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Taxonomy & naming

Trematocara macrostoma was described by the Belgian ichthyologist Max Poll in 1952, from material collected during the Belgian Hydrobiological Mission to Lake Tanganyika of 1946–47. The holotype (MRAC 111714) came from the coast of Moba, on the Congolese shore. The published description appeared in the Bulletin de l'Institut Royal des Sciences Naturelles de Belgique (vol. 28, no. 49).

The genus name comes from the Greek tremata (hole, opening) and kara (head) — a reference to the conspicuous sensory pores that perforate the head of these fishes, part of an enlarged mechanosensory lateral-line system suited to a lightless, deep-water life. The species epithet macrostoma means 'large mouth', and it is apt: the deeply cleft, oversized jaw is the fish's most obvious feature.

That unusual mouth caused taxonomic turbulence. In 1991 Maréchal and Poll moved the species into a new monotypic genus, Telotrematocara, on the strength of its distinctive jaw morphology, a placement still used by Poll & Gosse (1995). Later work by Takahashi (2002) concluded that these traits fall within the natural variation of Trematocara, and the species was returned to its original genus — a view followed by Konings (2015, 2019) and by Eschmeyer's Catalog of Fishes, which lists Telotrematocara macrostoma as a junior synonym. The valid name today is Trematocara macrostoma Poll, 1952. The genus Trematocara, erected by Boulenger in 1899 around the type species T. marginatum, now contains nine valid species and forms part of the deep-water tribe Bathybatini (Trematocarini).

Appearance

This is a slender, modest-sized cichlid. FishBase gives a maximum of 9.5 cm (3.7 in) standard length; field references round it to roughly 10 cm (4 in) total length. The body is the silvery, large-eyed build common to Tanganyika's deep-water cichlids, an adaptation to dim light at depth.

Where T. macrostoma departs from its relatives — and from essentially every other Tanganyikan cichlid — is the dorsal fin of mature males, which is extraordinarily elevated, reported as more than twice the height of the female's. This 'sail' is the basis for the informal common name sailfin cichlid (Andersen 2013). Males are also more strongly marked: darker overall, with black ventral fins, dark dorsal-fin markings, and black gill membranes, while females have largely clear fins with at most a dusky anal fin. The sexual dimorphism is described as very pronounced.

The oversized, deeply cleft mouth, set with fine villiform (brush-like) teeth, is diagnostic and immediately separates this fish from its congeners. It was striking enough that early workers used it to justify a separate genus before later studies folded the species back into Trematocara.

Range & habitat

Trematocara macrostoma is endemic to Lake Tanganyika and, unlike several wide-ranging members of its genus, has a restricted distribution. Every confirmed record falls in the southwestern part of the lake, between Moba in the Democratic Republic of the Congo and Chituta Bay in Zambia (Bailey & Stewart 1977; Andersen 2013); FishBase summarizes this as the southern half of the lake. The IUCN assessment lists it from the DRC and Zambia.

It is a deep-water, benthopelagic fish of the soft profundal — muddy or silty bottoms well below the rocky littoral that most aquarium cichlids inhabit. The IUCN gives a depth band of 60–100 meters (about 200–330 ft), matching Poll's original collections and the survey work of Bailey and Stewart. That places it in a cold, dark, low-oxygen world far removed from the sunlit reefs; the species has reportedly never been seen alive, only retrieved in nets and trawls. Among Tanganyikan cichlids, only a handful of lineages have colonized this deep benthic zone, which makes the genus Trematocara, and this species in particular, a useful window onto how the lake's famous radiation pushed into its least hospitable habitats.

Ecology & diet

What this fish actually eats remains genuinely unresolved, and the honest answer is that nobody is certain. Every specimen ever brought to the surface and dissected has had an empty stomach — almost certainly because the rapid pressure change on the way up causes regurgitation, a common problem with deep-caught fishes. FishBase places it at trophic level 3.3, squarely in the carnivore range, and its rarity has prompted speculation that it may be a predator. The large mouth and villiform teeth have fuelled several competing hypotheses: plankton-feeding by swimming open-mouthed through dense prey layers, scale-eating, or outright piscivory, possibly even on other, smaller Trematocara.

