Taxonomy & naming
Otopharynx decorus was originally described as Haplochromis decorus by Ethelwynn Trewavas in 1935, in her studies of the cichlid fishes of Lake Nyasa (now Lake Malaŵi); the parentheses in the current binomial mark that later transfer from Haplochromis to Otopharynx. Eschmeyer's Catalog of Fishes lists it as valid as Otopharynx decorus (Trewavas, 1935), within the family Cichlidae, subfamily Pseudocrenilabrinae. The genus Otopharynx Regan, 1920 is defined chiefly by a melanic colour pattern of three discrete dark blotches — a suprapectoral spot, a supra-anal spot and a caudal-peduncle spot. The generic assignment of decorus is, however, contested: Snoeks and Hanssens (2004), in their treatment of the Malaŵi haplochromines, considered it probably better placed in the related genus Mylochromis, and the Catalog of Fishes flags that view. The specific epithet decorus is Latin for 'comely' or 'elegant'. The type locality is Vua, a point on the far northwestern shore of the lake (the site of an old lake-steamer fuelling station, roughly 16 mi northwest of Chilumba); the species is now known from both the northern and southern ends of the lake.
Morphology
Otopharynx decorus is a medium-sized, moderately deep-bodied haplochromine reaching about 7 in total length, with females only a little smaller than males. It has the laterally compressed build and generalised haplochromine head and jaws of a sand-and-rock-dwelling 'hap', with a notably small mouth — among the lake's sand-dwelling cichlids, that small mouth combined with its overall size makes live decorus fairly recognisable in the field. In hand or from photographs, the more reliable marks are meristic: an unusually high 13 or 14 soft rays in the dorsal fin (most other haplochromines have fewer than 12) and a flank scale count higher than in any other known Otopharynx. As in the rest of the genus, the diagnostic colour markings are three dark melanic blotches — suprapectoral, supra-anal and on the caudal peduncle — that distinguish Otopharynx (and the closely allied Mylochromis) from genera with an unbroken midlateral stripe; in decorus these blotches are often indistinct and, in the subdued light around 66 ft depth, can be almost invisible, never as bold or black as in the sympatric Otopharynx (Ctenopharynx) nitidus, with which it is easily confused at a glance. Body colour otherwise follows the sand it lives over — a light beige. The species' uncertain generic home reflects how subtly these Lake Malaŵi haps grade into one another: the characters that would settle whether decorus belongs in Otopharynx or Mylochromis are fine details of dentition, squamation and head morphology rather than anything obvious to the eye. Breeding males, in the manner of the flock, develop brighter metallic coloration than females and non-territorial individuals, though genuinely territorial males have rarely if ever been documented in the field.
Habitat
The species is endemic to Lake Malaŵi, and its range probably spans the whole lake: specimens have been taken at Vua on the far north-western shore and in the southern part of the lake alike, an unusually broad north-to-south distribution for a Malaŵi hap, many of which are restricted to single shores or islands. FishBase characterises it as a freshwater, demersal fish, with young observed around an average depth of about 20 ft at Namalenje Island in the south; field surveys report adults foraging over open sand at around 66 ft depth in subdued light, so the species likely ranges deeper than the juvenile record alone suggests. It lives in small groups, usually of no more than about six fish, though solitary individuals are frequently seen too, foraging over the sandy and intermediate (sand-meeting-rock) zone characteristic of so many predatory and benthic-feeding haps — ground it shares with the small-mouthed sand-sifter Maravichromis sp. 'Double Spot' and with Otopharynx (Ctenopharynx) nitidus. Lake Malaŵi's inshore waters are warm, clear and strongly alkaline, with stable temperatures in the mid-20s Celsius and a high, well-buffered pH — the conditions under which this and the rest of the species flock evolved.
