Taxonomy & naming
George Albert Boulenger of the British Museum described this fish in 1901 as Tilapia trematocephala, in the Annals and Magazine of Natural History (series 7, volume 7), as part of his work-up of the cichlids brought back from John E. S. Moore's Tanganyika expeditions. That provenance produced the species' most notorious feature before anyone ever discussed the fish itself: the holotype was recorded as coming from "the north end of Lake Tanganyika," yet the specimen is unmistakably a Lake Malawi haplochromine. The error — most likely a mislabelled or commingled jar in a large collection that also drew on the Usisi River and other sites — went uncorrected for decades; Max Poll formally fixed the type locality to Lake Malawi in 1987, and Eschmeyer's Catalog of Fishes now lists the type locality as "Lake Malawi [Lake Nyasa] … [not north end of Lake Tanganyika]." The holotype, a unique ripe male about 3 in in standard length, is held at the Natural History Museum, London (BMNH/NHMUK 1906.9.6.151). Generic placement has been a moving target. Eccles & Trewavas, in their 1989 reclassification of the Malawi haplochromines, transferred the species to Trematocranus; Konings and subsequent workers have since treated it within Aulonocara Regan, 1922 — the genus of Lake Malawi "peacock" cichlids, diagnosed by the conspicuously enlarged lateral-line canals and pores on the head — and Eschmeyer's Catalog accepts it as valid as Aulonocara trematocephalum. Because Aulonocara is neuter, the epithet takes the neuter ending trematocephalum; the feminine trematocephala (the original spelling, under Tilapia) and the masculine -us forms used while it sat in Trematocranus persist as orthographic variants in older literature and databases. Crucially, that placement is not settled. M. K. Oliver's malawi.si database notes that the holotype has only moderately enlarged infraorbital pores while the preorbital pores are enlarged as in typical Aulonocara, and that the apparent absence of anal-fin egg-spots — a near-universal feature of the sand-dwelling peacocks — "casts doubt on its placement among sand-dwelling Aulonocara," raising the possibility that it is a cave-dwelling form whose genus remains uncertain. Whether trematocephalum is even a distinct species, or a senior name for a fish already known under a different name, cannot be resolved from one old specimen.
Morphology
Almost everything that can be said about the appearance of this fish comes from the single holotype. It is a small cichlid: the type is a mature male of roughly 3 in standard length, and FishBase records a maximum of about 3.5 in total length — there is no series from which to estimate how large the species actually grows, only this one fish. Boulenger's original meristics give a dorsal fin of about XVII spines, an anal of III spines and 9 soft rays, around 40 scales in a longitudinal series and 28 in the lateral line, over a body whose depth is contained several times in the standard length — in other words, a comparatively slender, elongate haplochromine rather than a deep-bodied one. That elongate build and the dark coloration of the mature male led Konings to compare it to the so-called "Chitande Type" Aulonocara, a group of slim, dark, rock- and cave-associated peacocks, rather than to the stockier yellow sand-dwellers. The defining feature is the head, and it is what gives the fish its name: the cephalic lateral-line system opens through markedly widened sensory pores. Within that, the diagnostic detail recorded by Oliver is a mismatch — the preorbital pores are enlarged in the typical Aulonocara fashion, but the infraorbital pores are only moderately so, an intermediate condition that sits awkwardly between the peacocks and other Malawi genera. Equally telling is a negative character: the holotype appears to lack the egg-spots (ocelli) on the anal fin that almost all male Aulonocara carry. Because only one preserved male exists, nothing can be said about live colour, female appearance, or the extent of sexual dimorphism beyond inference from related fish — sexual dichromatism is the rule across the genus, with bright nuptial males and plain, silvery-brown females, and trematocephalum is presumed to follow that pattern, but this has never been observed.
