Taxonomy & naming
Albert Günther described the species in 1894, in a report published in the Proceedings of the Zoological Society of London (volume for 1893, part 4, page 624). The original combination was Chromis williamsi; the material had been transmitted to the British Museum by Harry Hamilton Johnston from his British Central Africa survey, collected from Lake Nyasa — the name by which Lake Malawi was known to colonial-era naturalists. The holotype (BMNH 1893.1.17.6) is housed at the Natural History Museum in London. The type locality in the original description is simply 'Lake Nyasa, southeastern Africa'; later sources, including the Cichlid Room Companion profile curated by Patrick Tawil, and Konings' field records, associate the species most strongly with the southeastern arm of the lake and with islands including Chizumulu.
The species moved through two intermediate placements — Tilapia williamsi appeared in older checklists — before settling as Pseudotropheus williamsi. Eschmeyer's Catalog of Fishes (updated May 2026) confirms this as the current valid combination, supported by Maréchal (1991), Stauffer & Kellogg (2001, 2002), Konings (2016), Stauffer et al. (2016), and Li et al. (2016). A misspelling, Pseudotropheus williamsii (with double i), also circulates in older literature but is not a valid name.
The species is the type species of the genus Pseudotropheus — the 'Williamsi complex' (informally grouping williamsi, Pseudotropheus galanos, Pseudotropheus tursiops, Pseudotropheus sp. 'williamsi north', and Pseudotropheus sp. 'williamsi south') has been discussed as a potential candidate for formal generic revision, but as of the most recent major treatments no separate genus has been erected for this group. FLAG: CoF (May 2026) and FishBase both retain it in Pseudotropheus without qualification; anyone writing about this fish should note the informal complex but not imply a genus-level rearrangement is established fact.
A parallel name, Pseudotropheus sp. 'williamsi north', circulates in the hobby for the northern lake population (Makonde to Nkhata Bay), while the Cichlid Room Companion also lists Pseudotropheus sp. 'williamsi south'. Konings introduced this notation in Malawi Cichlids in their Natural Habitat (3rd edition, 2001) to capture the colour-morph diversity without asserting separate species status. The described species, Pseudotropheus williamsi (Günther 1894), anchors the southern and central localities including the islands, and is the taxon evaluated by IUCN.
Morphology
FishBase records a maximum of 6.5 in total length for unsexed adults; the Stauffer & Kellogg (2002) redescription gives the holotype a standard length of 3.5 in (roughly 4 in TL), and community keepers in the United States consistently report males topping out around 7–7.5 in TL under aquarium conditions — somewhat above Günther's type material but consistent with the broad range Konings documents across the lake. FishBase fin-count data: 17 dorsal spines, 8 dorsal soft rays, 3 anal spines, 8 anal soft rays.
Sexual dimorphism is conspicuous. Breeding males are a vivid medium blue on the body, deepening toward royal blue on the head and flanks, with the lips distinctively pale — a blue-white to near-white colouration that gives the fish its common trade name, 'Blue Lips.' The orange pigment in the dorsal fin intensifies in dominant males; at some localities (particularly the northern 'williamsi north' form at Makonde and Nkhata Bay) the dorsal can show a bold orange-red flush. Females and non-breeding fish are brownish-yellow to grey, with two rows of black spots running along the flanks — this two-spot-row pattern is the most reliable field mark distinguishing Pseudotropheus williamsi females from other mbuna females that share the same rock. Egg spots are present on the anal fin of males.
The head is relatively large and deep (head depth ~102% of head length in the holotype; Stauffer & Kellogg 2002), with large eyes and a somewhat upturned lower jaw that positions the mouth for feeding in the upper water column as well as from rock surfaces.
Habitat
Pseudotropheus williamsi is a Lake Malawi endemic. It has no populations outside the lake and is absent from the rivers that drain it. Within the lake, it occurs along much of the western and eastern shorelines and around the major islands: documented localities include Deep Bay, Vua (Tanzania), Nkhata Bay, Likoma Island, Chisumulu (Chizumulu) Island, Mbenji Island, Maleri Island, and the southeastern arm (FishBase; Cichlid Room Companion). The GBIF occurrence data and Konings' surveys extend this further — the species appears nearly lake-wide wherever suitable rocky habitat exists, though the different colour forms at each locality have led to informal recognition of northern and southern 'populations.'
The habitat is strikingly narrow in the vertical plane. Pseudotropheus williamsi lives almost entirely in the top two metres of the water column, at exposed rocky sites where wave action keeps the water oxygenated and turbulent. FishBase records it as benthopelagic, and the 'turbulent habitat' label that appears on cichlid-forum.com's profile matches Konings' field descriptions of it foraging at wave-washed boulder faces. This is one of the shallowest depth distributions among Lake Malawi cichlids — deeper than the splash zone but shallower than most mbuna congeners that operate from 10–49 ft.
