Taxonomy & naming
Protomelas marginatus was first described by Ethelwynn Trewavas in 1935, in the Annals and Magazine of Natural History, as Haplochromis marginatus — one of a large batch of Lake Malawi cichlids she named from material collected at the south end of the lake. It was transferred to the genus Protomelas by Eccles and Trewavas in their 1989 revision, which grouped together non-mbuna haps sharing a continuous dark midlateral stripe. Catalog of Fishes lists it as valid as Protomelas marginatus (Trewavas 1935), the parentheses signalling its move out of the original genus; the lectotype (BMNH 1935.6.14.745) was designated by Eccles and Trewavas. For a time two subspecies were recognised — Protomelas marginatus marginatus at the southern end of the lake and Protomelas marginatus vuae at the northern end — but the species is now generally treated without that subdivision. The specific epithet marginatus is Latin for 'bordered' or 'margined,' a reference to a marginal band of colour on the fins typical of the fish.
Morphology
Protomelas marginatus is a medium-sized hap reaching about 6.5 in total length. Its meristics are fairly variable: 15–18 dorsal spines with 10–11 soft rays, and 3 anal spines with 9–11 anal soft rays. The body is the typical fusiform, laterally compressed shape of a Protomelas, and the genus-defining feature — a continuous dark midlateral stripe along the flank — is present. The epithet marginatus points to a coloured margin on the fins, the most readily noticed cosmetic feature of the fish. As with most members of the lake's species flock, breeding males develop brighter nuptial coloration than females and non-breeding fish, though the original 1935 description and the standard reference works give only limited detail on live colour. Its relatively unspecialised mouth and jaws are consistent with its plant-grazing diet rather than with the crushing or predatory adaptations seen in some congeners.
Habitat
Protomelas marginatus is endemic to Lake Malawi and, having once been recognised in both a southern and a northern form, is distributed across much of the lake. It is a fish of shallow, vegetated areas — the inshore weed beds and plant-fringed shallows rather than the open rocky reefs. This ties it ecologically to the soft, well-lit margins of the lake where aquatic and emergent vegetation grows, the habitat that supplies the algae and plant material it feeds on. The type series came from the south end of the lake, but the species' presence at both ends indicates a broad distribution within suitable vegetated inshore habitat around Lake Malawi.
Feeding
Protomelas marginatus is a herbivore and detritivore — one of the more plant-dependent members of its genus. It feeds on algae and on the debris of vascular (higher) plants, grazing in the shallow weed beds it inhabits. Any small invertebrates it ingests are taken incidentally, swallowed along with the plant material rather than actively hunted. FishBase reflects this with an unusually low trophic level of about 2.0 — the value expected of a primary consumer feeding directly on plants and algae, and notably lower than the carnivorous and paedophagous members of the genus. This makes marginatus a useful counterpoint within Protomelas, illustrating how a single Malawi cichlid genus can span feeding modes from plant-grazing to snail-crushing to brood-robbing.
Mating
Specific courtship behaviour has not been documented in detail for Protomelas marginatus, but it follows the polygynous, maternal-mouthbrooding pattern that is near-universal among Lake Malawi haplochromines. Males in breeding colour establish and defend a display site or small territory and court passing females with lateral displays and quivering; a receptive female spawns and immediately takes the eggs into her mouth, with no lasting pair bond formed. The brighter nuptial coloration of males relative to females is the visible expression of this system, in which females choose among displaying males. Finer details of marginatus courtship — display structures, spawning sites, the choreography of the spawning run — are inferred from the wider flock rather than recorded specifically.
Breeding
Protomelas marginatus is a maternal mouthbrooder, in keeping with its genus and the broader Lake Malawi assemblage. The female incubates a clutch of large, yolk-rich eggs in her buccal cavity for roughly three weeks, fasting through much of that period, then releases free-swimming fry and continues to gather them back into her mouth for protection when threatened for a short while afterward. Detailed clutch sizes and incubation times specific to marginatus have not been widely published, but its reproductive mode is well established by analogy with its many close relatives. As a vegetated-shallows species it would spawn within or near the weed beds it inhabits.
In the aquarium
Protomelas marginatus is recorded by FishBase as being of commercial interest both as a food fish and in the aquarium trade, but it is not a prominent or widely kept hobby species in the way that some of its showier relatives (such as the 'spilo' group of Protomelas) are. Where it is kept, it suits the standard Lake Malawi hap setup: a large aquarium with sand, rockwork and open swimming space, hard alkaline water (high pH and hardness) at warm rift-lake temperatures, and robust filtration to cope with the high stocking and feeding levels typical of Malawi tanks.
Its herbivorous biology has a practical implication that distinguishes it from carnivorous tankmates: a plant-grazer like marginatus does best on a vegetable-rich diet — spirulina-based foods, algae and similar plant matter — and can suffer on an over-rich, protein-heavy regime. As a medium-sized, comparatively peaceful grazer for its size, it would be a reasonable, if not especially flashy, member of a mixed Malawi community of similarly sized non-mbuna.
Conservation
The IUCN Red List assessed Protomelas marginatus in 2018 and classified it as Least Concern, reflecting its broad distribution across Lake Malawi and the absence of evidence for any major decline. As a widespread, vegetation-associated generalist grazer rather than a narrow-range specialist, it is among the more secure members of its genus. That said, its Least Concern status sits within the larger story of Lake Malawi, whose extraordinary cichlid flock is under steadily growing pressure from intensive fishing, sedimentation from catchment deforestation, declining water levels and a warming climate. Inshore weed-bed species like marginatus are sensitive to the degradation of those vegetated shallows in particular, through siltation, nutrient loading and shoreline disturbance. For now the species is not of conservation concern in its own right, but its fortunes — like those of the whole flock — depend on the continued health of the lake.