Taxonomy & naming
The rainbow happy was described by the Belgian-British ichthyologist George Albert Boulenger in 1905 as Paratilapia carlottae, in the Annals of the South African Museum, from a specimen taken in the Zambezi River immediately above Victoria Falls, on the Zambia–Zimbabwe border. The holotype (SAM 8788, originally in the South African Museum and now held at the Albany Museum, Grahamstown) anchors the name. The species was later moved into Serranochromis Regan, 1920, and treated as a member of the small-mouthed subgenus Sargochromis — a grouping that Bell-Cross (1975) and Greenwood (1979) recognised within the largely piscivorous Serranochromis, and which Paul Skelton (1993) raised to full genus rank. The Catalog of Fishes and FishBase both now list the fish as Sargochromis carlottae (Boulenger, 1905). The genus belongs to the haplochromine cichlids (subfamily Pseudocrenilabrinae) and contains roughly half a dozen species concentrated in the upper and middle Zambezi system; carlottae is one of the most widespread. As Winemiller's (1991) Barotse floodplain survey makes clear, the Upper Zambezi drainage — the river above Victoria Falls together with the Okavango and the Cuando/Chobe — is the centre of diversity for the whole Serranochromis/Sargochromis radiation.
Morphology
Sargochromis carlottae is a moderately deep-bodied, laterally compressed cichlid with the comparatively small, slightly subterminal mouth that separates the snail-eating Sargochromis from their big-jawed piscivorous cousins. FishBase records a maximum of about 13.5 in standard length and a published weight of roughly 1 kg, with a maximum age near six years; in practice most fish encountered on the floodplain are far smaller — adults in Winemiller's Barotse samples ran to only about 5–7 in SL, and the species matures at around 4–5 in. The common name refers to the colour: living fish are an olive to brassy-grey ground broken by iridescent green, blue and gold reflections across the flanks and fins, brightest in ripe males. Like other happies the species carries the haplochromine signature of egg-spot-like markings on the anal fin of males. Sexual dimorphism is real but modest — males grow larger and mature at a greater size than females (Winemiller estimated maturation at about 6–7 in SL in males versus 4–5 in in females) and are the more intensely coloured sex, while ripe females are deeper-bodied and, as mouthbrooders, carry a distended buccal cavity when holding a brood.
Habitat
The rainbow happy is endemic to the warm, soft, tea-stained waters of south-central Africa's great inland drainages: the Okavango River and Delta, the Upper Zambezi above Victoria Falls, the Cuando/Chobe, the Kafue, and Lake Kariba and other impoundments of the Middle Zambezi to which Upper Zambezi fishes have spread. Native range thus spans Angola, Namibia, Botswana, Zambia and Zimbabwe. FishBase characterises the species as favouring deep permanent floodplain channels and lagoons with sandy bottoms and aquatic vegetation, and Winemiller's fieldwork bears this out: large carlottae were taken from lagoons, slow tributaries and quiet backwaters, almost always close to weed beds, submerged branches or other cover rather than in open water. These are tropical floodplain waters laid over ancient, nutrient-poor Kalahari sand. In-situ measurements from the Barotse floodplain during Winemiller's 1989 survey give a realistic envelope: water temperatures from about 64 °F (deep in a canal in mid-winter) up to 82 °F (surface in late spring), Zambezi mainstream water of 70–82 °F, and pH ranging from roughly 5.5 in acidic floodplain pools to 7.0 in tributaries — clear, well-oxygenated and often stained brown with dissolved organics. The species' life is governed by the annual flood pulse, dispersing across the inundated plain at high water and concentrating into permanent channels and lagoons as the water falls.
Feeding
The genus Sargochromis is famous as the snail-crushing wing of the happies — molluscivores armed with robust pharyngeal mills, and FishBase summarises carlottae's diet as aquatic insects, crustaceans and snails, noting its value as a snail-control agent. The detailed dietary work, however, reveals carlottae as the odd one out. In Winemiller's (1991) Upper Zambezi study, where the true molluscivore Sargochromis giardi fed mainly on bivalves and Sargochromis codringtonii on snails and seeds, Sargochromis carlottae fed mostly on aquatic insects (chiefly mayfly and caddisfly larvae) and — strikingly — on fish scales, which made up about half the diet by volume in larger adults. Small carlottae took more insects and microcrustacea; as the fish grew, scales came to dominate. Whether the species actively rasps scales from living fish (lepidophagy) or gleans shed scales from the substrate was left unresolved, but the reliance on scales is unusual among the happies and gives carlottae the broadest measured diet breadth of the three Upper Zambezi Sargochromis. Notably, almost no carlottae stomachs were empty (3 of 54 examined), marking it as a persistent, generalist forager of the vegetated lagoon margins. It is best described, then, as an invertebrate-and-scale feeder built on the molluscivore plan rather than a dedicated snail specialist.
