Taxonomy & naming
The species was described by the British ichthyologist Ethelwynn Trewavas in 1935 as Haplochromis pleurostigma, on material from Lake Malawi, during the foundational work that first laid out the lake's vast haplochromine flock. Eschmeyer's Catalog of Fishes fixes the type locality at Chilumba (Deep Bay), Lake Malawi, with the holotype held at the Natural History Museum, London (BMNH 1935.6.14.1982). It was later transferred to the genus Copadichromis, which Eccles & Trewavas erected in 1989 to gather the utaka and their allies out of the old catch-all 'Haplochromis'. The Catalog of Fishes records the current status as valid as Copadichromis pleurostigma (Trewavas, 1935), and FishBase and the IUCN both use the same parenthetical authorship, the parentheses signalling that the species was originally placed in a different genus. The utaka remain a taxonomically tangled group: Copadichromis as currently constituted is probably not monophyletic, and several nominal species are very similar to one another. Copadichromis pleurostigma is part of a small cluster of three-spot-derived utaka and is most easily confused with Copadichromis pleurostigmoides (Iles, 1960), a separate species named for its resemblance to it; the two are separated by the shape of the side spot and by gill-raker counts (about 17-18 in pleurostigma versus 21 or more in pleurostigmoides). M.K. Oliver has also suggested that the nominal Copadichromis trimaculatus may prove to be a synonym of pleurostigma, a question not yet resolved.
Morphology
Copadichromis pleurostigma is one of the largest of the utaka. FishBase gives a maximum of 7.5 in total length, and Oliver records wild fish reaching at least 6.5 in standard length and 8 in total length. The body is the fusiform, laterally compressed shape typical of an open-water plankton-feeder. Its defining mark is a single melanic side spot: a large suprapectoral blotch that is longer than it is tall (horizontally elongate) and extends slightly above the upper lateral-line tubules, with no supra-anal spot present. This spot shape, together with the low gill-raker count of roughly 17-18, is diagnostic against the squarish, non-extending spot and higher raker count of the look-alike Copadichromis pleurostigmoides. Internally the fish is built for an algal diet, with a notably long intestine and pharyngeal jaws bearing many small, crowded teeth. Coloration is sexually dimorphic in the usual haplochromine fashion: females and non-breeding males are a plain silvery-beige showing the dark side spot, while dominant, territorial males develop brighter breeding dress. Juveniles are strikingly different — bright yellow over the whole body — and they lose the yellow on the flanks as they mature.
Habitat
The species is endemic to Lake Malawi and, despite an early belief that it was confined to the northern part of the lake, it is now known to have a lake-wide distribution that spans all three riparian countries — Malawi, Mozambique and Tanzania. The IUCN assessment lists records from Chilumba, Chiwanga Bay in Mozambique, Khande, the northern ends of the south-east and south-west arms, Domira Bay, the Maleri Islands (to about 151 ft depth), Namiasi, Chembe and Chintheche. It lives over sand, in the open sandy habitat and in the intermediate habitat where stretches of sand are studded with scattered, sediment-covered rocks, rather than on continuous rock. Lake Malawi's water in these zones is warm, alkaline and moderately mineralised: Konings's in-situ measurements for the lake give a pH of roughly 7.7-8.6 and surface temperatures of about 75–82 °F, with a hardness on the order of 7-30 dH; depth records for the species reach about 151 ft at the Maleri Islands. As a shoaling open-water fish it is benthopelagic, foraging in the water column near the bottom rather than confining itself either to midwater or to the substrate.
Feeding
Although it belongs to the utaka — the zooplankton specialists of Lake Malawi — Copadichromis pleurostigma is widely regarded as an exception to the group's feeding habits. Its anatomy points firmly toward plant matter: Eccles & Trewavas (1989) and Konings (1995) noted that, almost uniquely among the utaka, it appears specialised to feed on phytoplankton rather than zooplankton, and the adaptations cited for this diet are a very long intestine (a hallmark of herbivory in fishes) and a pharyngeal dentition of many small, crowded teeth thought to function in rupturing the tough cell walls of algae. The IUCN assessment captures the same tension, stating that the species feeds on zooplankton 'although the pharyngeal jaws and dentition suggests it feeds on algae'. FishBase places it at a trophic level near 3.4. The weight of the anatomical evidence favours a phytoplankton/algae-based diet, which would make pleurostigma one of the few herbivorous members of an otherwise planktivorous assemblage; in either reading it is an open-water grazer of the lake's suspended primary production.
Mating
Like other utaka, Copadichromis pleurostigma is a shoaling fish for most of its life, with females and non-breeding males moving in loose open-water groups over the sand and intermediate habitats. Reproduction follows the lek-like pattern general to the haplochromine cichlids of Lake Malawi: dominant, territorial males in heightened breeding colour establish and defend spawning sites on the bottom, while females visit to spawn and then carry the brood away. Detailed bower-building and courtship behaviour have not been documented for this particular species in the literature as thoroughly as for some of its better-known relatives, but its membership in the utaka and its sandy-habitat lifestyle place it within this familiar Malawian breeding system, in which the male's coloration and display serve to attract conspecific females amid the many similar-looking utaka that can share the same water.
