Cichlid Atlas — the world's cichlid waters, in depth

Taxonomy & naming

The fish was described in 1951 by the Belgian ichthyologist Max Poll, working from material collected by the Belgian hydrobiological mission to Lake Tanganyika of 1946–1947. He named it Xenotilapia tenuidentata, with the type locality at Baie de Vua on the lake's western (Congolese) shore; the holotype is held at the Royal Museum for Central Africa in Tervuren (MRAC 109884). The species epithet tenuidentata means 'slender-' or 'fine-toothed,' a nod to the delicate dentition that sets this fish apart from its more robustly built relatives.

The name has since become a genuinely unsettled question, and it is worth being explicit about. In 1991 Maréchal and Poll moved the species into a new genus, Microdontochromis (Greek for 'small-toothed cichlid'), alongside its close relative M. rotundiventralis. Eschmeyer's Catalog of Fishes today lists the valid name as Microdontochromis tenuidentatus (Poll, 1951), and the IUCN follows that usage. FishBase, GBIF, and much of the hobby literature, however, continue to use Xenotilapia tenuidentata, treating Microdontochromis as a synonym — a position supported by DNA-based work that nests these plankton-feeders deep inside Xenotilapia rather than in a separate lineage. The cichlid specialist Ad Konings has gone back and forth on the question in print. The practical upshot: the two names refer to exactly the same fish, and a reader will meet both. Either way the species belongs to the tribe Ectodini, the large and behaviorally varied group of sand-, mud-, and open-water cichlids that also contains the showy featherfins and the sand-sifting Callochromis and the rest of Xenotilapia.

Appearance

This is one of the smallest cichlids in the lake — an elongate, silvery, faintly translucent fish built more like an open-water schooler than a bottom-dweller. Males reach about 3 in (8 cm) in total length; FishBase caps it there, while some hobby sources push the maximum to roughly 3.5 in (9 cm). Females are only marginally smaller, less than 5% shorter than males, and there is essentially no sexual dimorphism in color — both sexes wear the same pale, reflective dress, which is part of why telling pairs apart in a school is genuinely hard. Adults usually show a set of faint vertical bars along the flank, and the body is noticeably shallow and streamlined.

For anyone trying to separate it from its near-twin, M. rotundiventralis, the diagnostic details are small but consistent: tenuidentata carries a single row of teeth in each jaw, a pointed (rather than rounded) tip to the pelvic fin, a proportionally longer innermost pelvic-fin ray, a shallower body, and usually ten anal-fin soft rays against nine in its relative. The presence of those flank bars is itself a quick field clue — rotundiventralis lacks them. The fine teeth that gave the fish its name are the equipment of a planktivore, not a sand-sifter, and they reflect how far this little cichlid has moved from the typical Ectodine way of life.

Range & habitat

Xenotilapia tenuidentata is endemic to Lake Tanganyika, and here the sources genuinely disagree about how widespread it is. FishBase and the IUCN assessment describe it as restricted to the western, Congolese shore of the lake — the region of its type locality. Field-based hobby references such as the Cichlid Room Companion and tanganyika.si, by contrast, report it from many sites all around the lake and treat it as effectively lake-wide. The honest reading is that this is a small, easily overlooked, open-water fish whose true range is probably under-documented; the conservative published statements lag behind what divers and collectors actually see.

What the sources do agree on is the kind of place it lives. This is a shallow-water, open-column fish. By day it hangs in the upper part of the water column, typically within the top 30 ft (10 m), most often over intermediate habitat — a sandy bottom dotted with scattered stones — though it also ranges over rock. At night the schools appear to drop down and rest on sandy bottom at depths of around 65 ft (20 m) or more. The water is the warm, hard, alkaline water of the open lake, roughly 73–82 °F (23–28 °C) and strongly basic at around pH 8–9. Unlike most of its sand-dwelling tribe, it spends much of its time well off the bottom, out in the plankton.

Ecology & diet

Xenotilapia tenuidentata is a zooplanktivore — a specialist on the small drifting animals of the lake's nearshore water. It feeds by picking out individual prey with a protrusible mouth, the jaws shooting forward to seize one organism at a time rather than filtering the water indiscriminately. Stomach-content analyses have turned up copepods and other small crustaceans, diatoms, bacteria, assorted micro-organisms, and the remains of insect larvae; copepods are the staple, and FishBase places the fish at a trophic level of about 3.1, the low-carnivore band you would expect of a plankton-picker. The fine, slender teeth that name the species are the tools for this job.

Ecologically the fish is interesting precisely because it has stepped off the substrate. Most Ectodini are tied to the sand or rock they forage over; tenuidentata and its sister species have converted into something closer to the open-water Cyprichromis, exploiting the standing crop of zooplankton in the shallow water column. It does this in numbers — schools running from several hundred up to a couple of thousand individuals have been reported — which makes it, in the aggregate, a meaningful conduit channeling the lake's planktonic productivity into fish biomass in exactly the nearshore zone where many larger predators hunt.

