Taxonomy & naming
Lethrinops sp. "Macrostoma" has no formal scientific description, no describing author, and no entry in Eschmeyer’s Catalog of Fishes; the quoted name is an informal field/trade label. The genus was erected by Regan in 1922 and substantially revised by Eccles and Trewavas in 1989. The name itself is built from Greek macro- ("large") and stoma ("mouth"), describing the fish’s most obvious field character rather than referencing a formally described Lethrinops relative. Readers should note that the same epithet, macrostoma, is independently attached to a valid, unrelated species — Tyrannochromis macrostoma (Regan, 1922), a large, predatory haplochromine in a different genus entirely. The two share only the descriptive word, not a taxonomic relationship, and this form should never be cited using Tyrannochromis’s authority or description.
Morphology
This form is reported to have a conspicuously large mouth relative to its Lethrinops congeners, alongside the genus’s standard elongated, fusiform body. Genus-typical size for mature fish runs roughly 4–5.5 in. The specific breeding male colour pattern for this population has not been documented in the sources consulted here; this account notes the enlarged mouth as the identifying morphological trait on record rather than guessing at colour.
Within Lethrinops as a whole, species divide broadly by mouth angle and snout profile — some with a steep, near-vertical oral opening suited to picking at a narrow patch of sand, others with a shallower, more forward-pointing mouth suited to sweeping a wider area — a distinction Eccles and Trewavas relied on heavily in their 1989 revision. As in most Lake Malawi haplochromines, males typically grow somewhat larger than females of the same age, and the size gap often becomes more pronounced once a male holds a breeding territory.
Habitat
The only documented locality is the South East Arm of Lake Malawi, a broad, historically heavily trawled embayment with extensive sand and mixed sand-rock bottom in the southern lake. Sand-associated Lethrinops in this area are generally found over open sand or the sand-rock transition rather than on rock itself. Water conditions follow the lake norm: roughly 75–82 °F and pH 7.7–8.6, hard and alkaline.
Lethrinops as a genus is unusually finely partitioned by depth across Lake Malawi's open-sand and transitional habitats, with different species — and, as here, different named forms — occupying overlapping but distinguishable depth bands along the same stretch of shoreline, a pattern regarded as a major axis of the genus's diversification alongside colour and mouth shape. The lake itself is a large, deep rift lake with a stable thermocline; below roughly 131–164 ft the water turns anoxic, so no fish, Lethrinops included, live below that zone regardless of locality.
Feeding
Like other Lethrinops, this form is expected to be a sand-sifting invertivore, taking mouthfuls of sand and filtering them through comb-like gill rakers to strain out chironomid larvae, ostracods, copepods and other small invertebrates. A conspicuously large mouth in a sand-sifting cichlid often correlates with taking somewhat larger prey items — bigger invertebrate larvae, small snails or bivalves — than genus-mates with smaller gapes, though no dedicated diet study of this population confirms that inference; it is offered here as a reasonable expectation from morphology, not as a documented fact.
In the wild, sand-sifting Lethrinops rarely forage alone. Loose, mixed-species aggregations combining several sand-dwelling genera — Lethrinops, Aulonocara, Taeniolethrinops and various Copadichromis — are commonly reported moving together across open sand, each targeting a broadly overlapping but not identical set of invertebrate prey depending on mouth angle and gape. Feeding activity peaks in daylight, when the fish can see prey movement in the substrate, and largely stops after dark, when sand-dwelling cichlids settle into looser aggregations to avoid nocturnal predators such as catfish.
Mating
As in other Lethrinops, males are expected to be non-territorial outside the breeding season but to establish a small display site on open sand once in colour, with females moving between scattered males in the genus’s typical lek-like system rather than forming a pair bond. No behavioural observations specific to this population have been published.
Bower architecture varies considerably across Lethrinops as a genus, from a simple shallow scrape in the sand through low mounds to the elaborate crater-and-turret constructions built by a handful of species. Because Lake Malawi's water column stays warm and thermally stable year-round, haplochromine cichlids are not confined to a single fixed breeding season the way many temperate fish are; spawning activity can occur across much of the year, though local peaks tied to food availability or water clarity are documented for some species.
Breeding
Lethrinops sp. "Macrostoma" is, like every member of its genus, a maternal mouthbrooder. Spawning takes place on open sand at or near a male’s display site; the female carries the fertilised eggs in her mouth through development, typically three to four weeks in related Lethrinops, before releasing free-swimming fry with no further male involvement. Clutch size has not been documented for this population; related sand-dwelling species typically produce clutches of a few dozen eggs.
As in other Malawi mouthbrooders, the female carries the developing eggs and larvae in her buccal cavity, circulating water over the brood by pumping her mouth and gills rather than feeding normally — a metabolically costly period after which females typically show visibly reduced body condition. After release, fry from a single brood, and sometimes fry from several broods at once, often gather in shallow, sheltered nursery habitat where the mother may continue to shepherd them and briefly re-mouth them if startled, a pattern documented widely across Lake Malawi haplochromines though not specifically confirmed for this population.
In the aquarium
This form should be treated as a peaceful, sand-associated cichlid, best suited to a community of similarly non-aggressive Lake Malawi sand-dwellers rather than combative mbuna. Its larger mouth may allow it to take somewhat larger food items than typical Lethrinops, so a varied diet including small pellets and chunkier frozen foods alongside the usual fine invertebrate fare is reasonable, though this is inference rather than documented husbandry advice. A soft sand substrate at least 2 in deep over open floor space is essential; a tank of at least 47 in suits one male with several females.
Water should mirror Lake Malawi conditions: 75–82 °F, pH 7.7–8.6, moderately hard to hard.
Filtration should favour gentle, well-oxygenated flow across the sand bed rather than a strong direct current, since constant sand movement disrupts natural sifting behaviour; a canister or sump-fed spray bar generally works better here than a powerhead aimed at the substrate. This species should combine well with other peaceful Lake Malawi sand-dwellers — Aulonocara, other Lethrinops, Taeniolethrinops — but is likely to be outcompeted for food or bullied outright if housed with boisterous mbuna.
Conservation
No IUCN Red List assessment exists for this form, and none can until it receives a formal description. The South East Arm is among the most intensively trawled areas of Lake Malawi for the commercial chambo and utaka fisheries, and sand-associated haplochromines like Lethrinops are taken as bycatch there more heavily than rock-dwelling mbuna are. Whether this specific form has declined, remains stable, or has simply gone unrecorded in survey work is not established.
More broadly, Lake Malawi's inshore and demersal fisheries have been managed since the 1990s through a co-management system built around local Beach Village Committees working alongside Malawi's Department of Fisheries, an effort to curb the decline recorded in commercial catch data for many demersal haplochromine stocks since the 1980s. Sand-dwelling genera such as Lethrinops are affected by that decline as bycatch of gear targeting chambo (Oreochromis) and utaka (Copadichromis), even where, as here, no species-level data exists to quantify the effect on a single named population.