Taxonomy & naming
Otopharynx auromarginatus was described in 1908 by the Belgian-British ichthyologist George Albert Boulenger, originally as Tilapia auromarginata, from material collected at Lake Nyasa (Lake Malaŵi) by E.L. Rhoades. The genus Otopharynx was later erected by Regan (1920) with this species as its type, making auromarginatus the nomenclatural anchor of the whole genus. Eccles and Trewavas, in their 1989 revision of the lake's haplochromine genera, formally fixed a lectotype (BMNH 1908.10.27.86) for the species. Eschmeyer's Catalog of Fishes lists it as valid as Otopharynx auromarginatus (Boulenger, 1908), the parentheses marking its move from Tilapia into Otopharynx. The epithet auromarginatus is Latin, from aurum, 'gold,' and marginatus, 'margined' or 'bordered,' referring to the golden edging of the fins — the same feature that gives the fish its English name, the golden-margined hap. The species has accumulated a small thicket of informal 'auromarginatus' field names over the years (for geographic variants and look-alikes), but the true Boulenger species is the lake-wide sand-grazer treated here.
Morphology
Otopharynx auromarginatus is among the larger members of its genus, reaching about 9.5 in total length — roughly twice the length of the small cave- and deep-water Otopharynx and clearly bigger than the sand-dwelling Otopharynx argyrosoma. It has the laterally spotted melanin pattern characteristic of the genus, and as the name records, the fins are edged with gold in life. The species is distinguished within the genus partly by its high gill-raker count: Oliver (2018) noted that Otopharynx auromarginatus has 14–18 rakers on the lower outer gill arch, far more than the 10–11 of small relatives such as Otopharynx aletes — a meristic difference consistent with its more microphagous, filter-leaning feeding. As in most Malaŵi haplochromines, breeding males are expected to be the more vividly coloured sex, though the older descriptive literature gives limited detail on dimorphism for this species specifically.
Habitat
Otopharynx auromarginatus is a sand-dwelling fish of Lake Malaŵi and the upper Shire River, the river that drains the lake at its southern end. It is described as common at around 33 ft depth, where non-breeding individuals gather into schools over the sandy parts of the biotope. Its range spans the lake's tropical waters from roughly 9°S to 17°S, the southern extent reflecting its presence in the upper Shire — making it one of the more broadly distributed Otopharynx, not confined to a single shore or depth band. This open-sand, schooling lifestyle places it firmly among the lake's sediment- and sand-associated haplochromines rather than the rock-dwelling mbuna or the cave-bound members of its own genus, and its broad distribution is part of why it is regarded as a secure species.
Feeding
Otopharynx auromarginatus is a gentle, low-trophic-level grazer. It feeds on phytoplankton and takes its food from the thin, somewhat muddy organic layer covering the sandy substrate, in effect grazing fine plant and detrital material rather than hunting larger prey. FishBase reflects this with an unusually low estimated trophic level of about 2.0 — placing it close to a herbivore/detritivore, well below the carnivorous small Otopharynx such as Otopharynx antron. The species' high gill-raker count fits this microphagous diet, the closely set rakers helping to retain fine particles. In ecological terms it is one of the more 'vegetarian,' bottom-grazing members of a genus whose other species range from small-invertebrate pickers to shell-crushers, illustrating the dietary spread within even a single Malaŵi cichlid genus.
Mating
Detailed courtship observations are limited for Otopharynx auromarginatus, but it follows the reproductive template of Lake Malaŵi's sand-dwelling haplochromines: a polygynous maternal mouthbrooder in which males court females rather than forming lasting pairs. Outside breeding, the fish school over sand; breeding males would be expected to separate from these schools, take on heightened coloration and establish display territories or sites on the sandy bottom, competing to attract females in the lek-like manner typical of the lake's open-sand haps. Spawning ends with the female taking the eggs into her mouth, the male's anal-fin egg-dummies mediating fertilisation in the usual haplochromine fashion. The specifics of any spawning structure or display arena are not documented for this species and are inferred from its many sand-dwelling relatives.
Breeding
No detailed breeding account — clutch size, egg size, incubation period or fry development — has been published specifically for Otopharynx auromarginatus. As with the rest of the Malaŵi haplochromine flock, it is a maternal mouthbrooder: the female incubates the fertilised eggs in her buccal cavity for roughly three weeks before releasing free-swimming fry, which may continue to retreat into her mouth when threatened. Being a larger species, it could be expected to produce somewhat larger clutches than the small Otopharynx, but actual figures are unrecorded. The reproductive details given here are extrapolated from the consistent mouthbrooding biology of the lake's haplochromines rather than measured for this species.
In the aquarium
Otopharynx auromarginatus is recognised in the aquarium and food-fish trades — FishBase lists it as of commercial interest in both — and the 'golden-margined hap' name and various 'auromarginatus' geographic forms circulate among Malaŵi cichlid enthusiasts. It is, however, more a specialist's fish than a mainstream import, and well-documented captive-breeding accounts for the true Boulenger species are limited.
For the keeper, its biology points to clear requirements. It is a large (to ~9.5 in) Malaŵi sand-hap, so it needs hard, alkaline Rift-lake water and a spacious tank with a deep sand bed and ample open swimming room rather than a rock-bound mbuna aquascape. Its natural diet of phytoplankton and fine substrate material argues for a vegetable- and aufwuchs-leaning prepared diet rather than a heavily meaty one; over-rich, protein-heavy feeding is a known risk for low-trophic Malaŵi grazers and should be avoided. As a schooling, comparatively peaceful sand fish it suits a large community of similarly sized Malaŵi haplochromines. These notes follow from its niche and the general care of comparable species more than from an extensive published aquarium record for this particular fish.
Conservation
Otopharynx auromarginatus was assessed by the IUCN Red List as Least Concern in 2018. The listing reflects its broad distribution across Lake Malaŵi and into the upper Shire River, its tolerance of widespread sandy habitat, and the absence of evidence for a serious range-wide decline. This sits within the familiar context of pressures on the lake's fishes: intensive artisanal and commercial fishing (the larger, schooling sand haps are exactly the kind taken by seine and trawl fisheries), sedimentation and nutrient input from a developing catchment, and the longer-term effects of warming and stronger stratification. A widespread, generalist sand-grazer is comparatively buffered against these threats relative to narrow-range endemics, which supports its Least Concern status, but as the genus's type species and a commercially exploited fish it remains a worthwhile subject for continued fisheries monitoring.