Taxonomy & naming
The species was described in 1973 by the American aquarists and authors Warren E. Burgess and Herbert R. Axelrod, in their paper 'New cichlids from Lake Malawi' in Tropical Fish Hobbyist, where it was named Haplochromis stonemani — the original combination, not Aulonocara. (A note on the date: the fish is occasionally cited as 'Burgess, 1978', but Eschmeyer's Catalog of Fishes and FishBase both anchor the name to the 1973 Burgess & Axelrod description, and that is the authority used here.) The type locality is the open water off Monkey Bay at the southern end of Lake Malawi, where the holotype — a unique specimen, USNM 210697, held at the Smithsonian's National Museum of Natural History — was taken at a depth recorded as 43 fathoms, roughly 78 metres. Its generic home has been genuinely contested ever since. Eccles & Trewavas, in their 1989 reclassification of the Malawi haplochromines, moved it to Placidochromis, and Maréchal followed suit in the 1991 CLOFFA checklist, so for years it was widely listed as Placidochromis stonemani — the name the IUCN still carries as a synonym. Snoeks & Hanssens (2004) and then Ad Konings, in the fifth edition of his Malawi Cichlids in their Natural Habitat (2016), reassigned it to Aulonocara, which the Catalog of Fishes now treats as its valid placement. The disagreement is not trivial: it turns on whether the fish's enlarged cephalic sensory canals — the tube-and-pore system on the head that defines the peacock genus — outweigh a body plan and deep-mud habitat that look more like the broader haplochromine 'utaka' assemblage. Even committed to Aulonocara, Aulonocara stonemani sits at the genus's odd edge, closest in habit to the large sand-and-mud dweller Aulonocara rostratum but far smaller, and a reader should treat its peacock affinity as the current consensus rather than a settled fact.
Morphology
This is a very small cichlid. FishBase gives a maximum of 2 in standard length; the IUCN account states a maximum of 2.5 in total length, and the malawi.si database cites a typical adult around 3 in total length — minor differences that all place it among the smallest species in the genus. The body is slender and somewhat flattened along the lower profile, with a notably ventral (downturned) mouth, a build suited to working a soft bottom rather than picking from rock. Most of what is documented comes from preserved material, so colour in life is poorly recorded; the fullest description of nuptial dress, compiled on malawi.si, is of breeding males showing five or six dark vertical bars beneath the dorsal fin, a dark stripe through the eye, a yellow-spotted dorsal fin edged with white lappets tipped in black, black pelvic fins, a yellowish caudal fin, and a broad black margin on the anal fin carrying the pale egg-spots typical of mouthbrooding haplochromines. As in other Aulonocara, the diagnostic feature is on the head — the widened lateral-line canals and conspicuous sensory pores that give the genus its name — and it is precisely this feature that has kept the species in the peacock genus despite its unusual habitus. Sexual dichromatism almost certainly follows the genus pattern of bright males and plain females, but it has not been documented from living, wild-observed fish.
Habitat
Aulonocara stonemani is a Lake Malawi endemic, and within the lake it is a deep-water specialist — an unusual niche for a genus most associated with the sunlit sand and intermediate zones of the upper few tens of metres. The IUCN assessment records it from the southeastern and southwestern arms of the lake, trawled from depths of 55 to 80 metres over a bottom of mud or sand carrying a thick layer of fine sediment; malawi.si reports a specimen from off Monkey Bay taken at roughly 90 metres, and the original type was caught at about 78 metres. These are depths well below the seasonal thermocline, in the cool, dim, low-oxygen-tending water that fishery trawlers, not divers, reach. No species-specific in-situ water chemistry has been published for it, but it shares the general Lake Malawi medium — warm, hard and strongly alkaline (pH roughly 7.7 to 8.6) at the surface, grading to distinctly cooler water at depth: where surface temperatures reach about 75–82 °F, the metalimnion and hypolimnion through which this fish is trawled sit at roughly 72–73 °F (Vollmer et al. 2005), so it lives several degrees colder than the shallow-water peacocks. Because it lives below diving range and is known mainly from net hauls, its distribution is almost certainly under-recorded: the two arms in which it has turned up reflect where demersal trawl surveys operate as much as the fish's true range. This is a species defined by the deep, soft-sediment habitat of the lake's southern basins rather than by any single reef.
