Taxonomy & naming
Placidochromis longimanus was described in 1935 by the British ichthyologist Ethelwynn Trewavas, originally as Haplochromis longimanus, in her studies of the cichlid fishes of Lake Nyasa (Lake Malawi). For much of the twentieth century it remained in the sprawling genus Haplochromis, the holding bin for most of Malawi's non-mbuna haplochromines. When Eccles and Trewavas reorganised these fishes in 1989 and erected the genus Placidochromis for a set of generally calm-mannered species, longimanus was placed there, and Catalog of Fishes today lists it as valid as Placidochromis longimanus (Trewavas 1935). The genus name combines Latin placidus, 'placid', with the cichlid name-element chromis; the specific epithet longimanus derives from Latin longus, 'long', and manus, 'hand', a reference to long pectoral fins (the 'hand' of a fish in older descriptive usage). As one of the genus's earlier-named and more widely distributed members, it is better characterised ecologically than the obscure deep-water Placidochromis described decades later.
Morphology
Placidochromis longimanus is a medium-small cichlid, reaching about 5.5 in total length — larger than the diminutive deep-water members of the genus. It has the laterally compressed, moderately elongate body typical of a Malawi 'hap', and, as its name suggests, comparatively long pectoral fins. Beyond these general features the published morphometric detail is limited compared with the meticulously diagnosed modern species; its identity rests on the original Trewavas description and subsequent checklist treatments rather than on a recent revision. Its relatively long intestine is an ecologically informative trait, consistent with a diet that includes a good deal of plant and detrital material, since herbivorous and detritivorous fishes generally have longer guts than carnivores. Life coloration and sexual dimorphism are not richly documented in the literature available, though, as in most Malawi haplochromines, breeding males are likely to be the more strongly coloured sex.
Habitat
This species occupies the southern end of the Lake Malawi system: the south end of the lake itself (including the shallow south-east arm), the smaller Lake Malombe just downstream, and the upper Shire River that drains the lake. It is a shallow-water fish rather than a deep-water one, and its habitat is unusually specific — it seems to be restricted to the beds of rootless aquatic plants, the hornwort Ceratophyllum and the naiad Najas, that form floating tangles at a depth of around ten metres. These weed beds create a sheltered, structurally complex microhabitat over softer bottoms in the warm, productive shallows of the southern basin, a very different setting from the cold benthic plains favoured by the deep-water Placidochromis. The species' tie to this particular plant community is a notable feature of its ecology and sets it somewhat apart within its genus.
Feeding
Placidochromis longimanus is thought to feed mainly from the sediment that accumulates among the weeds of its habitat — the fine organic detritus, algae and associated micro-organisms that settle into the floating plant beds. Its long gut is taken as an indicator that the species is largely herbivorous or detritivorous, since a long intestine is characteristic of fishes that process plant material and detritus rather than animal prey. Consistent with this, FishBase gives it a notably low estimated trophic level of about 2.0, at the herbivore/detritivore end of the spectrum and well below the values for the carnivore-leaning deep-water members of the genus. Rather than hunting, it appears to graze and sift the rich sediment trapped within its weed-bed home.
Mating
Specific courtship behaviour has not been described in detail for Placidochromis longimanus, but as a Lake Malawi haplochromine it is a maternal mouthbrooder, in keeping with the near-universal reproductive mode of the lake's cichlid flock. Males court females — typically with lateral displays and quivering over a chosen site — and females select among them before taking the fertilised eggs into the mouth. Living in shallow, structured weed-bed habitat, longimanus has ready access to sheltered spawning sites within or near the plant beds. The fine details of male breeding coloration, display form and seasonality are not well recorded for this species, and this account reflects the general pattern of the Malawi haplochromines applied to a fish of its habitat.
Breeding
There is no detailed published account of clutch size, egg development or brood-rearing for Placidochromis longimanus, but its reproductive mode is not in doubt: like other Lake Malawi haplochromines it is a female mouthbrooder. The female collects the fertilised eggs into her mouth and incubates them through hatching, sheltering the larvae and then the free-swimming fry in her buccal cavity for several weeks; young fry may dart back to her mouth when threatened. For a species of around 5.5 in, clutch sizes are likely to be larger than in the tiny deep-water Placidochromis but still modest by the standards of egg-scattering fishes, reflecting the heavy parental investment that mouthbrooding entails. The precise figures, however, are not on record for this species, and the shallow weed-bed setting would make its broods less observed than those of popular reef cichlids.
In the aquarium
FishBase notes both a commercial fishery and a commercial aquarium interest for Placidochromis longimanus, but it is not a widely kept or readily available aquarium fish, and there is little dedicated hobby literature on it; in its home range it is more likely to be encountered as part of the artisanal catch than as an export ornamental. The husbandry notes here are therefore largely extrapolated from the genus and from the species' known ecology. A weed-associated, largely herbivorous hap of around 5.5 in would, in principle, suit a spacious aquarium with the hard, alkaline water of Lake Malawi (high pH, moderate-to-high hardness), clean and well-oxygenated conditions, open swimming room and, ideally, some robust plant or structural cover that echoes its natural weed beds. A diet leaning toward vegetable matter — spirulina-based foods, blanched greens and quality cichlid preparations — would better match its long gut and low trophic level than a heavily protein-rich regime. As a calm, non-mbuna species it would be a relatively peaceful tankmate by Malawi standards.
Conservation
Placidochromis longimanus was assessed by the IUCN Red List as Least Concern, with the assessment dated 20 June 2018, and it has no CITES listing. Its occurrence across the southern lake, Lake Malombe and the Shire River gives it a reasonably broad range within the system, supporting the Least Concern rating. That said, the southern Lake Malawi system is among the most heavily exploited and human-impacted parts of the lake: Lake Malombe in particular has experienced severe fishery decline and habitat change, and the shallow southern shallows face intense fishing pressure, catchment erosion, sedimentation and nutrient enrichment, while the weed beds the species depends on are sensitive to shifts in water level, clarity and nutrient load. The Lake Malawi conservation literature consistently flags these pressures on the shallow southern habitats. For a weed-bed specialist of this region, maintaining the health of those shallow plant communities and managing the southern fisheries sustainably are the key conservation concerns, even though the species is not currently considered threatened.