Behavior & breeding

Socially this is a peaceable schooling fish, not a territory-holder. It forms very large, cohesive shoals and is mild toward both its own kind and other species — the aggression that defines so many Tanganyikan cichlids is largely absent here. The interesting biology is in how it breeds. Xenotilapia tenuidentata is a biparental mouthbrooder, a comparatively rare strategy in which both the male and the female carry developing young in the mouth. Within the Ectodini, brood care ranges from maternal-only to fully shared, and this lineage sits at the biparental end — a system that appears to have evolved more than once within the tribe, which is part of why these small fish attract evolutionary interest out of proportion to their size.

The mechanics are well documented from the wild. The eggs are very small, around 2 mm, and the clutch is modest — commonly on the order of 6 to 15. The female takes the eggs first; once the fry reach about 5 mm (after roughly a week), she transfers about half the brood to the male, and from then on both parents brood and continue to feed while doing so. Incubation runs about three weeks, and by the time the young are released at around 15 mm each parent is typically carrying only a couple of fry. Aquarium observations sometimes show a different sequence — the female taking all the eggs initially and handing them off to the male later — but the shared, two-parent outcome is consistent. It is a low-output, high-investment way to reproduce, and it depends on a stable pair bond holding together through the weeks of mouthbrooding.

In the aquarium

This is a specialist's fish, not a beginner's, and the reasons are size and temperament rather than any exotic water chemistry. Xenotilapia tenuidentata is small, delicate, and a natural schooler, so it has to be kept as a group — single fish or tiny clusters become chronically stressed. Plan on a sizeable shoal in a tank that gives them open swimming room: published hobby guidance puts the practical minimum around 50 gallons (about 200 L), and more is better given that the fish naturally forms very large schools and is built to cruise. A fine sand bottom with a few scattered stones and plenty of open water above it matches the wild setting and lets the fish behave normally.

The real husbandry trap is competition. Because it is so small, peaceful, and slow to push for food, it is easily outcompeted and bullied; tankmates must be chosen from among the smaller, gentler Tanganyikans — sand-dwellers and mild open-water species — and it should never be stocked with the larger, pushy rock cichlids that dominate the trade. Water should track the lake: hard, alkaline (pH roughly 8–9) and warm (mid-70s to low-80s °F / about 24–28 °C), with the strong filtration and consistent water changes any Tanganyikan setup needs. Feed small, plankton-appropriate foods — frozen cyclops, daphnia, baby brine shrimp, and finely crushed dry foods — offered in small, frequent portions so this deliberate feeder actually gets its share. Breeding the biparental mouthbrooders is achievable but demanding, and it hinges on a settled group within which stable pairs can form.

Conservation

At the species level, Xenotilapia tenuidentata (assessed by the IUCN under its current name, Microdontochromis tenuidentatus) is listed as Least Concern. The original assessment was published in 2006 by Bigirimana and Nzeyimana, and the species was re-evaluated and again rated Least Concern in 2025. The justification is straightforward: it is endemic to Lake Tanganyika but apparently widespread within it, with no known major threats specific to the species. There is some collection for the aquarium trade, but it is a minor export rather than a heavily targeted one. The Cichlid Room Companion adds a telling point about why this particular fish is relatively insulated from fishing pressure — it is so small that it slips straight through the mesh of regulation-size seines, and it lives deep and close enough to structure that even the illegal mosquito-net fishing that scours other shallow species has little effect on its numbers. By the lake's reckoning, the one thing that would genuinely threaten it is a collapse in copepod abundance, and a problem on that scale would be either a local anomaly or a lake-wide catastrophe affecting the entire planktivore guild.

That last clause matters, because the lake itself is under strain even where this fish is not. Lake Tanganyika is warming, and the consequences fall on exactly the planktonic food base tenuidentata depends on. Long-term work led by Catherine O'Reilly (2003, Nature) found that rising surface temperatures have strengthened the lake's stratification and weakened the wind-driven mixing that lifts deep nutrients into the sunlit surface waters, with sediment-core evidence pointing to a roughly 20% decline in primary productivity over the twentieth century and an inferred drop in fish yields on the order of 30%. Andrew Cohen and colleagues (2016, PNAS) extended that picture with a paleoecological record spanning centuries, linking sustained recent warming to reduced algal production and an estimated 38% contraction of oxygenated benthic habitat in their study areas, alongside measurable declines in fishery species. A planktivore whose entire livelihood is the standing crop of copepods in the shallow water column is, by definition, exposed to a less productive lake.

The second basin-wide pressure is sedimentation: deforestation and farming across the catchment wash eroded soil into the nearshore, and the IUCN assessment names habitat deterioration through sedimentation, together with fishing, as the threats acting on this species. Sediment bears more heavily on bottom-spawning, substrate-tied fishes than on an open-water plankton-picker, but it degrades the whole littoral system within which tenuidentata feeds and rests. These pressures play out across a lake shared by four nations — Burundi, the Democratic Republic of the Congo, Tanzania, and Zambia — whose coordinated stewardship of fisheries and watersheds, through the Lake Tanganyika Authority, remains a work in progress. The honest summary is the one the data support: Xenotilapia tenuidentata is secure today and rightly listed as Least Concern, but it lives in a warming, increasingly silted lake whose productivity — the very thing this small planktivore eats — is slowly being eroded.

Xenotilapia tenuidentata

About this species