Feeding
The species is a benthic micro-invertivore of the deep sediment. FishBase, drawing on Fryer & Iles' classic 1972 study of the Great Lakes cichlids, records its diet as benthic entomostracans — small crustaceans, particularly ostracods (seed shrimps) and chydorid cladocerans (a family of tiny water fleas) — and the IUCN account similarly describes it feeding on small invertebrates taken from the sediment. Its estimated trophic level is about 3.3, squarely that of a low-level invertebrate predator rather than a piscivore or grazer. The ventral mouth and slender body fit a fish that works methodically over soft mud, and the enlarged cephalic sensory pores it carries as an Aulonocara are exactly the apparatus its shallow-water relatives use to detect invertebrates moving within the substrate — a plausible adaptation for hunting prey it can barely see in near-dark deep water, though the feeding mechanics of this particular species have not been observed directly. In the deep community of the southern arms it occupies the small-invertebrate-feeder role alongside the lake's deep-water haplochromine and Diplotaxodon assemblages.
Mating
Nothing has been published on the courtship or social behaviour of Aulonocara stonemani in the wild — a direct consequence of its living far below the depths where its breeding habits could be watched. What can be said rests on inference from the genus and on the preserved nuptial males described on malawi.si. Like all Aulonocara and indeed all Lake Malawi haplochromines, it is expected to be polygynous with no pair bond, the males developing the brighter colour and the egg-spot-bearing anal fin used to draw females during spawning, and females remaining plainer. The dark vertical bars, yellow dorsal markings and pale anal-fin egg-spots recorded on breeding-coloured museum males are consistent with a conventional haplochromine lek-style courtship in which a coloured male displays and a ripe female is drawn in to spawn. Whether males hold territories on the deep mud, and how a fish this small finds and signals to mates in dark water, are open questions. Any statement firmer than 'presumed to follow the genus pattern' would be speculation.
Breeding
Breeding has not been documented in this species, in the wild or in captivity, and the honest position is that it is inferred entirely from the genus. Aulonocara — and the Lake Malawi haplochromine radiation as a whole — are maternal (ovophilous) mouthbrooders: the female lays a small clutch, immediately takes the eggs into her mouth, and is induced to pick at the egg-spot markings on the male's anal fin, taking up milt that fertilizes the clutch held in her buccal cavity. She then broods the developing eggs and larvae in her mouth, fasting for several weeks, and releases free-swimming fry with no further parental care and no male involvement. The pale egg-spots visible on the anal fin of breeding-coloured Aulonocara stonemani males are physical evidence that this same egg-dummy mechanism operates here. Clutch size, brooding duration and fry development for this species specifically are unrecorded; for a fish of barely 2.5 in a clutch of only a few to a couple of dozen eggs would be expected, smaller than the broods of larger peacocks. The IUCN assessment gives a generation length of one to two years, implying early maturity and a short life cycle — but the particulars of its reproduction remain a blank that no source can responsibly fill.
In the aquarium
Aulonocara stonemani is, to be direct, essentially unknown in the aquarium hobby. It is rarely if ever collected for the ornamental trade — the IUCN notes this explicitly — and no captive husbandry records exist in the standard Malawi cichlid literature. Any keeper who claims to hold the species should be met with healthy scepticism: the fish is small, drab in non-nuptial dress, lives below recreational diving depth, and offers little commercial incentive to collect or export. What follows is therefore peacock-genus husbandry extrapolated from closely related Aulonocara, stated plainly as generalisation, with the honest caveat that it has not been tested on this species.
If a verified specimen were to enter captivity, a tank of 30 US gal (roughly 31.5 in in footprint) would be a minimum for a small group, and 55 US gal or more would be preferable. The substrate should be fine, light-coloured sand — soft-bottom Malawi species like this one probe sediment rather than graze rock, and the enlarged sensory pores that define the genus are adapted for detecting prey movement through a loose substrate. Rockwork can be provided at the perimeter for visual breaks, but the open sand floor should dominate. Filtration must maintain the hard, alkaline chemistry of Lake Malawi: pH 7.8–8.6, general hardness 10–20 °dH, carbonate hardness 6–10 °dKH. Temperature is more nuanced than for shallow-water peacocks — this fish is naturally trawled from water in the range of 72–75 °F, several degrees cooler than the 79–82 °F often recommended for Aulonocara generally, so the cooler end of that range (75–77 °F) is more defensible.
