Cichlid Atlas — the world's cichlid waters, in depth

Taxonomy & naming

Placidochromis trewavasae was described by Mark Hanssens in 2004, in his chapter "The deep-water Placidochromis species" within The Cichlid Diversity of Lake Malawi/Nyasa/Niassa, the identification monograph edited by Jos Snoeks and published by Cichlid Press. It sits in the family Cichlidae, subfamily Pseudocrenilabrinae, tribe Haplochromini — the haplochromine radiation that accounts for the overwhelming majority of Malawi's several hundred cichlid species. FishBase and the Catalogue of Life both carry the name as valid (FishBase species code 62377; GBIF taxon key 2372653), with no synonyms in current use.

The genus name Placidochromis pairs the Latin placidus, "calm" or "tranquil," with the Greek chromis, an old name for a perch-like fish — a nod to the unhurried bearing of these mostly mild-mannered haplochromines. The species epithet honors Dr. Ethelwynn Trewavas (1900–1993), the British Museum ichthyologist whose work on African cichlids spanned more than six decades and who is often called the founding figure of Malawi cichlid taxonomy. Within Hanssens' treatment the fish falls into his deep-water "group 3," a cluster of species defined in part by a moderate gill-raker count and a basic melanin pattern of vertical bars.

Appearance

This is a small cichlid: the largest measured specimen reached just 6.5 cm (2.6 in) standard length, so total length including the tail is only a little more. The body is fusiform — an ordinary, mildly streamlined cichlid silhouette with no exaggerated features. Fin counts run to 15 dorsal spines and 10 soft rays, with 3 anal spines and 8–9 anal soft rays, and there are 21–24 gill rakers on the lower limb of the first gill arch.

Much of what defines P. trewavasae is comparative anatomy rather than flashy color, because it was diagnosed against its close relatives in the deep-water Placidochromis group. Hanssens separated it from the similar P. longirostris by its shallower lachrymal (the bone below the eye), shorter snout, and proportionally larger mouth; the head makes up roughly 35–36% of standard length. Honesty compels a caveat here: there is no widely available photograph of a living, fully colored adult. The reference images are of preserved museum material — including specimens from Kadango — which fade to a uniform tan and lose exactly the breeding hues that would make a field description vivid. Its assignment to the vertical-barred "group 3" tells us the living fish likely shows faint vertical bars, but its full life coloration, and any difference between males and females, are essentially undocumented.

Range & habitat

Placidochromis trewavasae is a Lake Malawi endemic with one of the more sharply restricted known ranges in the genus. Every recorded specimen comes from the southeastern arm of the lake, collected along a transect between Mazinzi Bay and Kadango. Whether that pinpoint distribution reflects the fish's true range or simply the path of the survey trawls that found it is an open question — deep, soft-bottom habitat is rarely sampled, and the species is genuinely hard to observe.

The defining feature of its habitat is depth. Records span roughly 50–70 m (about 165–230 ft), placing it among the benthopelagic, low-light specialists of the lake rather than the brightly lit rocky shallows where mbuna and most aquarium favorites live. Lake Malawi is a deep, permanently stratified (meromictic) rift lake; limnological work divides its water column into a productive, oxygenated mixed layer above roughly 100 m and a vast, anoxic deep water below. At 50–70 m, P. trewavasae lives near the lower edge of that habitable upper layer, over sand and fine sediment in subdued green light — a quieter, dimmer world than the postcard reefs but still a fully functioning part of the lake's ecosystem.

Ecology & diet

No diet study has been published for P. trewavasae specifically, and the honest answer is that its feeding ecology is inferred rather than observed. FishBase places it at a trophic level of about 3.2 — squarely that of a small carnivore or omnivore — but flags this as an estimate derived from the size and relatives of the fish, not from gut-content analysis.