The genus as a whole offers context. Trematocara are small benthic and bathypelagic cichlids that feed on a mix of invertebrates, fish larvae, and phytoplankton, and they are best known for a striking behavior: following the upward nightly migration of zooplankton, many species ascend the lake slopes from deep water into the littoral after dark, then retreat to depth by day (Kirchberger et al. 2012). Whether T. macrostoma makes the same vertical commute is plausible but not directly documented for this poorly known species. Its large eyes are consistent with feeding in very low light. In short: a deep-water carnivore or planktivore whose precise diet is inferred from its anatomy and its relatives rather than from gut contents.

Behavior & breeding

Direct behavioral observations of T. macrostoma in the wild are essentially nonexistent, so most of what can be said is inference from the genus and from the fish's anatomy. Like all members of the Bathybatini, Trematocara are maternal mouthbrooders — the female carries the fertilized eggs and developing fry in her mouth — and on that basis T. macrostoma is presumed to brood the same way.

The species' pronounced sexual dimorphism points to a sexually selected breeding system. The male's exaggerated, sail-like dorsal fin is thought to function in courtship display, the kind of conspicuous male ornament that typically signals quality to females in mouthbrooding cichlids. Beyond that, breeding triggers, spawning site, brood size, and the duration of mouthbrooding are all unknown for this fish; there are no breeding reports in the hobby literature, and the field data are too thin to fill the gap. This is a species defined as much by what we don't know as by what we do.

In the aquarium

For practical purposes, Trematocara macrostoma is not an aquarium fish. It is a rarely collected, deep-water specialist that has never been observed alive in its habitat, carries no breeding reports, and does not appear in the ornamental trade. Even within the small circle of hobbyists who chase obscure Tanganyikan oddities, this is a museum-cabinet species rather than a tank subject.

The obstacles are fundamental, not a matter of skill. Fish adapted to 60–100 meters experience large, often fatal pressure and temperature changes on capture, which is precisely why every netted specimen arrives dead with an empty, everted stomach. Its natural diet is uncertain, its cold, dark, low-light profundal habitat is impractical to replicate, and it is geographically and ecologically remote from the rocky-shore and shell-bed cichlids that fill Tanganyika community tanks. If you keep Tanganyikans, the realistic takeaway is simply this: T. macrostoma is a fish to appreciate for what it reveals about the lake's deep-water radiation, not one to plan a tank around. Anyone offered it under that name in the trade should be skeptical.

Conservation

Trematocara macrostoma is assessed by the IUCN Red List as Least Concern (most recently 23 February 2025; assessor Haambiya, reviewed by the Tanganyika cichlid specialist Ad Konings), a status unchanged from its 2006 evaluation. The justification is straightforward: it is a widespread deep-water species with no known major threats. Its population trend is listed as unknown, and it is described as only rarely caught alongside other Trematocara by local food fishermen — there is no targeted fishery and no ornamental collection pressure on it. So the species-level signal is reassuring.

The caveat is the lake it lives in. Lake Tanganyika itself is under real strain, even where individual species are not yet flagged. O'Reilly et al. (2003, Nature, doi:10.1038/nature01833) documented that climate warming has strengthened stratification and weakened the deep mixing that drives the lake's productivity, with an estimated decline of around 20% in primary productivity and a comparable drop in fish yields over recent decades. Cohen et al. (2016, PNAS, doi:10.1073/pnas.1603237113) used paleoecological records to show that warming since the 19th century has been accompanied by an inferred loss on the order of 38% of the lake's oxygenated benthic habitat, hitting deep-living endemics hardest, with sedimentation from deforestation and shoreline development further degrading habitat (building on Cohen et al. 1993). A clupeid-and-Lates pelagic fishery feeds millions across the four riparian nations (DRC, Tanzania, Zambia, Burundi), now coordinated through the Lake Tanganyika Authority.

Those basin-scale pressures bear directly on this fish's guild. A benthopelagic species that lives at 60–100 meters and may depend on cold, oxygenated deep water and on plankton production is exactly the kind of fish exposed to a warming, more stratified lake with a shrinking oxygenated floor — even if it is buffered for now by being widespread and largely overlooked by fishers. The accurate statement is the careful one: T. macrostoma is Least Concern as a species, but it inhabits a lake whose deep-water habitat is measurably contracting, and it is poorly enough known that we would be slow to notice if that changed.

Trematocara macrostoma

About this species