Breeding
Copadichromis pleurostigma is a maternal (ovophilous) mouthbrooder, the standard reproductive mode of Lake Malawi's haplochromines: the female takes the eggs into her mouth, they are fertilised in the haplochromine manner as she mouths at the male, and she alone incubates the eggs and larvae buccally. What is distinctive is what happens at release. At Msuli Point, Konings (1995) observed mouthbrooding females in November releasing their free-swimming fry among the small schools of other utaka juveniles that gather above the nests of the kampango, the large catfish Bagrus meridionalis, which guards its own young vigorously. By depositing her fry into this protected nursery aggregation, the female effectively outsources their early defence; it appears the parents do not guard their own offspring after release. The newly released young are bright yellow all over, fading on the flanks as they grow. The IUCN gives a generation length of about two to three years.
In the aquarium
Copadichromis pleurostigma is rarely kept in the hobby. It does appear occasionally in specialist Lake Malawi collections, but it has never become a common trade fish — the IUCN notes explicitly that it is only rarely taken by the ornamental trade, and hobbyist records of it are thin compared to more colourful utaka such as Copadichromis borleyi or Copadichromis azureus. Potential keepers should be prepared to source it from specialist breeders or importers dealing in unusual Malawian cichlids rather than from general aquarium retailers.
As a species that can reach nearly 8 in and is naturally a shoaling, open-water fish, it requires a genuinely large aquarium. A tank of at least 71–78.5 in in length is the realistic minimum for a group, with as much horizontal swimming space as possible. The substrate should be fine sand — Lake Malawi's shoreline habitats are predominantly sandy — and a few scattered rocks can be placed against the back or sides to break line of sight and provide cover, but the priority is open water. Dense planting or crowded rock-work suited to mbuna is inappropriate; utaka are not rock-dwellers and will be stressed by confinement.
Water chemistry must reflect Lake Malawi: alkaline and moderately hard, with pH 7.7–8.6, hardness up to around 20–30 °dH, and temperature 75–82 °F. Many tap waters are too soft or too acidic straight from the tap, and dedicated Malawi salt mixes or sodium bicarbonate/calcium carbonate buffering are used by most keepers to achieve and stabilise the required parameters. Consistent water quality matters: like most large Lake Malawi cichlids, Copadichromis pleurostigma is susceptible to Malawi bloat — the common name for a potentially fatal digestive disorder linked to inappropriate diet and, probably, stress. A high-quality cichlid pellet or flake supplemented with spirulina or other vegetable-based foods is more appropriate than heavy protein feeding, given the species' apparent specialisation on phytoplankton and algae in the wild.
Suitable tankmates are other calmer Lake Malawi haplochromines — peacocks (Aulonocara spp.) and large haps that share broadly similar water requirements and temperament. Aggressive mbuna (rock-dwelling cichlid groups such as Melanochromis or Labeotropheus) are generally poor companions for utaka: mbuna occupy habitat differently, tend to be more territorially aggressive, and can bully open-water fish persistently. Mixed-species Malawi communities work best when all inhabitants are drawn from the calmer end of the Malawian spectrum and the tank is large enough to give each fish room. Keeping a group of conspecifics — rather than a lone individual — is advisable, as shoaling behaviour appears to be important to the species' sense of security.
Breeding in captivity follows the standard utaka pattern for those who achieve it. Males develop breeding colour and will attempt to establish a territory and excavate a shallow depression or bower in the sand; they court passing females with lateral displays. Spawning occurs on or near the sand, and the female immediately takes the small clutch into her mouth for buccal incubation. Genus-typical clutch sizes for Copadichromis of similar size run in the range of roughly 50–90 eggs. The female should be given a quiet, undisturbed area or, if the tank is not large enough to provide this naturally, moved to a separate holding tank for the brooding period of roughly three to four weeks. Fry are released free-swimming and are large enough at release to accept finely crushed food from the outset. Because this species is uncommon in the hobby, successful breeding represents a genuine contribution to its availability in specialist circles.
Conservation
The IUCN Red List assesses Copadichromis pleurostigma as Least Concern. The current listing (Konings, Kazembe, Makocho & Mailosi 2019, an amended version of the assessment last evaluated on 22 June 2018 and reviewed by Jos Snoeks) cites the species' wide, lake-wide distribution and the absence of any major widespread threats. The population trend is recorded as unknown, and no information is available on population size. The only threat flagged is subsistence fishing, treated as a potential rather than a demonstrated pressure; the species is only rarely taken by the ornamental aquarium trade. No conservation actions are in place, and it carries no CITES listing. As with the rest of Lake Malawi's endemic flock, its long-term security is tied to the health of the lake as a whole, which faces broad pressures from shoreline development, sedimentation and intensifying fisheries, but none of these is documented as a specific danger to this widely distributed utaka.