What can be said comes from the genus and its guild. Placidochromis are generally benthic foragers of the lake's "hap" radiation, and their deep-water members are small, bottom-oriented fishes built for picking food from sand and soft sediment. By analogy with better-studied relatives, a diet of small benthic invertebrates — insect larvae, micro-crustaceans, and other meiofauna sifted from the substrate — is the most reasonable expectation, with the gill-raker count and modest mouth fitting that kind of foraging rather than active piscivory. In the lake's food web it would function as a small mesopredator on the deep sand floor, itself potential prey for larger predatory cichlids and the lake's fish-eaters. Any reader wanting more than that should treat it as a gap in the science, not settled fact.

Behavior & breeding

Direct behavioral and reproductive observations of P. trewavasae are, again, lacking — a recurring theme for a fish known mainly from nets and jars. What we can state with confidence rests on its membership in the Lake Malawi haplochromine radiation, every well-studied member of which is a maternal mouthbrooder. In that breeding system a female collects her eggs into her mouth, often picking up sperm by nosing at the male's anal fin, then carries the developing eggs and larvae for several weeks before releasing free-swimming fry. There is no parental defense of a fixed nest in the substrate-spawner sense; the protection travels with the mother.

FishBase lists a short generation length of one to two years and a "high" resilience rating, consistent with the fast life history typical of small lake cichlids. Social structure, courtship behavior, whether males hold territories or display on the open sand, and any breeding coloration all remain unrecorded for this species. Anything more specific would be extrapolation from its relatives, and it is more useful to flag the blank than to fill it with confident guesswork.

In the aquarium

For practical purposes, Placidochromis trewavasae is not an aquarium fish. The IUCN assessment states plainly that the species is not targeted by the ornamental trade, and there is no body of hobbyist keeping experience — no forum breeding logs, no care threads, no imports under this name. This is the single most important thing an aquarist should know about it.

A point of frequent confusion is worth heading off: the fish sold across the hobby as the "deep water hap" is Placidochromis electra, a larger, blue, widely bred and genuinely popular Malawi cichlid — a different species entirely. So is "Star Sapphire," P. sp. "phenochilus." Searches for trewavasae quickly drift to those trade fish, and the resulting care sheets do not apply to it. If a deep-water Placidochromis ever did reach a tank, its biology argues for a large, dimly lit aquarium with a deep sand bed, gentle lighting, and the hard, alkaline water (roughly pH 7.7–8.6, high mineral content) and stable warmth of Lake Malawi — and given a maximum length near 6.5 cm (2.6 in), it would be a small, mild fish easily bullied by the boisterous mbuna that dominate the hobby. But that is a hypothetical: the realistic care advice is that this is a wild species to read about, not a fish to shop for.

Conservation

The IUCN Red List assessed Placidochromis trewavasae as Least Concern in 2018 (assessment by Konings and the FishBase/RMCA team, reviewed by Snoeks). The reasoning is straightforward: it is endemic to Lake Malawi but faces no identified major threat, is not fished commercially, and is not collected for the trade. Its population size and trend are simply unknown, and the assessment says so — "Least Concern" here means "no evidence of a problem," not "confirmed secure." The flip side of its obscurity is that its very narrow recorded range, in one arm of one lake at one depth band, leaves little margin if conditions there were to change.

That caution comes into focus when the species is placed against the wider state of its lake, rather than read off the IUCN line alone. The basin review by Chavula and colleagues (Journal of Great Lakes Research, 2023) describes a lake under real and growing strain: heavy and increasingly unsustainable fishing pressure, marked by the long decline of the prized chambo tilapia; rising sediment and nutrient loading washed in from catchments stripped by deforestation, burning, and erosion; warming of the order of about 0.7 °C in the shallow water, which strengthens the lake's permanent stratification, slows the mixing that lifts nutrients into the productive layer, and thereby threatens to cut the lake's overall productivity; and the looming risk of invasive species. For a deep-water, soft-bottom fish like P. trewavasae living near the lower edge of the oxygenated zone, the most relevant of these is the warming-and-stratification story — anything that shrinks or starves the productive layer reaches down to the deep sand floor it depends on — followed by the steady rain of sediment off the land. None of that targets this fish directly, and it should not be overstated: P. trewavasae remains Least Concern. But it is a poorly known endemic in a lake whose health is no longer something to take for granted.

Placidochromis trewavasae

About this species