Taxonomy & naming
The black tilapia was described by Ethelwynn Trewavas in 1941 and, like the other maternal-mouthbrooding 'Tilapia' of her era, was later placed in the genus Oreochromis as part of her comprehensive revision of the tilapiines, formalised in the 1983 monograph 'Tilapiine Fishes of the Genera Sarotherodon, Oreochromis and Danakilia.' It is accepted as valid as Oreochromis placidus in Eschmeyer's Catalog of Fishes and FishBase. The epithet placidus is Latin for 'calm' or 'placid,' a fitting reference to the quiet, sluggish pools the fish favours. Historically two subspecies were recognised — the nominate Oreochromis placidus placidus of the Zambezi and southern coastal rivers, and Oreochromis placidus ruvumae of the Ruvuma system on the Tanzania–Mozambique border — though the subspecies are not consistently upheld in modern usage. The species belongs to the southeastern African coastal-river assemblage of Oreochromis.
Morphology
Oreochromis placidus is a medium-large tilapia, reaching about 14 in in total length and a published maximum weight of around 1.1 kg — a respectable food-fish size. It is a deep-bodied, laterally compressed cichlid of conventional tilapia proportions, with the meristics of the genus: roughly 16–18 dorsal spines and 10–13 soft rays, with three to four anal spines and eight to eleven anal soft rays. The common name 'black tilapia' points to the dark coloration the species can assume, particularly in territorial and breeding males, over an otherwise typical countershaded tilapia ground. As in the genus generally it has fine grazing dentition and the breeding-dependent colour changes characteristic of tilapias, with males darkening and intensifying when holding a spawning territory. It is a solidly built, unspecialised tilapia of slow lowland waters rather than a niche specialist.
Habitat
The black tilapia ranges across the lower Zambezi and southward and eastward through the lower reaches of a series of southeastern African rivers — the Sabi (Save), Lundi (Runde), Pungwe and Buzi of Mozambique and Zimbabwe — and northward to the Ruvuma River on the Tanzania–Mozambique border, with records from Lake Rutumba and associated systems. It is a fish of quiet water: FishBase notes a preference for calm pools with marginal vegetation, the placid backwaters its name evokes. It occurs in both fresh and brackish water, consistent with a lowland coastal-river fish that ranges down into the tidally influenced, slightly saline lower reaches and coastal lakes and lagoons. This is a benthopelagic tilapia of warm, slow, vegetated lowland habitats — riverine pools, floodplain lakes and brackish coastal margins — rather than fast or rocky water.
Feeding
Detailed diet studies are limited for Oreochromis placidus, but its morphology and generic placement mark it as a typical Oreochromis microphage: a grazer and detritivore taking algae, phytoplankton, fine plant material and the organic detritus of soft sediments, with the fine teeth and gill-raker filtering apparatus of the genus. In the quiet, vegetated, often nutrient-rich pools and backwaters it favours, such fine plant and microbial material is abundant, and the species works low on the food web as a primary consumer rather than a predator. This broad-spectrum herbivore-detritivore habit is the ecological signature of the genus and underpins the species' value as a productive food fish in the systems where it is netted commercially.
Mating
As an Oreochromis, the black tilapia is a maternal mouthbrooder with the genus's lek-and-pit mating system. Territorial males darken into their nuptial coloration, clear and defend spawning pits in the substrate of the shallows or quiet margins, and court females moving through the breeding area; spawning occurs over the male's pit, after which the female immediately takes the fertilised eggs into her mouth. While species-specific breeding observations are sparse, this pattern is universal among the well-studied members of the genus, and nothing in the ecology of placidus suggests a departure from it. Breeding in its lowland-river habitat would be expected to follow the warm, high-water productive season, when shallow vegetated breeding habitat is most extensive and food most abundant.
Breeding
Reproduction is by maternal mouthbrooding: the female incubates the eggs and yolk-sac larvae in her buccal cavity, aerating and protecting them, and shelters the free-swimming fry in her mouth when threatened until they become independent. As in related Oreochromis, fecundity scales with female size and runs to several hundred eggs in a well-grown female — the mouthbrooder's trade of quantity for the high survival of guarded young. Specific clutch counts, incubation periods and growth data for placidus are not prominently published; the figures from better-studied congeners are the closest guide. The species' tolerance of brackish water suggests, as in other estuarine-capable tilapias, that it can breed across a span of salinities in its lower-river and coastal habitats, contributing to its broad distribution and its persistence as a fishery resource.
In the aquarium
Oreochromis placidus is a regional food and fishery fish, not an ornamental, and the aquarium hobby will rarely if ever encounter it. Where it is held it is in aquaculture and fisheries settings, and the requirements follow its wild ecology: warm tropical fresh water, with tolerance of (or a mild preference for) hard to slightly brackish conditions reflecting its lower-river and coastal origins; a large tank or pond suited to a fish of around 14 in and up to a kilogram; quiet, vegetated or open margins with soft or open substrate over which males can dig spawning pits; and a vegetable-, algae- and detritus-based diet rather than high-protein animal feeds. As with all Oreochromis, breeding males become territorial and need space and sight-breaks. The species is too large and too plain to recommend as an ornamental; its interest lies in its role as a hardy, productive native tilapia of southeastern African rivers.
Conservation
The IUCN Red List assesses Oreochromis placidus as Least Concern (assessed 1 May 2018). The status rests on the species' relatively wide distribution across the lower Zambezi and a series of southeastern African coastal rivers, its tolerance of both fresh and brackish conditions, and its generalist habits — a habitat generalist of quiet lowland waters rather than a narrow-range endemic, which buffers it against localised disturbance. That said, the familiar pressures on African lowland freshwaters apply: pollution and agricultural runoff, habitat alteration and damming of its river systems, fishing pressure, and — a recurring concern across the genus — competition with and possible hybridisation from introduced tilapias, especially Nile tilapia, spreading through southern African waters. None of these is currently documented as a population-level threat sufficient to change its status, and it carries no CITES listing; it remains one of the more secure members of a genus that includes some of Africa's most imperilled freshwater fishes.