Taxonomy & naming
Haplochromis crassilabris was described by the Belgian-British ichthyologist George Albert Boulenger in 1906, in a paper titled 'Descriptions of new fishes discovered by Mr E. Degen in Lake Victoria' (Annals and Magazine of Natural History, ser. 7, vol. 17, p. 445). The type locality is Entebbe, on the Ugandan shore of Lake Victoria (Victoria Nyanza), and Boulenger's syntype series — eleven specimens, BMNH 1906.5.30.419-428 — is held in the Natural History Museum, London. Eschmeyer's Catalog of Fishes, FishBase and the CLOFFA check-list (van Oijen et al. 1991) all treat the name as valid as originally combined, Haplochromis crassilabris.
The species sits within the vast Lake Victoria haplochromine 'species flock' — several hundred closely related cichlids that radiated explosively from a single ancestor. Modern workers place it in the subgenus or genus-group Paralabidochromis, the thick-lipped lineage characterised by enlarged, fleshy lips and a robust lower jaw. Seehausen (1996), in his monograph on Lake Victoria rock cichlids, recognises a 'Haplochromis (Paralabidochromis) crassilabris group' that, unlike most of its relatives, lives away from rock and on macrophyte-rich sand bottoms. We retain the conservative heading Haplochromis here, as FishBase and the Catalog of Fishes do, while noting that the literature also writes the fish as Paralabidochromis crassilabris. One point of confusion is worth flagging: although a Ugandan fauna list has tagged the species 'endemic to Lake Edward', the original description, the type material and every taxonomic authority anchor crassilabris firmly in Lake Victoria — the Lake Edward attribution appears to be an error.
Morphology
Haplochromis crassilabris is a small cichlid, reaching about 6 in in total length (FishBase, after CLOFFA). It has the typical compressed, moderately deep-bodied haplochromine build, but its defining feature is announced in the name: crassus, 'thick', plus labrum, 'lip'. Boulenger singled out its 'very thick' lips — markedly thicker, he noted, than those of the related Haplochromis retrodens — and these fleshy, mobile lips are the hallmark of the whole Paralabidochromis group to which it belongs. The lower jaw and dental arcades are comparatively broad; the outer teeth are stout and the head is relatively short-snouted compared with the lake's slender-jawed rock-pickers. As in nearly all Lake Victoria haplochromines, breeding males would have carried the brightest nuptial colours and the egg-spot-bearing anal fin, while females and non-breeding fish were duller and more cryptically marked. Detailed colour notes for this particular species are scarce in the accessible literature, a reflection of how little of the flock was studied in life before its collapse.
Habitat
Haplochromis crassilabris is endemic to Lake Victoria, the largest tropical lake in the world, shared by Tanzania, Uganda and Kenya. Within the lake it is an inshore, shallow-water fish: Seehausen (1996) records the crassilabris group from macrophyte-rich sand-bottom habitats in the Mwanza Gulf at the lake's southern end, explicitly noting that — unlike most members of the Paralabidochromis lineage — it has not been found on rocky shores or reefs. FishBase classifies it as a freshwater, benthopelagic species of the tropical zone (roughly 1 degrees N to 3 degrees S). Lake Victoria is a large, shallow, warm equatorial lake; its open waters are well-buffered and alkaline, with surface temperatures generally around 23-28 degrees C and pH on the alkaline side of neutral (commonly cited in the roughly 7.2-9.0 range for the lake as a whole). Precise in-situ chemistry for this species' weed-bed microhabitat is not separately published, so the figures here describe the lake environment it occupies rather than measurements from a crassilabris catch.
