Taxonomy & naming
Haplochromis molossus was described in 2019 by Nathan Vranken, Maarten Van Steenberge and Jos Snoeks in the journal Hydrobiologia, as part of a revision titled 'Grasping ecological opportunities: not one but five paedophagous species of Haplochromis (Teleostei: Cichlidae) in the Lake Edward system.' That paper untangled the egg- and larvae-eating haplochromines of Lakes Edward and George: before it, only a single paedophage — Haplochromis taurinus — was formally recognised from the system, but a morphometric study revealed that what had been treated as one variable lineage was in fact five species, including three new ones (Haplochromis gracilifur, Haplochromis molossus and Haplochromis relictidens) alongside the redescribed Haplochromis taurinus and Haplochromis paradoxus. The holotype (Royal Museum for Central Africa, MRAC 2016.036.P.0046) is a female of 4 in standard length taken at Mukutu Kihinga, a rocky site offshore of Mweya in Lake Edward, on 23 October 2016; paratypes are held at the MRAC and at the Royal Belgian Institute of Natural Sciences (IRSNB) and span both Lake Edward and Lake George. The species is placed in Pseudocrenilabrinae. As with most members of the East African haplochromine radiation, the catch-all genus Haplochromis is widely regarded as an artificial, non-monophyletic assemblage, and many former 'Haplochromis' have been reassigned to other genera; the describers nonetheless retained molossus in Haplochromis, and that remains its valid combination, so the genus heading is kept here.
Morphology
This is a small cichlid: the type series reaches a maximum of about 4.5 in (4.5 in) standard length, with most specimens between roughly 75 and 4.5 in SL. The body is relatively slender, the head short and concave in profile, and the snout slopes steeply (45–55°). The defining feature is the mouth: the oral jaws are long, very wide and broadly rounded, isognathous (upper and lower jaws of equal projection), with a gentle gape inclination of only 20–30°. It is this flat-faced, broad-jawed 'pug-head' look that gives the fish its name. The lips and oral mucosa are firmly thickened, and the small outer teeth are deeply embedded in the mucosa and hidden behind a lower lip that folds over them — an arrangement that distinguishes molossus from its relatives. The fins carry 15–16 dorsal spines and 9–10 dorsal soft rays, 3 anal spines and 8–9 anal soft rays, over 28–30 vertebrae. The describers separated molossus from the other Lake Edward paedophages on a suite of fine characters: broader oral jaws (67–87% of lower-jaw length, versus 42–73% in the others), the steeper snout and gentler gape, smaller lachrymal bones, a deeper caudal peduncle, more infraorbital cheek scales (3–4) and an intermediate count of outer upper-jaw teeth (22–34). Coloration is sexually dimorphic and is itself diagnostic: mature males are green over the body and flanks and carry six to eight vertical bars (described as blue in life), a pattern unlike that of any other paedophage in the system. Females and immatures are plainer.
Habitat
Haplochromis molossus is endemic to the Lake Edward system, which the describers treat as a single hydrographic unit comprising Lake Edward, the smaller and shallower Lake George, the Kazinga Channel that links them, and associated rivers such as the Semliki. Lake Edward straddles the border between the Democratic Republic of the Congo and Uganda; Lake George lies wholly within Uganda. The type material was collected at offshore localities over both rocky bottoms (the holotype came from a rocky site off Mweya, and other specimens from hard substrate at the mouth of the Kazinga Channel) and soft substrate (for example off Katwe and at Akika island in Lake George), indicating that the species ranges across the sublittoral offshore zone rather than specialising on a single bottom type. Precise capture depths are not tabulated in the description, and FishBase classes the fish simply as benthopelagic; the depth band given here reflects the shallow offshore sublittoral of these lakes — Lake George is only a few metres deep throughout, and Lake Edward is comparatively shallow for an African Great Lake — rather than a measured range, and should be read as approximate. Published in-situ chemistry specific to the collecting sites was not reported; the values in the fact box are the broadly typical surface conditions of these warm, productive, moderately alkaline equatorial lakes.
