Taxonomy & naming
Copadichromis diplostigma was described by Jay R. Stauffer Jr. and Ad Konings in 2006, in their 'Review of Copadichromis (Teleostei: Cichlidae) with the description of a new genus and six new species' (Ichthyological Exploration of Freshwaters 17(1): 9-42). The holotype, catalogued as PSU 3654, was taken at Maingano Island just off Likoma Island in the northern reaches of Lake Malawi (12 deg 02.818' S, 34 deg 45.547' E), with a paratype series of 18 specimens (PSU 3655). Eschmeyer's Catalog of Fishes lists the name as valid as Copadichromis diplostigma Stauffer & Konings 2006. The 2006 revision was a substantial reorganization of a difficult assemblage of zooplankton-feeding haplochromines: it split several long-confused 'utaka' lineages, erected the new genus Mchenga, and named six new Copadichromis, of which diplostigma was placed in the Copadichromis mbenjii species group of sand-dwelling, bower-building utaka. Before it was formally described, the fish circulated in the aquarium hobby as Copadichromis sp. 'Likoma Blue', a provisional trade label tied to its island of origin; it has no settled English common name and is otherwise sold by its scientific name. The genus Copadichromis itself was established by Eccles & Trewavas in 1989 to receive the open-water plankton-pickers formerly lumped under Haplochromis.
Morphology
Copadichromis diplostigma is a small, moderately deep-bodied utaka reaching about 4.5 in in standard length. Like other members of its genus it has the protrusible, highly distensible jaws of a midwater plankton-picker, allowing the mouth to be shot forward into a tube to seize individual prey items from the water column. The species' diagnostic feature, and the source of its name, is a pair of dark lateral spots on each side of the body: an anterior 'supra-pectoral' spot set above and behind the pectoral fin, and a posterior spot on the caudal peduncle. Outside of breeding, the fish are silvery and rather plain, which is typical of the shoaling, cryptically coloured females and immature males of the utaka. Breeding males transform: they take on the metallic blue head and body of a courting Copadichromis, set off by a yellow breast and a narrow yellow marginal band along the anal fin. This combination of two flank spots plus the yellow breast and anal margin separates nuptial diplostigma males from the other blue-headed bower-builders of the mbenjii group.
Habitat
The species is an inhabitant of the intermediate zone, where sandy bottoms meet rocky outcrops, in the clear offshore waters around Likoma and the adjacent Maingano Island. Territorial, breeding males are found over sand at the sand-rock interface at depths of roughly 12 to 18 metres, where they clear and defend their spawning bowers. Non-territorial fish — females and immature males — are not bound to the bottom: they form small groups that range up into the open water column. Lake Malawi is a warm, alkaline rift lake; its surface waters are typically around 24 to 28 deg C with a pH near 7.7 to 8.6 and moderate hardness, the chemical envelope in which all of its endemic cichlids, including this one, live. So far as is documented, Copadichromis diplostigma is restricted to the Likoma Island area and is not known from the main Malawian or Tanzanian shorelines, making it one of the lake's many narrowly localized island endemics.
Feeding
Copadichromis diplostigma is a planktivore in the classic 'utaka' mould — the Chichewa name 'utaka' refers collectively to the open-water, zooplankton-feeding Copadichromis that have long supported an important Malawian fishery. Females and immature males form small groups and feed on plankton in the water column, picking copepods, cladocerans and other suspended zooplankton one item at a time with their protrusible jaws, and they supplement this with benthic invertebrates taken from the bottom. This dual habit — drifting midwater to graze the plankton, then dropping to the substrate for small bottom invertebrates — is characteristic of the bower-building utaka and reflects the seasonally variable availability of lake plankton. The species feeds low on the food web, converting the lake's pelagic zooplankton production into fish biomass.
Mating
Reproduction in Copadichromis diplostigma centres on male-built sand bowers, the hallmark of the mbenjii species group. When ready to breed, a male moves to the sand at the sand-rock interface and constructs and defends a bower — a cleared and shaped crater in the sand that serves as a courtship arena and spawning site. There he assumes full nuptial colour, the blue head and body with yellow breast and yellow-edged anal fin, and displays to passing females. Several males may hold neighbouring territories so that the spawning ground takes on the character of a loose lek, with females moving through to select a mate. Females and the non-breeding males, by contrast, remain in their feeding shoals in midwater and over the substrate until a female is ripe and descends to spawn.