Habitat
The species is endemic to Lake Malawi, the southernmost and second-deepest of the African Rift lakes, shared by Malawi, Mozambique and Tanzania. Beyond that, its distribution is effectively a blank: the IUCN assessment states plainly that it is known only from the holotype and that the precise collection site is unknown, the original "north end of Lake Tanganyika" label being a documented error. No diver or collector has reported it from a named reef, sandbank or bay, so the lake's familiar mosaic of rocky, intermediate and sandy biotopes cannot be mapped onto this fish with any confidence. The one circumstantial clue comes from malawi.si: the holotype is thought to have been taken alongside sand-dwelling cichlids, possibly in a beach seine, which has led to the cautious suggestion that it was associated with open sand — though the elongate, dark, possibly egg-spotless morphology hints just as plausibly at a rock- or cave-dwelling habit, and the two readings have never been reconciled. In-situ water chemistry was never measured for this specimen, but the lake itself is warm, hard and strongly alkaline throughout the upper water column: surface temperatures broadly in the 75–82 °F range, pH typically 7.7-8.6, and high carbonate hardness, the standard envelope for Malawi cichlids. Those figures describe the water this fish lives in; they are not measurements taken from it.
Feeding
No stomach contents, no field observations and no captive feeding records exist for Aulonocara trematocephalum — its diet is inferred entirely from its anatomy and from what its relatives do. FishBase summarises it as probably feeding on soft invertebrates and places it near trophic level 3.5, the value typical of the peacock radiation. The genus Aulonocara is built around a single elegant foraging trick: the greatly enlarged sensory pores on the head form an array of pits that detect the faint movements of small invertebrates buried in sand, allowing the fish to hover over open substrate, sense hidden prey, and lunge down to take a mouthful of sediment that it then sorts, swallowing the animals and expelling the grains. The very name trematocephalum — "hole-head" — advertises that this fish possessed the hardware for that lifestyle. Whether it actually used it in the same way is the open question: if the species really is a cave- or rock-associated form rather than a sand-sifter, as its body shape and the apparent lack of egg-spots might suggest, its prey capture could differ from the classic Aulonocara model. With one preserved fish and no behavioural data, the honest statement is that it was almost certainly an invertebrate-feeder equipped with the genus's sensory-pore system, and that the details remain unknown.
Mating
There are no observations of courtship, territoriality or social structure in this species — none have ever been possible, because the fish has never knowingly been seen alive. What can be offered is the well-established pattern for the genus and for Lake Malawi haplochromines generally, against which trematocephalum's one clue stands out. Aulonocara, like the lake's other haplochromines, is polygynous and lek-like: there is no pair bond, brightly coloured males hold and defend small territories or spawning sites, and females move among them, visiting to spawn. The holotype is a ripe male, which at least confirms that males reach sexual maturity at a small size in this species. But here the single specimen complicates the easy assumption. The apparent absence of anal-fin egg-spots is significant for mating behaviour, not just taxonomy: in the typical Malawi mouthbrooding sequence those ocelli are the male's lure, drawing the female to nip at his anal fin and thereby take up milt to fertilise the eggs already in her mouth. A male lacking them would have to court and fertilise by some other means, as a few Malawi lineages do. Whether that reflects a genuinely different reproductive strategy or simply an artefact of one faded, century-old specimen is unknown, and it is the most intriguing loose thread the fish leaves behind.
Breeding
Breeding in Aulonocara trematocephalum has never been documented — there are no eggs, larvae, brooding records or captive spawnings, and the species is essentially absent from the aquarium hobby despite FishBase's generic note of "aquarium: commercial" interest in the genus. The expected mode, by strong inference from every well-studied congener, is maternal (ovophilous) mouthbrooding: the female takes the eggs into her mouth immediately after laying, broods them there for roughly three to four weeks without feeding, and releases free-swimming fry that receive no further parental care, with no male involvement after spawning. Clutches in peacocks of this size class are modest, on the order of a few dozen eggs. IUCN gives the species a generation length of one to two years, consistent with a small, fast-maturing haplochromine. All of this is genus-typical expectation, not observation. The one feature that would most directly bear on breeding — the apparent lack of male anal-fin egg-spots — is precisely the character that cannot be checked without fresh material, and it leaves even the assumption of standard egg-spot-mediated mouthbrooding open to question for this particular fish.
In the aquarium
Aulonocara trematocephalum is, to be straightforward, essentially absent from the aquarium hobby. No confirmed captive specimens are on record, and it is not available in the trade under this name. The fish is known only from a single century-old museum specimen whose precise collection location is unknown, so there is no source population to export and no collector who has knowingly kept it. Any fish sold as "trematocephalum" should be treated with considerable scepticism; the name occasionally appears on livestock lists attached to unverified or misidentified peacocks.