Mating
Like all members of the genus, Sargochromis carlottae is a mouthbrooder, and its reproduction is tied to the seasonal calendar of the floodplain. Through the falling- and low-water months the fish concentrate in permanent channels and lagoons, and Winemiller found the adult population ripening through September to December — small fractions of fish showing mature gonads in September, the proportion rising steadily toward the rains, so that most appeared poised to spawn just before the annual flood. This timing is adaptive: a female that broods through the start of the flood can carry her young onto the newly inundated plain and release them into a vast, food-rich, low-predator environment. Courtship in the genus centres on a male-tended spawning site. FishBase describes carlottae as building a small "sandscrape" nest — a shallow depression excavated in sand into which the eggs are first laid before the female takes them up — and classes the species as ovophilic and agamous, meaning eggs are brooded but no lasting pair bond forms. Beyond the breeding concentration, carlottae is a cover-loving fish of the weed beds rather than an open-water shoaler.
Breeding
Sargochromis carlottae is a maternal (female) mouthbrooder. The male clears and defends a shallow sandscrape nest; eggs are deposited there and then gathered into the female's mouth, where they are incubated and the larvae sheltered through their most vulnerable days before release. Fecundity is modest and, fittingly for a mouthbrooder, the eggs are large and few: Winemiller counted an average of about 362 mature oocytes in ripe Barotse females (the lowest fecundity of the seven Serranochromis he examined), with mean mature-oocyte diameters near 0.5 in. Spawning is timed to the onset of the flood, and growth is unhurried by aquarium standards — the species reaches sexual maturity at roughly two years and about 4–5 in, and lives to around six years. Parental care follows the standard haplochromine pattern: the female alone broods the eggs and guards the released fry in her buccal cavity, snapping them back to safety at any threat, with no documented male involvement after spawning. There is no published record of the species being bred deliberately in captivity, so what is known of its reproduction comes from field study of wild populations rather than the aquarium.
In the aquarium
The rainbow happy is essentially absent from the ornamental hobby. It is a large, plainly-coloured floodplain fish of subsistence and commercial importance in Zambia, Namibia, Botswana and Zimbabwe — sold for the table and pursued as a light gamefish — rather than an aquarium subject, and it has no established place in the trade. Any honest husbandry note must therefore be extrapolated from its biology and from experience with related southern-African cichlids rather than from a body of keeping reports. A fish that reaches some 13.5 in and a kilo would need a very large tank — on the order of two metres of length and several hundred litres — with a deep sand bed it can scrape and sift, generous structure of driftwood and robust plants or rockwork to break sightlines, and strong, well-oxygenated filtration that nonetheless leaves areas of calm water, mirroring the still lagoon margins it favours. Water should be kept warm and on the soft side of neutral (roughly 72–82 °F, pH about 6.5–7.5) in keeping with its Zambezi-Okavango origins. Diet would present few problems: this is an unfussy generalist that in the wild eats insects, crustaceans, snails and even scales, so a varied menu of meaty frozen and live foods with sinking prepared diets would suit it. As a maternal mouthbrooder it could in principle be spawned in the haplochromine manner, but with no real-world precedent any attempt would be experimental. For nearly all keepers this is a fish to appreciate in its native floodplains, not in glass.
Conservation
The IUCN Red List assesses Sargochromis carlottae as Least Concern (assessed 1 March 2007 by Denis Tweddle and Brian E. Marshall, with an unknown population trend). The assessment cites a wide distribution across the upper Zambezi, Okavango and Kafue systems, presence in Lake Kariba, and establishment in Middle Zambezi impoundments, and records no known threats to the species. It carries no CITES listing and FishBase rates its fishing vulnerability as low. The fish is a real if minor component of the floodplain gillnet and subsistence fisheries — it appears routinely, in small numbers, in catch surveys from the Barotse plain, the Caprivi/Zambezi-Chobe system and Lake Liambezi — but at a level that does not appear to threaten its populations. The broader risks to the species are the basin-scale pressures now bearing on the Okavango and Zambezi wetlands generally: flow regulation and damming, altered flood-pulse regimes, habitat loss and rising fishing effort across these floodplains. None is documented as a specific danger to the rainbow happy, but they form the backdrop against which the status of this and every Upper Zambezi floodplain cichlid should be read.