Taxonomy & naming
Haplochromis maxillaris was described by the British ichthyologist Ethelwynn Trewavas in 1928, in the Annals and Magazine of Natural History, from material taken in the Emin Pasha Gulf on the southern shore of Lake Victoria (type locality approximately 2°31.5′S, 31°43.5′E, in what was then Tanganyika Territory, now Tanzania). The original series held several specimens; Peter Humphry Greenwood, the foremost reviser of the Lake Victoria flock, selected a lectotype (BMNH 1928.5.24.486) and recognized eight paralectotypes (BMNH 1928.5.24.480–485) when he treated the species in 1959 during his monumental multi-part revision of the lake's Haplochromis. The Catalog of Fishes and FishBase both retain the name as valid in the form Haplochromis maxillaris Trewavas 1928. Greenwood allied it with the small-mouthed congener Haplochromis microdon, from which the more exposed upper-jaw bone of maxillaris helped distinguish it. The genus Haplochromis has long been a catch-all for the East-African lacustrine flocks, and many Victoria species have since been shuffled into segregate genera (Greenwood himself erected several); maxillaris has not been formally moved, so the traditional Haplochromis heading is kept here while noting that the genus as applied to the Victoria radiation is widely regarded as an artificial assemblage awaiting revision. The species sits within the explosively speciose haplochromine radiation of Lake Victoria, one of the fastest vertebrate adaptive radiations known.
Morphology
Haplochromis maxillaris is a moderately sized haplochromine reaching about 6.5 in in standard length, the larger end for the genus in Lake Victoria. The body follows the generalized deep-bodied, laterally compressed haplochromine plan, but the head and mouth are the defining region: the jaws are relatively large with a notably exposed maxillary, the trait Trewavas singled out in the name. This mouth architecture is consistent with its paedophagous habit — a wide, distensible gape used to engulf the broods of mouthbrooding cichlids. As is typical of the Victoria flock, the genus shows strong sexual dichromatism: breeding males develop bright nuptial coloration while females and non-breeding fish are a more cryptic silvery-brown, and males carry yellow or orange egg-spots (ocelli) on the anal fin. Detailed in-life colour notes for this particular species are thin in the public literature, reflecting how little of the Victoria flock was documented in colour before the Nile-perch collapse; descriptions rest largely on preserved type and revision material rather than aquarium observation.
Habitat
The species is endemic to Lake Victoria, the largest tropical lake in the world (the Lake Victoria Basin freshwater ecoregion), shared by Kenya, Tanzania and Uganda. It is a fish of the littoral and sublittoral zones, recorded to about 30 ft depth. According to the CLOFFA treatment most specimens were caught over sand or shingle bottoms, with comparatively few over mud — a substrate preference that places it in the inshore, well-lit margins rather than the deep offshore. Lake Victoria is a warm, well-buffered tropical lake; its surface waters are alkaline, typically in the order of pH 7.2 to 9 and temperatures around 73–81 °F, the broad envelope within which the entire haplochromine flock evolved. As with most members of the flock, maxillaris has a narrow native range confined to this single basin.
Feeding
Haplochromis maxillaris is a paedophage — a specialist predator on the eggs and larvae of other cichlids. In Lake Victoria most paedophagous haplochromines obtain their prey by raiding the buccal cavities of mouthbrooding females, either by ramming the brooding fish to make her spill the clutch or by seizing and engulfing the young directly; the large, exposed-maxillary jaws of maxillaris are the morphological signature of this guild. FishBase records that it feeds on cichlid embryos or larvae, and assigns it a high trophic level near 4.0, consistent with a predator of other fishes' offspring. Its diet is not rigidly fixed, however: the smallest specimen examined in Greenwood's work was exceptional in having a stomach filled with copepods and blue-green algae, suggesting that juveniles take plankton and microphytes before recruiting to the brood-robbing habit of the adults. Paedophagy evolved repeatedly and convergently in the Victoria flock, and maxillaris is one of several species exploiting the abundant broods of a lake once dominated by mouthbrooding cichlids.
Mating
Like the rest of the Lake Victoria haplochromine flock, Haplochromis maxillaris is a polygynous maternal mouthbrooder, and its mating system follows the pattern documented across these cichlids rather than being described in detail for the species itself. Males establish and defend small courtship territories and signal to females with their nuptial colours and with the egg-spots on the anal fin; a receptive female is led through a circling, quivering courtship over the male's site. Mate choice in this radiation is strongly visual, driven by male breeding colour, and the breakdown of that visual signalling under reduced water clarity is thought to have eroded the reproductive barriers between species. Outside of courtship the sexes are unremarkable shoaling fish; the male contributes only gametes and a defended spawning site, with all subsequent parental investment falling to the female.
Breeding
Reproduction is by maternal (female-only) mouthbrooding, the universal mode of the Lake Victoria flock. Spawning follows the classic haplochromine sequence: the female lays a small clutch of large, yolky eggs and takes them immediately into her mouth, then nips at the egg-spots on the male's anal fin, prompting him to release milt that she draws in to fertilize the clutch already in her buccal cavity. She broods the developing eggs and larvae in her mouth for roughly two to three weeks (the genus-typical interval), not feeding, then releases free-swimming fry that may continue to shelter in her mouth when threatened. Clutch sizes have not been published specifically for maxillaris; Victoria haplochromines of this size typically carry on the order of a few dozen eggs per spawning, a small fecundity offset by the high survival that prolonged oral brooding affords. No captive breeding account is available, so these details are extrapolated from the well-documented behaviour of congeners rather than direct observation of this species.
In the aquarium
Haplochromis maxillaris is essentially never seen in the aquarium hobby. It is an obscure, narrow-range Victoria endemic with no established trade and, by all indications, no captive population; it does not appear in standard hobby husbandry references. Were it ever maintained, the practical obstacle would be its biology: an obligate or near-obligate paedophage that in the wild feeds on the broods of other cichlids would be difficult to feed and to house, since it preys on the very fish it would share a tank with, and a single-species group would deny it both its natural prey and the brooding females it raids. General requirements would follow those for Lake Victoria haplochromines — hard, alkaline water around pH 7.5–8.5 and 75–81 °F, a spacious tank with open sand and shingle and visual breaks, and the harem-style sex ratios (one male to several females) used to manage haplochromine aggression — but this should be read as informed inference, not a tested protocol. For the overwhelming majority of keepers this is a museum-and-literature fish rather than a candidate for the home aquarium, and conservation-minded sourcing matters more than husbandry technique.
Conservation
The IUCN Red List assesses Haplochromis maxillaris as Vulnerable under criterion D2 (assessed 31 March 2010 by F. Witte, M.P. de Zeeuw and E. Brooks), reflecting a very restricted situation that makes the species susceptible to sudden decline; the population trend is listed as unknown. It is endemic to Lake Victoria and recorded from the Kenyan, Tanzanian and Ugandan waters of the lake. The assessment names the lake's well-known pressures: predation by the introduced Nile perch (Lates niloticus), whose explosive increase from the 1980s drove the haplochromine community to collapse and extirpated or pushed many species to extinction; hybridization arising from decreased water transparency, as eutrophication blurs the colour signals that keep species reproductively distinct; and ongoing fishing and harvesting alongside the broader suite of invasive-species impacts. Paedophages, which depend on a healthy community of brooding cichlids for their food, are doubly exposed — their prey base collapsed along with the rest of the flock. The species carries no CITES listing. As a Victoria endemic it shares the precarious outlook of the lake's surviving haplochromines, for which the partial recovery seen in some taxa offers cautious but not assured hope.