For a keeper who genuinely had this fish, or who keeps the closely related sand-dwelling Aulonocara as a reasonable proxy, the following is sound general guidance based on the genus and the known biology of the species. The information is explicitly generalised from peacock-cichlid husbandry and should be read as such. A tank of at least 40–55 US gal (roughly 47 in in length) is the minimum sensible footprint for one male and a small group of females; with Lake Malawi peacocks, bigger is always better, because males defend spawning territories and will harass females in confined space. At roughly 3.5 in total length this is a small species, but the social requirements of the genus rather than the fish's body size should govern tank dimensions.
Water chemistry must replicate Lake Malawi conditions: temperature 75–82 °F, pH 7.7–8.6, and moderately hard to hard water with significant carbonate hardness (general hardness in the range of 10–20 °dH is typical for Malawi setups). Buffering is best maintained with crushed coral or aragonite substrate rather than with chemical dosing. Substrate should be fine or medium-grained sand — not gravel — because peacocks in general, and this species in particular given its elaborately developed cephalic pore system, are tactile sensory feeders that hover low over open sand and detect buried invertebrates through those pits. Hard substrates deny them their primary foraging behaviour. The aquascape should combine open sandy areas for foraging with a rockwork perimeter providing visual breaks, line-of-sight barriers, and hiding places for females pursued by dominant males; an entirely bare tank produces chronic stress.
Social structure should follow the standard peacock harem model: one male to three or more females at minimum. Keeping two males in the same tank without very substantial space and rock division typically results in the subordinate male being killed or chronically stressed. Hybridisation is a serious and documented problem across the entire peacock genus: different Aulonocara species readily interbreed in captivity, and Aulonocara males will also hybridise with some Utaka (Copadichromis) females. Because trematocephalum is a taxonomically uncertain fish whose wild identity needs to be preserved in any scientific context, mixing it with other peacocks or Malawi haplochromines would be especially irresponsible. Even in a purely hobby context, hybridisation degrades the genetic integrity of both species and produces offspring that misrepresent both parents.
Feeding in captivity should reflect the species' sensory-foraging biology. Peacocks accept high-quality sinking pellets and frozen invertebrate foods (Artemia, Mysis, chironomid larvae); the high-protein invertebrate foods match the wild diet and elicit natural foraging behaviour. Slow-sinking or live food scattered into the sand is particularly stimulating. The diet should be varied but lean — fatty or high-carotenoid foods pushed hard can promote colour in males but do nothing for health and can cause liver stress in repeated excess. Difficulty is assessed as moderate for the genus generally; this species specifically is effectively impossible to keep because it cannot be obtained, and any future captive culture would require careful sourcing, verification of identity, and a deliberate effort to keep the gene pool separate from related cichlids.
Conservation
The IUCN Red List assesses Aulonocara trematocephalum as Data Deficient (assessed 22 June 2018 by Ad Konings and Jacqueline Kazembe, reviewed by Jos Snoeks; errata version published 2019 to correct an assessor's name). The reasoning is straightforward and unusual: the species is known only from its holotype, the type locality is unknown, and it is not even certain whether it is a valid species or conspecific with a fish already described under another name. With no data on distribution, population, ecology or threats, no risk category can responsibly be assigned, and the assessment calls explicitly for further research on taxonomy, distribution and life history. There is no CITES listing, and the fish is not used in trade — there is nothing to trade. That near-total ignorance is itself the conservation story. Lake Malawi holds the largest species flock of any lake on Earth, the great majority of it found nowhere else, and the lake is under mounting pressure: warming surface waters and strengthened stratification are reducing the deep mixing that fertilises the food web, while sedimentation from catchment deforestation, nutrient loading and intense artisanal and commercial fishing degrade the shallow, near-shore habitats where most cichlid diversity concentrates. A fish like this one — possibly a narrow-range endemic, possibly tied to a specific sand or rock habitat, and so poorly known that it could decline or disappear without anyone noticing — embodies the basin's central conservation problem: the lake is losing, or could lose, species it has never properly catalogued. Until trematocephalum is rediscovered and its identity resolved, it stands as a reminder of how thin our knowledge of Malawi's endemic radiation still is.