The social structure for any Aulonocara in captivity is one male with three or more females: males are hard on females, particularly outside of spawning condition, and a harem of sufficient size both distributes aggression and maintains the polygynous dynamic that mirrors wild behaviour. A single male–female pair will result in a chronically harassed or dead female. Breeding, if it occurs, follows maternal mouthbrooding: the female lays a small clutch (likely only a handful of eggs given this fish's tiny adult size), picks up fertilised eggs after passing over the egg-spot markings on the male's anal fin, and broods them buccally for three to four weeks before releasing free-swimming fry. The male takes no part in brooding or rearing. For a fish this small, fry will be tiny at release and will require appropriately fine food — crushed flake or newly hatched brine shrimp — from day one.
The most important — and most commonly violated — rule for keeping any Aulonocara, including a prospective Aulonocara stonemani, is to avoid housing it with other peacock species unless hybridisation is acceptable. Aulonocara species interbreed readily in captivity, and the peacock community is already awash with undocumented hybrids sold under invented colour names. Because Aulonocara stonemani has no established trade presence and is seldom identified with confidence, the risk of an already-blurred identity being further compromised by a mismatched tankmate is real. If the goal is anything like species integrity, it should be kept either alone (as the only Aulonocara in the tank) or in a species-only setup. Peaceful, non-competitive Malawi species of similar size — small Copadichromis, smaller Placidochromis, or similarly sized non-cichlid species — may be compatible, but aggressive mbuna should be excluded entirely; they will shred the fins and disrupt the feeding behaviour of a small, open-water sand sifter.
Difficulty should be assessed honestly as moderate for experienced Malawi keepers and challenging for beginners, less because the fish has unusual physiological demands than because it is essentially uncharted: no keeper community has accumulated the trial-and-error knowledge that makes a fish like Aulonocara jacobfreibergi or Aulonocara stuartgranti straightforward to maintain and breed. Suitable food in captivity would be small live and frozen invertebrates — bloodworm, Daphnia, brine shrimp, small mysis — given on or close to the substrate to match the fish's natural feeding posture; it is not a surface or mid-water feeder by inclination. Dry food can supplement but should not dominate. Given that the species has no established captive population, anyone who does acquire confirmed specimens has an unusual opportunity — and an unusual responsibility — to document its husbandry from scratch.
Conservation
The IUCN Red List assesses Aulonocara stonemani as Least Concern, last assessed on 22 June 2018 by Ad Konings and Jacqueline Kazembe (errata version published 2019 to correct an assessor's name; reviewed by Jos Snoeks), an unchanged carry-over from its 2006 Least Concern listing. The rationale is straightforward: it is endemic to Lake Malawi but widespread across the lake's deep southern basins with no major threats identified, and its population trend is recorded as unknown for want of data. Tellingly for a peacock, it is described as rare and is not targeted by the ornamental aquarium trade — its deep, drab, net-only existence keeps it off the collectors' lists that have devastated showier shallow-water peacocks such as Aulonocara baenschi. The one threat the assessment flags is incidental: as a bottom-dwelling fish of trawlable depths, it can be taken as bycatch in the commercial demersal trawl fishery that works the southeast and southwest arms, the same fishery whose seasonal and spatial catch patterns have been studied directly in the southwest arm. That risk has to be read against Lake Malawi's wider strain — a basin under growing pressure from over-fishing, catchment deforestation and sedimentation, nutrient loading, and a warming, more strongly stratified water column that threatens the deep-water oxygen and productivity on which the lake's demersal community depends. None of these is documented as a present danger to this particular fish, but a small endemic confined to deep, soft-bottom habitat in heavily trawled arms sits squarely in the path of both the nets and the slower changes overtaking the lake.