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

Taxonomy & naming

George Albert Boulenger described this fish in 1898 from material collected by J. E. S. Moore during the first scientific expeditions to Lake Tanganyika, publishing it as Plecodus paradoxus in the Proceedings of the Zoological Society of London (the holotype, MRAC 189, is held in Tervuren). It belongs to the tribe Perissodini, the lake's lineage of scale-eaters and their plankton-feeding relatives. The species epithet paradoxus — 'contrary to expectation' — fits a fish whose diet was genuinely paradoxical to the naturalists who first opened its stomach.

The genus name is where things get complicated, and it is worth being honest about the disagreement rather than papering over it. Boulenger's Plecodus has come to be regarded by many authors as a synonym of the older genus Perissodus (also Boulenger, 1898; Greek perissos, 'uneven', plus odous, 'tooth', for the irregular teeth). Eschmeyer's Catalog of Fishes and FishBase now list the species as valid under Perissodus paradoxus, following Konings (2015, 2019). The GBIF taxonomic backbone, by contrast, still carries Plecodus paradoxus as the accepted name with Perissodus paradoxus as a synonym. Both binomials refer to the same fish; hobbyist sources and older literature lean toward Plecodus, while current taxonomic authorities favour Perissodus. We use Plecodus paradoxus here for continuity with the aquarium trade, but readers chasing the primary literature will find it under both names.

Appearance

This is a fairly large, laterally compressed cichlid with a deep but streamlined body built for cruising open water rather than hugging rock. FishBase gives a maximum of about 30 cm (12 in) total length; field references for Lake Tanganyika put typical adults at roughly 29 cm (11 in), and aquarium-oriented sheets cite an average closer to 25 cm (10 in) with the largest males reaching the 30 cm mark. Coloration is muted and silvery compared with the lake's gaudy rock-dwellers — an adaptation for ambushing prey over pale sand. Non-breeding fish commonly show a narrow mid-lateral stripe that ends in a dark spot on the caudal peduncle, just ahead of the tail.

Sexual dimorphism is reported inconsistently. Lake survey material notes no obvious difference between the sexes in size or color, while some hobbyist sheets claim dominant males grow larger; the honest reading is that any dimorphism is slight and not a reliable way to sex the fish. Within the scale-eater guild, P. paradoxus is separated from its relatives by a combination of meristic and proportional traits — more dorsal-fin spines than the rare deep-water Perissodus eccentricus, a notably more slender body than the chunky P. straeleni, and differences in oral tooth counts and anal-fin rays from P. multidentatus and P. elaviae. None of these are field-obvious, which is exactly why the genus has historically been a taxonomic headache.

Range & habitat

Plecodus paradoxus is endemic to Lake Tanganyika, the long, deep rift lake shared by Burundi, the Democratic Republic of the Congo, Tanzania, and Zambia; the Catalog of Fishes also notes records from the Lukuga, the lake's single outflow. It is distributed lake-wide along the shores, and is regarded as the most frequently encountered of Tanganyika's scale-eaters — a fish divers routinely see over sand throughout the lake.

Unlike the rocky-reef specialists that dominate Tanganyikan aquaria, this species favors sandy bottoms, often near rocks but not confined to them. It is most often seen swimming in loose hunting groups roughly a meter above the substrate, scanning for targets. Its vertical range is wide: it has been recorded from the well-lit shallows down to about 250 m (820 ft), and FishBase notes that it can aggregate in very large numbers at depth. That depth tolerance matters for its biology and its conservation, because much of the population lives below the zone most exposed to shoreline disturbance. In-situ, it inhabits the lake's characteristically hard, alkaline water — roughly pH 7.8-8.8 and around 23-25 °C (73-77 °F) in the surface layers it shares with its prey.

Ecology & diet

Plecodus paradoxus is a lepidophage — a scale-eater — and one of the clearest examples of the trophic specialization that makes Tanganyika's cichlid radiation so remarkable. Adults feed primarily on scales and small pieces of skin stripped from the flanks of other fish, and stomach-content analyses reveal neat, stacked rows of swallowed scales; fish fry turn up occasionally as well. FishBase places its trophic level at about 4.4, squarely predatory. The fish does not kill its prey so much as parasitize it: a successful strike removes a mouthful of scales and the victim swims on, scarred but alive.

Prey size scales with predator size. Large individuals are reported to harass big-bodied cichlids — even the giant Boulengerochromis microlepis, the largest cichlid in the lake — while smaller scale-eaters work over smaller sand-dwelling species. The genus is best known to science for the asymmetry work on its relative Perissodus microlepis, in which 'left-mouthed' and 'right-mouthed' individuals attack opposite flanks of their prey, a system Michio Hori's 1993 Science study made into a textbook case of frequency-dependent selection. P. paradoxus belongs to the same lineage and shares the basic scale-eating toolkit, though the famous laterality experiments were done on its cousin, not on this species.

