Taxonomy & naming
Nyassachromis purpurans was described by Ethelwynn Trewavas in 1935 as Haplochromis purpurans, in her synopsis of the cichlid fishes of Lake Nyasa (Lake Malawi) in the Annals and Magazine of Natural History. The type material — a lectotype (BMNH 1935.6.14.895) and a substantial series of paralectotypes in London, Vienna and Paris, the lectotype fixed by Eccles & Trewavas (1989) — came from Mwaya, at or near the Mbasi River mouth at the northern end of the lake, in present-day Tanzania. The species remained in the broad 'Haplochromis' until the 1989 revision of the Malawi haplochromines by David Eccles and Trewavas, which established the genus Nyassachromis for a distinct assemblage of slender, sand-associated, plankton-feeding species and transferred purpurans into it. The Catalog of Fishes lists it as valid as Nyassachromis purpurans (Trewavas 1935). The specific epithet is descriptive, from Latin purpurans ('becoming purple', 'purple-coloured'): as the ETYFish Project records from Eccles & Trewavas, it refers to the purple- or dark-coloured appearance of the preserved male type specimens — a coloration of the museum material rather than necessarily of the living fish.
Morphology
Nyassachromis purpurans is a medium-small cichlid, reaching about 7 in total length (FishBase) — larger than the short-headed Nyassachromis microcephalus but smaller than the male-biased Nyassachromis nigritaeniatus. It has the slender, laterally compressed body and small, somewhat protrusible mouth characteristic of the genus, suited to taking plankton from open water. The name points to a purplish, dark cast in the preserved male type material, but living coloration is not well documented; as in other Nyassachromis, breeding males are expected to be the more strongly coloured and the larger sex, with females and non-breeding fish plainer and more silvery. Beyond Trewavas's original account and the Eccles & Trewavas re-diagnosis, the species has not been the subject of a detailed modern morphological or colour description, and it is infrequently photographed alive, so its appearance in nuptial dress remains largely uncharacterised in the accessible literature.
Habitat
The species is endemic to Lake Malawi. Its type locality lies at the far northern end of the lake, near the Mbasi River mouth at Mwaya in Tanzania, and FishBase gives a northern latitudinal range of about 9°S to 11°S — distinguishing it from the southern, Monkey Bay-centred members of the genus. It is a shallow-water sand specialist: FishBase reports it from shallow sandy areas, where it feeds on plankton in open water. This places it in the warm, well-lit sand-floor shallows of the northern lake, away from the rocky mbuna zones. Lake Malawi is a large, ancient, hard-alkaline rift lake with stable chemistry — pH generally between about 7.7 and 8.6, surface temperatures cycling roughly between 24 and 82 °F through the year, and moderate carbonate hardness. The shallow sand of the northern basin provides both the open substrate the males need for nest-building and the productive surface waters that supply the plankton the species feeds on.
Feeding
Nyassachromis purpurans is an open-water plankton feeder. FishBase records that it 'feeds on plankton in the open water' and assigns it a trophic level of about 3.4 — placing it among the more carnivorous, zooplankton-leaning members of the genus rather than at the phytoplankton-grazing end occupied by Nyassachromis breviceps. The small, protrusible mouth is the anatomical correlate of this feeding mode, suited to quick suction strikes at planktonic crustaceans suspended above the sand flats. Picking plankton from the open shallows is the classic 'utaka' strategy shared across this group of Lake Malawi cichlids, exploiting the lake's seasonally abundant suspended prey. As with most of the genus, this characterisation rests on the compiled lake-level ecological literature rather than on a dedicated modern gut-content study of the species itself.
Mating
Nyassachromis purpurans is a sand bower-builder, and its nest architecture is documented as a reference point for the genus. Territorial males move onto the open sandy floor and construct a sand turret topped by a spawning platform; field accounts comparing the genus note that the turret of Nyassachromis microcephalus is similar in general form to that of purpurans but carries a more sharply slanted spawning dish — implying that the purpurans nest has a comparatively less inclined platform. Males defend these structures during the breeding period, and the genus characteristically forms temporary loose breeding colonies in which neighbouring males each hold and display from their own nest. Courtship takes place at the turret, where the coloured male displays to females cruising over the sand and draws a receptive female down to the spawning platform; female mate choice is shaped by male coloration and by the quality of the constructed nest.
Breeding
As with all Lake Malawi haplochromines, Nyassachromis purpurans is a maternal mouthbrooder. Spawning takes place on the platform atop the male's sand turret: the female deposits her eggs there, takes them into her mouth, and incubates the fertilised eggs and larvae in her buccal cavity for roughly three weeks before releasing free-swimming fry. The male's contribution is limited to building and defending the nest and to courtship; all parental care thereafter is the female's, who broods without feeding and shelters the fry for a period after they become free-swimming. The construction of the spawning turret is the documented part of this species' reproduction; precise clutch counts and captive breeding records specific to Nyassachromis purpurans do not appear to have been published, so the mouthbrooding details given here are inferred from the consistent biology of the genus and the wider Malawi flock.
In the aquarium
Nyassachromis purpurans is rarely kept and is not an established aquarium fish, so the notes here are generalised from the care of open-water, sand-dwelling Lake Malawi haplochromines rather than drawn from a documented body of keeping experience for the species.
The genus norm applies: a tank of at least 47 in length and around 90 US gal, with the priority placed on a broad, unobstructed floor of fine sand deep enough for a male to heap up his turret and shape the spawning platform. Rockwork should be a modest background only, and rooted plants are unsuitable given the digging activity. Water should match Lake Malawi conditions — pH roughly 7.8–8.6, temperature 75–81 °F, hard, well-buffered and low in nitrate, maintained with strong filtration and regular partial water changes. As a plankton feeder it is well served by small frozen foods — cyclops, daphnia, mysis, brine shrimp — alongside quality dry foods, with the usual caution against an over-rich, meaty diet that can trigger 'Malawi bloat'. Males are territorial when breeding and a single dominant male with several females is the most workable arrangement for showing the species' nest-building; tankmates should be other peaceable open-water haps and peacocks rather than aggressive mbuna, which would harass and outcompete a sand-dwelling species of this temperament.
Conservation
The IUCN Red List assesses Nyassachromis purpurans as Least Concern (LC), in an assessment dated 20 June 2018. The LC rating indicates that, on current information, the species is not considered at significant risk of extinction — a more secure position than the Critically Endangered Nyassachromis breviceps or the Data Deficient Nyassachromis leuciscus. As with any Least Concern listing, this is a statement about present status rather than a guarantee of the future: the shallow sand habitats this species uses at the northern end of Lake Malawi are exposed to the same broad pressures affecting the lake's sand cichlids generally, including artisanal and commercial fishing over sand flats and sedimentation driven by deforestation and land-use change in the catchment, which can smother the clean sand that bower-building cichlids need for spawning. The species is not listed under CITES, and no targeted conservation measures are in place or currently required. Lake-wide assessments of the Lake Malawi/Nyasa/Niassa catchment (e.g. Sayer and colleagues, 2019) document these stressors across the sand-cichlid community and provide the context for monitoring even a Least Concern endemic over the longer term.