Taxonomy & naming
Protomelas macrodon was described by Digby H. Eccles in the landmark 1989 monograph Malawian Cichlid Fishes: the Classification of some Haplochromine Genera, co-authored with Ethelwynn Trewavas (the species being attributed to Eccles within that joint work). That book established the genus Protomelas itself, carving a set of non-mbuna 'haps' out of the old catch-all genus Haplochromis on the basis of a shared melanin pattern — a continuous dark midlateral stripe. Catalog of Fishes lists the species as valid as Protomelas macrodon Eccles 1989, with the holotype housed at the Natural History Museum, London (BMNH 1906.9.6.145) and a single paratype at the Malawi Fisheries Research Unit. The specific epithet macrodon is a compound of Greek makros, 'large,' and odous/odontos, 'tooth' — 'big tooth' — referring to the conspicuously enlarged molariform teeth on the lower pharyngeal bone, a crushing dentition that sets it apart from its closest look-alike, Protomelas pleurotaenia.
Morphology
Protomelas macrodon is a small cichlid, the type material reaching only about 3.5 in standard length, with a small mouth and a generally fusiform, laterally compressed body. Its fin counts are 16 dorsal spines and 10–11 soft rays, with 3 anal spines and 9–10 anal soft rays. In overall body form and coloration it closely resembles Protomelas pleurotaenia, and the two are easily confused; macrodon is distinguished by its broader head and body, larger eye and longer premaxillary pedicel, and above all by its dentition. The diagnostic feature — and the source of the name — is a set of greatly enlarged, molariform teeth on the pharyngeal jaws, the specialised bony mill at the back of the throat that cichlids use to process food. Such a robust pharyngeal apparatus is the signature of a durophagous (hard-prey-eating) cichlid, built to crush snail shells and other hard-shelled invertebrates. Because the species is known from so few specimens, little can be said about live coloration, growth or sexual dimorphism beyond what the small type series shows.
Habitat
Protomelas macrodon is endemic to Lake Malawi and is one of the rarest-recorded fishes in the lake — FishBase notes that only two specimens have ever been documented. One of those came from an extensive shallow, sandy area with beds of the aquatic plant Vallisneria, which gives the best clue to its preferred habitat: the vegetated sand and weed flats of the lake's inshore zone rather than the rocky reefs favoured by the mbuna. The original description does not pin down a precise type locality beyond Lake Malawi, and the species' true range and depth distribution are simply unknown. What can be said is that, like its relatives, it is a fish of the lake's quieter sand-and-plant biotopes, where snails and other bottom invertebrates are abundant.
Feeding
Everything about Protomelas macrodon's anatomy points to a diet of hard-shelled invertebrates, especially snails. Its small mouth combined with massively enlarged molariform pharyngeal teeth is the classic toolkit of a mollusc-crusher: the fish picks up snails and uses the powerful pharyngeal mill at the back of its throat to crack the shells and extract the soft bodies. This is borne out by the limited direct evidence — the stomach of one of the two known specimens contained a gastropod (snail) and a ceratopogonid (biting-midge) larva. FishBase estimates a trophic level of about 3.2, in line with an invertebrate-eating carnivore. Snail-crushing is a well-known specialisation among Lake Malawi cichlids, and macrodon appears to be one of its smaller, less conspicuous practitioners.
Mating
No mating behaviour has been observed in Protomelas macrodon. As a member of the Lake Malawi non-mbuna assemblage, it is expected to follow the standard polygynous, maternal-mouthbrooding pattern of the flock: males in breeding dress court females at a display site, females spawn a small clutch and immediately take the eggs into the mouth, and there is no lasting pair bond. With only two specimens on record and no field observations, the courtship colours, display behaviour and territoriality of this species are entirely undocumented, and the description here is inferred from close relatives rather than recorded for macrodon.
Breeding
There is no published account of breeding in Protomelas macrodon. By the consistent biology of Lake Malawi haplochromines, it is almost certainly a maternal mouthbrooder, with the female incubating a modest clutch of large eggs in her buccal cavity for around three weeks before releasing free-swimming fry and continuing to shelter them briefly when danger threatens. Clutch size, incubation period and fry development are all unrecorded. Given how rarely the species has even been collected, no captive spawnings are known either, and its reproductive biology remains one of the many blanks in its profile.
In the aquarium
Protomelas macrodon is, for all practical purposes, not an aquarium fish. It is one of the least-collected cichlids in Lake Malawi, known to science from essentially two specimens, and it has no established place in the ornamental trade and no captive-breeding history. FishBase's note of 'aquarium: commercial' reflects the genus's general trade interest rather than any real availability of this species.
If it were ever kept, the husbandry would follow the Malawi-hap template: a spacious aquarium with sand and rockwork, hard alkaline water (high pH and hardness) at warm rift-lake temperatures, and a diet built around the hard-shelled invertebrates its pharyngeal mill is designed to process — snails and firm frozen foods rather than soft flake alone. But this is extrapolation. In reality macrodon is a fish to appreciate for its specialised snail-crushing anatomy and its obscurity, not one a hobbyist can expect to source or keep.
Conservation
The IUCN Red List assessed Protomelas macrodon in 2018 and classified it as Data Deficient — an honest acknowledgement that there is simply not enough information to judge its status. The species is known from so few records that nothing reliable can be said about its population size, range or trends; it may be genuinely rare, or merely overlooked among the lake's hundreds of similar haps. Its conservation prospects are best read at the scale of Lake Malawi as a whole, where the cichlid fauna faces intensifying fishing pressure, sedimentation from catchment deforestation, falling water levels and the effects of a warming climate. A small, specialised, rarely seen endemic like macrodon is exactly the kind of species that can decline unnoticed in such a setting, which is precisely why the Data Deficient listing matters — it flags a fish that needs basic survey work before its status can even be judged.