In-situ water chemistry measured across the lake's rocky shores: pH 7.7–8.6 (FishBase gives 7.8–8.3 for this species; the cichlid-forum profile extends to 8.6 for northern populations), hardness 10–20 dH, temperature 75–82 °F (75–79 °F per FishBase; the community profile records 78–82°F / 79–82 °F for the northern form, consistent with shallower, more sun-warmed water at northern latitudes). NOTE: depth data (0–7 ft) is from FishBase and field descriptions by Konings (habitat-typical for the wave zone); no published bathymetric survey gives a calibrated depth range specific to this species — the figure is habitat-inferred and flagged accordingly.
Feeding
Pseudotropheus williamsi feeds differently from most mbuna. Where the majority of rock-dwelling Malawi cichlids are dedicated aufwuchs scrapers — working the biofilm of algae and microinvertebrates off rock surfaces — williamsi feeds preferentially from the water column. FishBase and the ecology note attributed to Maréchal (1991, CLOFFA vol. 4) describe it as taking plankton when available, along with insect larvae and other invertebrates that enter the open water; it has also been observed picking at the biocover on rocks, but this appears secondary. The trophic level estimate is 3.37 (±0.41 se; FishBase), placing it firmly as an omnivore with a lean toward small animal prey rather than pure algae-grazing.
The feeding-type classification in FishBase is 'plants/detritus + animals (variable),' and the feeding habit is recorded as 'variable' — which is unusual enough to note. In practice, the wave-washed shallow zone where Pseudotropheus williamsi lives is where terrestrial insects and midge swarms contact the lake surface, and where zooplankton concentrations rise with upwelling; a fish that can exploit both the surface drift and the rock biofilm has a dietary breadth that most mbuna lack. In the aquarium this translates to acceptance of a wide range of foods, though, like all Malawi cichlids, it should not be overfed on high-protein animal diets that displace the plant matter its gut tolerates best.
Mating
Pseudotropheus williamsi is a polygamous, maternal-mouthbrooding cichlid, and breeding males hold territories on the rocky substrate. The courtship system follows the standard Lake Malawi haplochromine model: a dominant male intensifies his colour, displays to females with lateral spreading of fins and rapid shimmy movements, and defends a spawning site — typically a flat rock or shallow depression cleared of debris — against rival males and other intruders.
The 'williamsi north' forum profile from cichlid-forum.com notes that the species is 'mildly aggressive' toward both conspecifics and other species, which, for an mbuna, is relatively restrained. Multiple keeper accounts confirm that males are more aggressive toward each other than toward other species, and that the worst intraspecific aggression arises when two males are competing for the same territory in a tank without adequate visual barriers. Females without a mouthful of eggs are unremarkable in colour — the brownish-grey with two rows of spots blends into the rock — and seem to move through the reef largely unharassed.
Breeding
Spawning follows the standard haplochromine mouthbrooding sequence: the female deposits eggs on the substrate, immediately picks them up in her buccal cavity, then mouths the male's egg-spot-bearing anal fin to trigger sperm release. The fertilized eggs are incubated in the female's mouth for approximately three weeks at typical lake temperatures (75–79 °F). The female does not feed during incubation and should not be disturbed or netted during this period.
Clutch size data specific to Pseudotropheus williamsi is not published in the primary literature examined for this article; for the williamsi complex and similarly-sized mbuna, FishBase and genus-level literature on Pseudotropheus indicate clutches in the range of 20–60 eggs per spawn, consistent with the moderate fecundity figure (FishBase resilience: 'high, minimum population doubling time less than 15 months') and with community reports of broods typically running 20–40 fry at aquarium temperatures. NOTE: this figure is genus-typical and inferred from the fecundity resilience classification; a species-specific published clutch count for Pseudotropheus williamsi was not found in the literature surveyed.
Fry are brooded until they can swim freely and are large enough to evade predation; females release them after roughly 21 days, and in the lake the fry disperse into the shallow rock interstices. In the aquarium, keepers strip the female at about two weeks and finish the fry in a separate container, or allow the female to hold to term in a well-established tank with plenty of cover.
In the aquarium
Pseudotropheus williamsi is rarely encountered in the mainstream hobby. The fish most commonly sold under the name 'williamsi' or 'Blue Lips' in North American and European shops is typically the undescribed northern form, Pseudotropheus sp. 'williamsi north' (Makonde / Nkhata Bay), rather than the described species; the described Pseudotropheus williamsi itself appears sporadically at specialist Malawi cichlid vendors. A keeper sourcing this fish should confirm provenance with the seller, because the two forms differ slightly in colouration and the northern form appears to be the more readily bred trade line.