Feeding
Lake Victoria's haplochromines were sorted by Witte and van Oijen (1990) into some fifteen trophic groups, and Haplochromis crassilabris falls among the insectivores. The thick, fleshy, mobile lips that give the species its name are the functional tool of a prober: thick-lipped haplochromines press their cushioned lips against the substrate, sand grains and plant surfaces to seal over and extract small invertebrates — chironomid (midge) larvae and pupae and other soft-bodied insects hidden in the weed-bed and sand. The broad jaw of the Paralabidochromis lineage hints at robust feeding; some of its close relatives became snail-crushers with molariform pharyngeal teeth, but crassilabris itself is treated as an insect-feeder rather than a dedicated molluscivore. Like the rest of the flock, it foraged in the productive inshore zone, one thread in an extraordinarily fine-grained partitioning of food resources among hundreds of coexisting species.
Mating
No courtship account specific to Haplochromis crassilabris has been published, but the species belongs to a flock whose breeding biology is uniform and well documented. Lake Victoria haplochromines are polygynous maternal mouthbrooders with no lasting pair bond. Ripe males take on bright nuptial colours and defend a small temporary territory or display site, where they court passing females with quivering displays and lateral presentation, flashing the yellow or orange egg-spots on the anal fin. A receptive female is led through the spawning sequence; once eggs are laid and fertilised the male's role ends. The intense, colour-mediated female choice that drives this system is also the engine that built the flock — and, as water clarity fell, the breakdown of that visual mate-choice is thought to have hastened the loss and hybridisation of many species.
Breeding
Breeding has not been documented in the wild for Haplochromis crassilabris specifically; FishBase records only the genus-typical mode, 'mouthbrooding by female'. The species is a maternal (ovophile) mouthbrooder, as are essentially all members of the Lake Victoria haplochromine flock. The female lays a modest clutch on the substrate, immediately takes the eggs into her mouth — often picking at the male's anal-fin egg-spots, which helps ensure fertilisation inside the buccal cavity — and then broods the developing eggs and larvae orally for around two to three weeks without feeding. Genus-typical clutches for small Lake Victoria haplochromines of this size are on the order of a few dozen eggs (roughly 20-60). After release the fry are independent, though the female may continue to shelter them in her mouth at the first sign of danger for a short period. No species-specific fecundity count is available in the accessible literature.
In the aquarium
Haplochromis crassilabris is not an aquarium fish in any meaningful sense. It is a Critically Endangered, possibly near-vanished Lake Victoria endemic with no established presence in the ornamental trade; it is not bred for hobbyists and is essentially never offered for sale. The few Lake Victoria haplochromines kept by specialists are mostly hardier, more colourful species maintained through dedicated conservation-minded breeding networks rather than the commercial trade. Were the fish ever available, sound husbandry would follow the general template for the flock: hard, alkaline water (pH about 7.5-8.5) at roughly 24-27 degrees C, a sand bottom with plant cover to suit its weed-bed origins, a spacious tank, and a harem of one male to several females to spread the male's persistent breeding aggression. Mixing visually similar Victorian haplochromines is discouraged because they hybridise readily in captivity. In practice, this is a species to read about and to conserve, not to keep.
Conservation
The IUCN Red List assesses Haplochromis crassilabris as Critically Endangered (criterion C2a(ii)), assessed on 31 March 2010 by Witte, de Zeeuw and Brooks, with a decreasing population trend; the listed range covers Tanzania and Uganda. The assessment attributes the species' plight to the catastrophe that overtook the Lake Victoria cichlid flock in the late twentieth century: predation by the introduced Nile perch (Lates niloticus), which removed an estimated several hundred haplochromine species from large parts of the lake; eutrophication and the loss of water transparency, which both degraded habitat and broke down the visual mate-choice that keeps species reproductively distinct, driving hybridisation; pollution from domestic and urban wastewater; and fishing pressure. Inshore, soft-bottom insectivores like crassilabris were among the trophic guilds hit earliest and hardest. Some Lake Victoria haplochromines have shown partial recovery in cooler, rockier refuges since Nile perch numbers fell, but for this particular species there is no recent confirmation of healthy populations, and it has been flagged as possibly extinct. It carries no CITES listing.