Taxonomy & naming
Placidochromis hennydaviesae was first described in 1973 by Warren E. Burgess and Herbert R. Axelrod, in the aquarium magazine Tropical Fish Hobbyist, as Haplochromis hennydaviesae. The holotype — a unique specimen catalogued as USNM 210698 in the Smithsonian's national collection — was taken off Monkey Bay at the southern end of Lake Malawi, at a depth recorded as 43 fathoms (roughly 259 ft). For decades the species sat within the vast catch-all genus Haplochromis, the holding bin into which most of Malawi's non-mbuna 'haps' were once placed. Eccles and Trewavas, in their 1989 reorganisation of the Malawi haplochromines, erected the genus Placidochromis and transferred this species to it, a placement followed by later workers including Maréchal (1991), Hanssens (2004) and Konings (2016). The genus name combines Latin placidus, 'placid' or 'calm', with the cichlid-associated Greek chromis, a reference to the unaggressive demeanour of these fishes. The specific epithet honours Henny Davies, wife of Peter Davies; the couple were exporters of aquarium fish from Malawi in the mid-twentieth century, and Burgess and Axelrod credited the husband-and-wife team with making many new Malawian aquarium fishes available to science and the hobby. Catalog of Fishes lists it as a valid species, Placidochromis hennydaviesae (Burgess & Axelrod 1973).
Morphology
This is a modestly sized cichlid, reaching about 4 in total length. It has the generalised, laterally compressed body of a typical Placidochromis, without the exaggerated head or jaw features seen in some of its lake relatives. Meristically it carries 14–15 dorsal-fin spines and 9–10 dorsal soft rays, with 3 anal spines and 8–9 anal soft rays. The original and subsequent descriptions emphasise a set of proportional diagnostics rather than bold colour marks: a relatively long lower jaw (about 41–47 percent of head length), a fairly stout premaxillary pedicel, a moderately slender body (depth roughly 33–36 percent of standard length), and outer oral-jaw teeth that are tricuspid — three-pointed — a feature consistent with a fish that picks small invertebrates rather than scraping algae or crushing snails. As with most deep-dwelling Malawi haps known mainly from preserved material, life colour is poorly documented and any sexual differences in pattern are unrecorded in the formal literature.
Habitat
Placidochromis hennydaviesae is endemic to Lake Malawi and occurs in the southern part of the lake, on the deeper offshore bottom rather than along the shoreline. FishBase gives a depth range of roughly 233–328 ft, placing it among the lake's genuinely deep-water cichlids, and the type specimen came from about 259 ft off Monkey Bay. Its preferred ground is described as a muddy floor covered with a thick layer of sedimentary diatoms — the soft, organic ooze that accumulates on the lake's deep plains where light is dim and the water is cool and stable. This is a very different world from the wave-washed rocks of the mbuna zone: a low-energy, low-light habitat where food arrives as sinking plankton and where infauna burrow in the sediment. The species' tolerance of these conditions, and the strong thermal and oxygen stratification of deep Malawi water, mark it as a specialist of the lake's benthic deep rather than a generalist of the littoral.
Feeding
Feeding observations summarised in the literature indicate that Placidochromis hennydaviesae takes planktonic invertebrates from the water just above the bottom as well as insect larvae found within the substrate. This dual strategy — gleaning drifting zooplankton and probing the diatom-rich sediment for benthic larvae — fits both its tricuspid outer teeth and its deep, soft-floored habitat. FishBase places it at an estimated trophic level of about 3.3, in line with a small carnivore-leaning omnivore rather than a strict piscivore or a herbivore. There is no evidence that it is a specialised predator; instead it appears to be one of the many Placidochromis that make a living by careful, low-key foraging over and within the sediments of the deeper lake floor.
Mating
No direct account of courtship or spawning behaviour has been published for Placidochromis hennydaviesae. Like effectively all Lake Malawi haplochromines, however, it is expected to be a maternal mouthbrooder in which males court and females incubate. In the broader 'hap' radiation, males typically establish temporary territories or display sites — often a cleared patch of sand or sediment — and court passing females with lateral displays and quivering, while females select among them. Because this is a deep, dispersed, soft-bottom species rather than a colonial reef-dweller, any such display arena would lie on the open sediment plains well below the photic zone. The specifics of timing, male coloration and display intensity remain undocumented, and what is stated here is inferred from the consistent reproductive biology of the Malawi flock rather than recorded for this species.
Breeding
There is no published description of egg number, brood size or fry development for Placidochromis hennydaviesae. By the near-universal pattern of the Lake Malawi haplochromine cichlids, it is a female mouthbrooder: the female collects the fertilised eggs into her mouth, incubates them through hatching, and shelters the developing young in her buccal cavity for several weeks before releasing free-swimming fry, which may continue to dart back to her mouth when threatened. Clutch sizes in comparable small Malawi haps are modest, typically a few dozen eggs, reflecting the trade-off between fecundity and the intensive parental investment that mouthbrooding demands. For this particular species the details are simply not on record, a consequence of its deep-water habits and the difficulty of observing such fishes in the wild.
In the aquarium
Although FishBase notes a commercial aquarium interest for the species, Placidochromis hennydaviesae is not a familiar or commonly traded fish in the hobby, and there is little specific keeping experience to draw on; the historical link to the Davies' export business reflects mid-century collecting more than any modern trade. The husbandry notes below are therefore extrapolated from the genus and the species' known ecology, and should be treated as provisional. A deep-water, sediment-floored hap of around 4 in would, in principle, suit a roomy aquarium with a soft sand substrate, gentle structure and clean, well-oxygenated, alkaline, well-buffered water matching Lake Malawi's chemistry (a high, stable pH). As a calm, non-mbuna sand-dweller it would be a relatively peaceful tankmate by Malawi standards, best kept with other open-water or sand-sifting haps rather than boisterous rock cichlids. A varied diet of small frozen and live invertebrates plus a quality cichlid pellet would match its natural plankton-and-larvae feeding. In honesty, this is a fish of scientific and faunistic interest more than an established aquarium subject.
Conservation
Placidochromis hennydaviesae was assessed by the IUCN Red List as Least Concern (assessment dated 20 June 2018). As a deep-water species ranging across the southern part of a very large lake, it is not thought to face an immediate, species-specific extinction risk, and it has no CITES listing. That said, the deep benthic community of Lake Malawi is exposed to lake-wide pressures that affect all its endemics: heavy and partly unregulated fishing — including trawling and demersal fisheries that work the deeper bottom — together with catchment deforestation, increased sediment and nutrient input, and the temperature and oxygen changes driven by a warming climate, which can compress the habitable depth band for cool-water deep species. The lake-level cichlid literature (for example Konings' surveys and the broader Malawi conservation assessments) repeatedly flags these stresses. For a poorly studied, deep-dwelling endemic such as this one, sound conservation rests less on a species action plan than on protecting the integrity of Lake Malawi's deep-water ecosystem as a whole.