Feeding
Haplochromis molossus is a paedophage — it feeds on the eggs and larvae of other cichlids. In the Lake Victoria and Lake Edward haplochromine flocks, paedophages typically obtain this food by raiding the mouths of brooding females: the predator rams or startles a mouthbrooding female into releasing or partly spilling her clutch, then snaps up the eggs and fry. Stomach-content examination by the describers confirmed the paedophagous diet for molossus, and its morphology fits the role. The broad, rounded, firmly lipped jaws with their small, embedded teeth are not built for scraping algae or crushing snails but for engulfing soft, slippery clutches; the gentle gape and flat face let the fish work close against a brooding female's snout. Paedophagy is one of the more remarkable trophic specialisations to have arisen repeatedly within the East African Haplochromis radiation, and the Vranken et al. study is notable precisely for showing that the Lake Edward system supports a small cluster of such specialists partitioning the same unusual food source by subtle differences in jaw shape and tooth count.
Mating
No courtship or pre-spawning behaviour has been described for Haplochromis molossus specifically. As a member of the haplochromine radiation it is expected to follow the near-universal haplochromine pattern: polygynous, with no lasting pair bond, the sexes living apart outside of brief spawning encounters. The bright, barred green nuptial coloration of mature males is consistent with males displaying to ripe females and competing for them, as in related haplochromines, where males defend a small spawning site or simply court actively over the substrate. Beyond this general expectation, the social structure and territorial behaviour of the species in the wild remain undocumented — a fair reflection of how little is known about the day-to-day biology of most species in this recently revised flock.
Breeding
Breeding has not been directly observed in Haplochromis molossus, but its reproductive mode can be stated with confidence from its phylogenetic position: like essentially all members of the Lake Edward / Lake Victoria haplochromine flock, it is a maternal mouthbrooder. After spawning, the female takes the fertilised eggs into her mouth and broods them there through hatching and the early free-swimming stage, releasing the fry once they can fend for themselves and gathering them back in at the first sign of danger. There is an irony to this in the present case, since the same mouthbrooding habit is exactly what the species exploits in others: paedophages make their living by robbing brooding females of their clutches. Clutch size has not been reported for molossus; congeneric Lake Victoria–region haplochromines of comparable size typically carry only small broods, on the order of a few tens of eggs (commonly cited at roughly 20–80), reflecting the trade-off mouthbrooders make between egg number and parental investment. Parental care is by the female alone; males contribute nothing after spawning. Spawning seasonality and triggers in the wild are undocumented.
In the aquarium
Haplochromis molossus is essentially never kept in the aquarium hobby. It was described only in 2019, it has no ornamental trade, and it is restricted to a remote pair of equatorial lakes that are not a source of aquarium exports; there is no body of hobbyist husbandry to report. Anyone who did encounter it would be looking at a small (to roughly 4.5 in), warm-water haplochromine from moderately alkaline lake water — conditions broadly comparable to those used for the Lake Victoria 'furu' that some specialist keepers maintain (hard, alkaline water around pH 7.5–8.5 and 75–81 °F, a roomy tank with open swimming space and rock structure, and the robust filtration these active fish need). The critical caveat, however, is the fish's natural diet: as a dedicated paedophage that feeds by raiding the broods of mouthbrooding cichlids, it would be a poor and destructive community member, harassing and stripping the spawns of any mouthbrooders housed with it. This is a fish of scientific rather than aquarium interest, and that is how it should be regarded.
Conservation
Haplochromis molossus has not been assessed by the IUCN Red List — its status is Not Evaluated — which is unsurprising for a species described only in 2019. It carries no CITES or CMS listing. The Lake Edward system is home to an endemic haplochromine flock of roughly 60 cichlid species, the great majority of them found nowhere else, and crucially it was spared the catastrophic introduction of the predatory Nile perch (Lates niloticus) that devastated the much larger species flock of neighbouring Lake Victoria from the 1980s onward. That makes the Edward–George flock one of the more intact great-lake cichlid assemblages, but not a secure one: the basin's fisheries are heavily exploited, and the lakes face pressure from overfishing, catchment changes, pollution and other human activities, against a backdrop of poor monitoring data. As a newly recognised species with no population estimate, molossus should be regarded as data-deficient in the practical sense even though it carries no formal category; its prospects are tied to the general health of the Lake Edward system rather than to any threat documented for the species itself.