A species of this size — males reaching 6.5–7.5 in total length — requires genuine space. A single-species group (one male and three to four females) calls for a tank of at least 59 in / 5 ft in length with a usable footprint of roughly 150 × 19.5 in or larger; 180 × 23.5 in (6 ft × 2 ft) is better and becomes near-essential in a mixed-mbuna community. Lake Malawi water chemistry is non-negotiable: pH 7.8–8.6, general hardness 10–20 dH, temperature 75–82 °F (75–82°F), and consistent, high-quality biological filtration. Crushed coral or argonite substrate will buffer pH reliably; the tap water in most areas of North America and northern Europe is too soft and too acidic without supplementation. Stability matters more than hitting an exact number — wild swings in pH are more dangerous than a value of 8.0 versus 8.4.
Decor should replicate the wave-washed boulder-face habitat: large rocks piled to form caves, crevices, and visual barriers throughout the tank. Flat rocks of several sizes give a dominant male spawning sites to defend and allow less dominant fish to break line-of-sight. Open sandy patches in front of the rockwork are appreciated — this fish spends time in the upper water column and benefits from having open swimming space rather than a tank crammed floor-to-ceiling with stone. Fine or medium-grain sand works as substrate; coarse gravel is unnecessary and makes the tank harder to clean.
Tankmate selection follows the usual mbuna logic, complicated by this species being on the moderate end of mbuna aggression. In a spacious, well-stocked community it can coexist with other mid-sized mbuna (Labidochromis, Iodotropheus, Cynotilapia, other Pseudotropheus of similar temperament), but a single male Pseudotropheus williamsi will harass any conspecific male relentlessly — a second male should not be added unless the tank is large enough to hold territories simultaneously, which is rarely the case below 94.5 in. Peaceful or timid species (most Copadichromis, Aulonocara, small Julidochromis) are unsuitable companions because mbuna aggression outpaces what they can handle. Avoid any fish that visually resembles the male — blue-bodied mbuna with similar fin markings will be attacked as rivals.
Diet in the aquarium should be predominantly vegetable-based: high-quality spirulina flake or pellet as the staple, supplemented with blanched vegetable matter, Cyclops, and Daphnia. This species has a broader natural diet than most mbuna (plankton and invertebrates figure prominently in the wild), so it will accept frozen bloodworm and brine shrimp readily — but these should be treats, not regulars. Malawi bloat, the most common and most deadly keeper mistake with any Lake Malawi cichlid, is strongly associated with diets too high in animal protein or fat. It presents as rapid abdominal swelling, often with the fish sitting on the bottom, and is often fatal without swift intervention. The safest rule is a plant-forward diet with animal protein making up no more than a small fraction of weekly meals. Overfeeding and poor water quality compound the risk. Breeding in captivity follows the maternal mouthbrooding pattern described in the Breeding section; the female holds for approximately 21 days and should be moved to a separate holding tank or left undisturbed in a well-covered species tank to carry her brood to term.
Conservation
The IUCN Red List assesses Pseudotropheus williamsi as Near Threatened (NT), assessed 19 June 2018, under criterion B1a, with the assessment authored by Ad Konings. The basis for the Near Threatened ranking is geographic: the species is known from multiple localities but its extent of occurrence and area of occupancy across Lake Malawi's rocky shoreline are bounded by the limits of rocky habitat, and localized threats — particularly aquarium collection pressure and shoreline habitat degradation — are enough to warrant a flag short of a threatened category.
The broader context for any Lake Malawi endemic is the lake's own threatened status. Chavula et al. (2023, Journal of Great Lakes Research 49(6): 102241, DOI: 10.1016/j.jglr.2023.102241) provide the most comprehensive recent basin review, identifying climate change, fishery overexploitation, sedimentation from deforested catchments, nutrient loading, and invasive species as converging pressures. Shallow-water warming has been documented in the upper strata of the lake, the very zone Pseudotropheus williamsi occupies; increased turbidity from agricultural runoff directly degrades the algae-covered rock surfaces that define its habitat. The trade in wild-caught Malawi cichlids is ongoing, and williamsi — particularly the northern 'blue lips' form at Makonde and Nkhata Bay — circulates regularly in the hobby. That trade is unlikely to be at the volume that would push the species toward a higher threat category on its own, but it adds to habitat-level pressure.
The species is not confined to any single protected area, though some of its island localities (Mbenji, Maleri) fall within Lake Malawi National Park's waters. Populations across the main lake shorelines have no formal protection. The IUCN's NT listing is reasonable given its current condition, but the species warrants monitoring as lake conditions deteriorate.