Taxonomy & naming
Nyassachromis nigritaeniatus was described by Ethelwynn Trewavas in 1935 as Haplochromis nigritaeniatus, in her synopsis of the cichlid fishes of Lake Nyasa (Lake Malawi) in the Annals and Magazine of Natural History, from material taken at Monkey Bay in the southern part of the lake. The lectotype (BMNH 1935.6.14.875) and a series of paralectotypes are held at the Natural History Museum, London, the lectotype fixed by Eccles & Trewavas (1989). The species was carried in the broad 'Haplochromis' until the 1989 revision of the Malawi haplochromines by David Eccles and Trewavas, which erected the genus Nyassachromis for a distinct group of slender, sand-associated, plankton-feeding species and reassigned nigritaeniatus to it. The Catalog of Fishes lists it as valid as Nyassachromis nigritaeniatus (Trewavas 1935). The specific epithet is descriptive, combining Latin niger ('black') and taeniatus ('banded'): as the ETYFish Project records from Eccles & Trewavas, it refers to a black stripe, about as broad as one scale, running from the opercular (gill-cover) spot to the base of the caudal fin.
Morphology
Nyassachromis nigritaeniatus is among the larger species of its genus. FishBase records a maximum of 8.5 in total length for males and about 6 in TL for females — a clear size dimorphism, with males markedly bigger. The body is the slender, laterally compressed Nyassachromis form, and the diagnostic mark is the one that gives the fish its name: a narrow black longitudinal stripe, roughly one scale wide, extending from the dark spot on the gill cover back along the flank to the base of the tail. The mouth is small and suited to picking zooplankton from open water. Beyond the stripe and the size difference between the sexes, detailed colour notes for living fish are limited; as in other members of the genus, breeding males are expected to be the more strongly coloured sex, with females and non-breeding fish plainer. The species has not been the subject of a dedicated modern morphological or colour description beyond Trewavas's original account and the Eccles & Trewavas re-diagnosis.
Habitat
The species is a Lake Malawi endemic, with FishBase describing it as endemic to the Monkey Bay area in the southern part of the lake and giving a latitudinal range of about 14°S to 15°S. It is a fish of open sand at moderate depth: FishBase reports that it 'has been seen on open sand at a depth of about 49 ft', placing it deeper than the very shallow sand species of the genus and squarely within the lake's sand-floor community, away from the rocky mbuna habitat. Lake Malawi is a large, ancient, hard-alkaline rift lake with a stable epilimnion: pH typically between about 7.7 and 8.6, surface temperatures cycling roughly between 24 and 82 °F through the year, and moderate carbonate hardness; at 49 ft the water is a little cooler and more stable than the surface shallows. The clean open sand of the Monkey Bay area is the setting both for the fish's midwater feeding and for the males' nest-building.
Feeding
Nyassachromis nigritaeniatus is a midwater zooplankton feeder. FishBase records that it 'feeds on zooplankton in midwater' and assigns it a trophic level of about 3.4 — among the more carnivorous members of the genus, in contrast to the phytoplankton-grazing Nyassachromis breviceps. The small, somewhat protrusible mouth is the anatomical correlate of this feeding mode, suited to picking individual planktonic crustaceans from the water column above the sand. Hunting zooplankton in open water over the sand flats is the classic 'utaka' strategy shared across this group of Lake Malawi cichlids, exploiting the lake's seasonally abundant suspended invertebrate prey. As with most of the genus, this characterisation derives from the compiled lake-level ecological literature rather than from a dedicated modern gut-content study of the species.
Mating
Nyassachromis nigritaeniatus is a sand bower-builder like the rest of its genus. Territorial males descend to the open sandy floor and construct a spawning nest — a heaped sand turret or crater — which they defend as a territory, the genus characteristically forming temporary loose breeding colonies in which neighbouring males each hold and display from their own nest. Courtship takes place at the nest, where the larger, coloured male displays to females cruising over the sand and draws a receptive female down to the spawning site; female mate choice is shaped by male coloration and by the nest he has built and holds. Specific courtship observations for Nyassachromis nigritaeniatus have not been separately published, but as a member of Nyassachromis — and given the marked size advantage of males in this species — it is expected to follow the genus template, in which large, nest-holding males do the displaying and the spawning.
Breeding
As with all Lake Malawi haplochromines, Nyassachromis nigritaeniatus is a maternal mouthbrooder. Spawning takes place at the male's sand nest: the female lays her eggs on the spawning surface, 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 contributes only the nest and the courtship; all subsequent parental care is the female's, who broods without feeding and shelters the fry for a period after they become free-swimming. The pronounced size difference between the sexes — males to about 8.5 in, females to about 6 in — is consistent with this kind of polygynous, lek-style breeding system, in which large males compete for nest sites and matings while smaller females invest in brooding. No clutch counts or captive breeding records specific to this species appear to have been published, so the reproductive details here are inferred from the consistent biology of the genus and the wider Malawi flock.
In the aquarium
Nyassachromis nigritaeniatus 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.
This is one of the larger members of the genus, with males approaching 8.5 in, so it would need a sizeable tank — comfortably larger than the roughly 47 in / 90 US gal minimum suggested for smaller Nyassachromis — with the emphasis, as always for the genus, on a broad, unobstructed floor of fine sand deep enough for a male to build and defend a nest. Rockwork should be a modest background only, and rooted plants are unsuitable given the digging. Water should match Lake Malawi: pH roughly 7.8–8.6, temperature 75–81 °F, hard, well-buffered and low in nitrate, kept that way with strong filtration and regular partial water changes. As a zooplankton 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; 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 nigritaeniatus 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. Even so, Least Concern is a statement about present status rather than a guarantee: the open-sand habitats this species uses, in and around Monkey Bay, are exposed to the pressures affecting Lake Malawi's southern basin generally, including intensive artisanal and commercial fishing over sand flats and sedimentation from 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 supply the context for monitoring even a Least Concern endemic over the longer term.