Feeding
Otopharynx decorus feeds like a classic sand-sifting insectivore. Its large eyes let it scan the substrate closely as it moves, and only once it has located something promising does it bite into the sand, rather than simply picking prey off the surface. The diet consists mainly of invertebrates hidden within the substrate, and the centre of its lower pharyngeal-jaw dentition is somewhat enlarged — a trait shared by cichlids that take a good share of hard-shelled prey such as small crustaceans. This visual-scan-then-bite foraging style closely parallels that of the sympatric, small-mouthed Maravichromis sp. 'Double Spot,' with which decorus regularly shares sandy ground. No formal stomach-content study has been published, but field observation gives a reasonably clear qualitative picture of how the species feeds.
Mating
Field surveys of decorus have not found clearly territorial, breeding-dressed males defending a spawning site. Occasionally a single male within an otherwise foraging group shows partial nuptial colour — recorded in December — but such fish behave as foragers, not as territory holders. Members of the genus Otopharynx typically spawn near rocks, so it is plausible that decorus moves onto rocky substrate to breed and that the open-sand groups usually observed are non-breeding aggregations, but this remains inference rather than a documented spawning account. If the species follows the reproductive pattern universal to Lake Malaŵi's haplochromines, mating would still be maternal mouthbrooding with polygynous, lek-style spawning: a male in nuptial colour would court a receptive female with lateral displays and quivering, she would deposit a small clutch and immediately take the eggs into her mouth, his anal-fin eggspots mediating fertilisation as she snaps at them. That mechanism is inferred from the flock as a whole; the courtship sequence and actual spawning site of decorus itself remain unobserved.
Breeding
Breeding has not been directly observed in Otopharynx decorus, and even its spawning substrate is uncertain — the open sand where groups are usually seen, or the rocky ground the genus favours, as field observers have speculated but not confirmed. As a Lake Malaŵi haplochromine it is presumed to be a maternal mouthbrooder: the female would incubate fertilised eggs in her buccal cavity for roughly three weeks, fasting throughout, then release free-swimming fry that she may continue to shelter in her mouth at the first sign of danger. Clutch sizes in haps of this size are typically modest — a few dozen eggs — reflecting the survival premium oral incubation places on quality of care over raw numbers. Clutch size, incubation period and fry behaviour remain entirely unrecorded for decorus itself; the account here is drawn from the genus and the wider Malaŵi flock.
In the aquarium
Otopharynx decorus is of minor interest to aquarists — FishBase notes a commercial role both as a food fish and in the ornamental trade — but it is not a hobby staple and there is little keeping or breeding experience specific to it. Any husbandry guidance is extrapolated from comparable mid-sized Malaŵi haps.
A fish of around 7 in wants a spacious tank with a sand substrate and rockwork that recreates its natural intermediate zone, giving both open sand and broken cover. Standard Lake Malaŵi conditions apply: hard, alkaline water (pH roughly 7.5–8.5), stable mid-20s-Celsius temperatures and strong filtration with reliably clean water. It is best kept with robust, similarly sized tankmates rather than small or timid fish, as a breeding male will hold and defend territory in the usual hap manner. A varied carnivore/invertivore diet of quality pellets and meaty frozen foods suits it, avoiding the rich mammalian-based feeds that disagree with rift-lake cichlids. In short, an attractive but uncommon hap for the dedicated Malaŵi keeper rather than a readily available community fish.
Conservation
Otopharynx decorus has been assessed by the IUCN Red List as Least Concern. That status rests on its comparatively wide distribution — recorded from both ends of the lake — and the absence of any specific identified threat, rather than on detailed population monitoring. The wider context for Lake Malaŵi is more sobering: the lake holds the most species-rich freshwater fish fauna on Earth, almost entirely endemic, and the 2018 IUCN assessment of its fishes found a significant share at elevated risk from intense fishing pressure, inshore habitat degradation and sedimentation. A broadly distributed, generalised hap like decorus is not currently of concern, but its long-term prospects are bound up with the overall health of the lake's inshore ecosystem.