Breeding
Like all Copadichromis and the great majority of Lake Malawi's haplochromine cichlids, Copadichromis diplostigma is a maternal mouthbrooder. Spawning takes place at the male's bower: the female lays a clutch of eggs, takes them up into her mouth, and collects milt from the male — typically following his anal-fin markings — to fertilize the eggs inside the buccal cavity. She then leaves the spawning ground carrying the developing brood, and the male resumes courting other females. The female alone incubates the eggs and larvae in her mouth for the usual two-to-three weeks of haplochromine development, not feeding while she broods, and continues to shelter the free-swimming fry in her mouth for a period after release. There is no paternal care; the male's contribution ends at the bower. This female-only mouthbrooding, combined with male territoriality at the bower, is the standard reproductive system of the genus and was not reported to differ in the original description of the species.
In the aquarium
Copadichromis diplostigma is genuinely uncommon in the hobby. It circulated briefly under the trade label 'Likoma Blue' but has never secured a regular foothold in commercial supply chains, and most aquarists who keep it obtain fish through specialist cichlid breeders or import orders. Prospective keepers should be prepared to search; this is not a species that turns up on a retailer's rack alongside the more familiar utaka such as Copadichromis borleyi or Copadichromis mloto.
As a midwater planktivore from the open-water zone of Lake Malawi, Copadichromis diplostigma needs a tank large enough to give it room to cruise. A footprint of at least 59 in is the practical minimum for a small group; a longer tank is better, since these fish swim continuously and become stressed in cramped quarters. The floor should be furnished with a layer of fine white or buff sand — sand is part of the species' natural environment at the sand-rock interface, and bare glass or coarse gravel is unsuitable. A modest scatter of rocks along the back or sides provides reference points for territory boundaries and cover for females when males are displaying, but the bulk of the swimming volume should remain uncluttered; unlike mbuna, utaka are not rock-dwellers and will use open water when it is available. Subdued or natural-spectrum lighting suits them well.
Water chemistry must reflect Lake Malawi: hard and alkaline. A pH of 7.8–8.6, a temperature of 75–82 °F, and moderately hard to hard water (carbonate hardness of 10–20 °dKH is typical for a Malawi setup) are appropriate targets, matching the facts.water parameters derived from field data. Crushed-coral substrate or a small amount of coral sand in the filter is a practical way to buffer pH and maintain hardness in softer tap-water regions. Strong circulation and efficient filtration matter: these are fish from a clear, well-oxygenated lake, and they do poorly in organically loaded or poorly aerated water.
For tankmates, other Malawi haplochromines of similar size and temperament are the natural choice: peaceful to moderately assertive species from open-water or sandy-bottom habitats work well. Mbuna (rock-dwelling cichlids such as Melanochromis or Pseudotropheus) are generally unsuitable companions — they are often far more aggressive than utaka, dominate feeding, and are inclined to bully the less combative Copadichromis diplostigma. Peacock cichlids (Aulonocara spp.) of comparable size, or other mild-tempered open-water haplochromines, are better partners. Because males in breeding colour will chase females and rivals, keeping one male with two or three females reduces harassment; two males can coexist in a sufficiently large tank if territories can be established at opposite ends.
Diet in captivity should emphasize protein-rich planktivore foods: quality frozen cyclops, daphnia, artemia nauplii, and finely milled spirulina-based pellets or granules are well accepted. These fish should not be fed with high-fat or mammalian-protein content foods, which can cause the bloat condition (Malawi bloat, likely an opportunistic clostridial infection exacerbated by inappropriate diet) that periodically kills Lake Malawi cichlids in captivity. Breeding in the aquarium follows the natural pattern: the male adopts full colour, clears a depression in the sand as his bower, and courts females; spawning at the bower is followed by a two-to-three week oral incubation by the female. Brooding females will often accept a quiet, dimly lit retreat; removing the female to a dedicated brooding tank once mouthbrooding is confirmed is the simplest way to protect the fry from being consumed by tankmates. The species is not considered difficult to breed once a compatible group is established, but because it is rarely kept and seldom described in the hobby literature, most husbandry advice should be treated as a reasonable inference from Copadichromis-general and utaka-typical care rather than species-specific documentation.
Conservation
The IUCN Red List assesses Copadichromis diplostigma as Near Threatened (criterion B1a; assessed 22 June 2018). The principal concern is its very restricted range: as a species apparently confined to the Likoma Island area of Lake Malawi, it has a small distribution that makes it vulnerable to localized pressures even though the population is not documented as declining sharply. Lake Malawi as a whole faces mounting threats — heavy artisanal and commercial fishing pressure (the utaka have historically been a target fishery), sedimentation from catchment deforestation, and the long-term effects of a warming, increasingly stratified lake on its plankton-based food web. None of these is documented as an acute, species-specific danger to Copadichromis diplostigma, but a narrow-range planktivore tied to a particular island's intermediate zone has little geographic buffer against habitat change, which is the basis for its Near Threatened listing rather than a least-concern status.