galilaeus is a low-trophic-level grazer and filter-feeder — FishBase places it near trophic level 2 — and one of the more accomplished phytoplankton feeders among the tilapias. Its fine, closely spaced gill rakers and brush-like teeth let it strain microscopic algae and suspended organic particles directly from the water, and in the well-studied Sea of Galilee (Lake Kinneret) population it is an important consumer of phytoplankton, including the dinoflagellate blooms that dominate the lake; its filter-feeding is efficient enough that the species has been studied as a tool for improving water quality by cropping algae. The diet is broad and opportunistic, however, and shifts with age and place: gut analyses across the range record filamentous green algae (Spirogyra, Pithophora, Compsopogon), diatoms and blue-green algae (Phormidium), detritus, sand-bound benthic material, higher-plant fragments and incidental small invertebrates and insect parts. Young fish take proportionally more animal matter and have shorter guts; as they grow, the gut lengthens markedly (from roughly 1.4 times body length in fry to over 7 times in large adults), the hallmark of a shift toward a herbivorous, detritus- and algae-based diet. In the community it is a primary consumer that converts algal and detrital production into fish flesh, and it is in turn heavily preyed upon — Nile perch (Lates niloticus) and other piscivores take it readily.
Mating
Reproduction in Sarotherodon galilaeus inverts the usual cichlid script in a way that fascinated early researchers. Spawning takes place over a nest scraped in the substrate, but it is the female that takes the leading role: she is the more active partner in courtship, does much of the excavation of the breeding pit, and defends the mating territory. A pair forms for spawning, and the bond lasts only as long as it takes to get the eggs into the parents' mouths — once the clutch is taken up, the pair dissolves. During spawning the pair circle the nest in the classic cichlid manner, the female depositing eggs that the male fertilises in passes over the pit; because the eggs are large and do not adhere to the substrate, they are quickly gathered into the mouth rather than left to be guarded in place. There is no protracted territorial pair-bond of the kind seen in substrate-brooding cichlids; the social system is organised instead around the brief, intense spawning event and the long mouthbrooding that follows.
Breeding
Sarotherodon galilaeus is a biparental mouthbrooder — the trait that, more than any other, separates Sarotherodon from the maternal mouthbrooders of Oreochromis. After spawning, both parents take eggs and developing young into the buccal cavity; in this species the load is genuinely shared, with the male carrying a substantial portion of the clutch, though observations of captive pairs show the division can vary and clutches of the largest females may be brooded jointly. The eggs are large for a tilapia (about 0.5 in, up to ~0.5 in long) and, unlike the small adhesive eggs of true Tilapia, do not stick to the substrate — an adaptation to being mouth-incubated from the moment of spawning. Fecundity is modest relative to the egg-scattering Oreochromis, reflecting the heavy investment in each egg and in brooding: clutch and ripe-egg counts scale with female size, running from roughly 500–1,000 eggs in fish around 6.5–8.5 in to several thousand (up to ~5,000) in the largest females near 12 in. The brooding parents do not feed while carrying, and after the young are released they continue to shelter in the parents' mouths when threatened for a period before becoming independent. The species spawns repeatedly through a warm season and matures young — breeding females as small as about 6 in have been recorded in Lake Chad — traits that, combined with hardiness, underpin its success in the wild and its value in pond culture.
In the aquarium
Sarotherodon galilaeus is far better known as a food and aquaculture fish than as an ornamental, but it is occasionally kept, and its biparental mouthbrooding makes it a genuinely interesting subject for hobbyists with the space for it. It is not a community fish: at close to 15.5 in it is large, powerful and, like most tilapias, destructive to rooted plants and prone to redigging the substrate. A single fish or a breeding group needs a big tank — on the order of 120 US gal and upward for adults — with robust filtration to cope with the heavy waste load of a large, well-fed herbivore, and generous, frequent water changes.
The species' great virtue in captivity is its toughness. It is strongly eurythermal and tolerant of a wide chemical range, doing well anywhere from roughly 72–86 °F and across pH from about 6.5 to 9.0; it handles hard, alkaline and even brackish water without difficulty, which makes it forgiving of tap-water variation. A sandy or fine-gravel substrate suits its digging and spawning behaviour, with rockwork or other structure to break up sightlines. Feeding is easy: as a natural algae- and detritus-grazer it thrives on a vegetable-based diet — spirulina and other plant-based prepared foods, blanched greens, and good-quality sinking pellets — accepting flake and frozen foods readily but needing the plant matter to stay in long-term condition.
Be realistic about temperament and scale. Tilapias of this size are boisterous and can be aggressive toward conspecifics outside of spawning, so a group needs room to spread out, and tankmates should be similarly large and robust rather than delicate. The reward, for keepers set up for a fish of this size, is the chance to watch one of the classic shared-mouthbrooding systems play out — both parents cycling eggs and fry through their mouths — a behaviour rarely seen in the home aquarium. Local laws should be checked before keeping any tilapia, as several are regulated or prohibited because of their potential as invasive species.
Conservation
The IUCN Red List assesses Sarotherodon galilaeus as Least Concern (assessed 23 April 2020, published 2020, by Diallo and colleagues for the Pan-Africa freshwater biodiversity assessment), on the strength of its very large range, broad ecological tolerance, abundance and ongoing use as a fisheries and aquaculture species; the global population trend is listed as unknown. It carries no CITES listing. As one of the most important food fishes across its range it is heavily exploited — by artisanal and commercial fisheries throughout West Africa, the Nile and the Levant, and as a widely cultured pond species — and it has been moved and stocked far beyond its native waters, becoming naturalised in places. That very adaptability cuts both ways: while the species itself is secure, its introduction outside its range carries the ecological risks common to tilapias. In its type waters, the Sea of Galilee, the celebrated St. Peter's fish fishery has at times declined, and studies there have linked changing catches to factors including water-level management, the lake's shifting plankton community and the stocking of competing cichlids — a reminder that even an abundant, hardy fish is tied to the health of the water body that produces it.