Taxonomy & naming
Diplotaxodon sp. "Bigeye Blackdorsal" has no describing author, no year of publication, and no FishBase entry, because it has never been the subject of a published taxonomic description. It is known only by an informal field/trade name that follows the standard open-nomenclature convention for undescribed Malawi cichlids: the genus Diplotaxodon, the abbreviation "sp.", and a descriptive name in quotation marks. Diplotaxodon is itself among the least-resolved genera in the lake — a large radiation of open-water haplochromines in which the great majority of the roughly 15-plus recognised forms are informally distinguished rather than formally described, because the fish are difficult to collect, rarely seen alive by scientists, and separated mainly by subtle traits such as eye size, body depth, and fin pigmentation. "Bigeye" and "Blackdorsal" name exactly the two characters field workers have used to separate this form from its many silvery congeners. No formal revision of the genus has resolved its status, and none should be implied.
Morphology
This form follows the standard Diplotaxodon body plan: an elongate, laterally compressed, torpedo-shaped fish built for sustained open-water swimming rather than the deep-bodied profile typical of rock-dwelling Malawi cichlids. As in the whole genus, the eye is large relative to the head — an adaptation shared by the deep-water Diplotaxodon community for gathering light in the dim, blue-shifted twilight zone many of them inhabit — and in this form the eye is reported as conspicuously large even by genus standards, the trait behind the first half of its trade name. The second field mark is a distinctly dark, blackish dorsal fin, standing out against the plain silvery flanks and pale ventral colour typical of pelagic Diplotaxodon. Genus-typical size runs roughly 4–6 in SL; no species-specific measurements have been published for this form.
Habitat
Diplotaxodon sp. "Bigeye Blackdorsal" is recorded from open water off Chilumba and Nkhata Bay, both in the northern half of Lake Malawi on the Malawian shore. Unlike the mbuna and utaka that dominate most of the lake's cichlid diversity, Diplotaxodon does not associate with rock or sand substrate at all — it is a pelagic fish, living in loose schools over open, deep water well offshore. The genus as a whole spans a wide depth envelope, from moderate depths down into the dim, cool waters of the lake's deep mixed and transition layers, and many species undertake a nightly vertical migration toward the surface to feed before retreating to depth by day. Water chemistry follows Lake Malawi's well-mixed upper layer: warm, hard, and alkaline, though the deeper water this genus favours runs cooler and less oxygenated than the shallows most aquarists associate with the lake.
Feeding
Diplotaxodon as a genus is built around zooplanktivory: the fish forage in open water for copepods, cladocerans, and other zooplankton, picking individual prey items from the water column rather than grazing a substrate or sifting sediment. Several species are known to feed near the surface at night, coinciding with their vertical migration, and some also take insects and other items available in open water after dark. No dedicated dietary study exists for "Bigeye Blackdorsal" specifically, so its diet is presented here as genus-typical zooplanktivory rather than a documented fact for this exact form.
Mating
Reproductive behaviour in Diplotaxodon is poorly documented for the genus as a whole, and essentially undescribed for this particular form. Pelagic living makes courtship and spawning difficult to observe directly, since the fish do not hold conventional rock or sand territories the way most Lake Malawi cichlids do. What social structure has been inferred for the genus points to open-water, likely lek-like breeding rather than fixed pair bonds, consistent with the maternal mouthbrooding shared across every Malawi haplochromine, but the specific courtship sequence, territory type, and timing for this form have not been recorded in the literature available to this article.
Breeding
As with every Lake Malawi cichlid, Diplotaxodon sp. "Bigeye Blackdorsal" is presumed to be a maternal mouthbrooder: the female takes fertilised eggs into her mouth and carries the developing brood there until the fry are free-swimming. Diplotaxodon is notable within the lake's cichlid fauna for unusually low fecundity in the species that have been studied in any detail — broods of only a handful of well-developed, comparatively large fry rather than the dozens typical of rock-dwelling mbuna. Whether that pattern holds for this specific form has not been confirmed; it is presented here as a genus-typical expectation, not a documented count.
In the aquarium
Diplotaxodon sp. "Bigeye Blackdorsal" is not an established aquarium fish. Pelagic Diplotaxodon species are collected far less often than rock and sand cichlids, are more delicate in transport, and require the kind of large, open, well-oxygenated midwater space that few home aquaria can provide; the genus is essentially absent from the ordinary hobby trade. Any keeper who did encounter this form would need to approach it as a schooling open-water fish rather than a territorial cichlid — a large, uncluttered tank, strong water movement and oxygenation, and the standard hard, alkaline Lake Malawi water chemistry (roughly 75–82 °F, pH 7.7–8.6 or a little higher) — but specific husbandry experience with this form has not been documented and none should be invented.
Conservation
Diplotaxodon sp. "Bigeye Blackdorsal" has no IUCN Red List assessment, since formal evaluation requires a validly published scientific name and this form has never received one. More broadly, Lake Malawi's pelagic Diplotaxodon community is exposed to pressures distinct from the inshore rock-dwellers: it overlaps with the open-water usipa (Engraulicypris sardella) and utaka fisheries, which use fine-mesh nets capable of taking cichlids as bycatch, and the genus's dependence on deep, cool, well-oxygenated water makes it potentially sensitive to the lake's documented warming and strengthening thermal stratification, which reduces the mixing that resupplies oxygen to deeper layers. No population-level data exist for this specific form to say more than that it shares those general exposures.