The evolutionary backdrop is well resolved. A mitochondrial-and-AFLP phylogeny by Koblmüller and colleagues (2007) found that the Perissodini split from their nearest relatives roughly 3.3-4.6 million years ago — among the last tribes to emerge in Tanganyika's primary radiation — with most living species diverging 1.5-2 million years ago. Strikingly, the group appears to have originated in deep water and colonized the shallows only recently, which fits the broad depth range and offshore habits of P. paradoxus.

Behavior & breeding

Socially, P. paradoxus is a roving, group-living hunter outside of breeding, but it is far from peaceable: its entire feeding strategy is built on aggression toward other fish, and even conspecifics are attacked unless a stable pair has formed. This is not territorial display so much as predatory intent.

Its reproduction sits at an interesting halfway point in cichlid evolution — described as intermediate between substrate brooding and mouthbrooding. Spawning takes place on sand or rock, and the female then takes the eggs into her mouth; clutches are reported at roughly 200-300 very small eggs. After about nine days the fry are released, and here the species departs from the lake's many maternal mouthbrooders: both parents collect, herd, and guard the brood, defending the fry for at least six weeks. Biparental care of free-swimming young is a hallmark of the Perissodini and a useful reminder that 'Tanganyikan cichlid' covers a wide spread of breeding strategies, not a single template.

In the aquarium

Be honest up front: this is a specialist's fish that rarely appears in the trade, and for good reason. A scale-eater's natural diet is, definitionally, the bodies of other living fish, and that creates two problems no clever aquascaping solves. First, it is persistently aggressive toward tankmates — it is wired to approach other fish and bite them — so a community of mixed cichlids is a steady source of nipped, scarred, and stressed targets. Second, its feeding is genuinely difficult to satisfy: keepers and care references agree that it disdains flakes and freeze-dried foods and that some individuals never convert to prepared diets at all, which makes long-term husbandry a real commitment rather than a curiosity purchase.

Where it is kept, the consistent advice is open water and space. Published recommendations range widely — from a practical minimum around 250 L (about 65 US gal) for a single fish or a settled pair, up to far larger volumes for any attempt at a group — but the theme is the same: a long tank with generous swimming room, modest rockwork rather than a reef, and hard, alkaline Tanganyikan water (high pH, moderate-to-high hardness, stable temperature in the mid-70s °F / mid-20s °C). The realistic approach is a single fish or a monitored pair kept essentially alone, with any tankmates watched closely; throwing it into a typical mbuna-style community is a recipe for chronic injury to everything else. It is best regarded as a project for an experienced Tanganyikan keeper who is genuinely interested in the behavior, not as a display fish.

Conservation

At the species level, Plecodus paradoxus is not currently at risk. The IUCN Red List assessed it as Least Concern on 27 February 2025 (assessment by L. Mabo, reviewed by the Tanganyika specialist Ad Konings), under the valid name Perissodus paradoxus and consistent with its earlier Least Concern listing. The rationale is straightforward: it is widespread throughout the deeper waters of the lake, with no known major threats, although the population trend is formally recorded as unknown. There is no targeted fishery or notable collection pressure on this scale-eater. In short, the fish itself sits in a comfortable place — the concerns lie one level up, in the lake it cannot leave.

Lake Tanganyika is under broad, well-documented strain. O'Reilly and colleagues (2003, Nature) showed that twentieth-century warming has strengthened the lake's stratification and weakened the wind-driven mixing that lifts deep nutrients into the sunlit surface layer, reducing primary productivity and implying a substantial drop in fish yields. Cohen and colleagues (2016, PNAS) extended that picture with sediment records, tying sustained warming to declining fish abundance and a roughly 38% contraction of oxygenated bottom habitat in their study areas. On top of that sit sedimentation and nutrient loading from deforested catchments — which degrade the rocky littoral (Cohen et al. 1993) — and the intense commercial fishery for the pelagic clupeids (Stolothrissa and Limnothrissa) and the Lates predators that feed on them, a fishery supporting four nations and managed, imperfectly, through the Lake Tanganyika Authority.

For P. paradoxus, these basin-scale pressures bear on it more gently than on a shallow rocky-shore endemic, because much of its population lives offshore and at depth, away from the most disturbed inshore zone. But it is not insulated. The contraction of oxygenated deep-water habitat is precisely the kind of change that touches a fish willing to range down to 250 m, and as a parasite of the wider fish community its fortunes ultimately track the abundance of the sand- and rock-dwelling cichlids it feeds on. None of this makes the species threatened today, and Least Concern is the correct call — but, like everything in Tanganyika, its long-term security is tied to a lake whose trajectory on warming, productivity, and oxygenated habitat is pointing the wrong way.

Plecodus paradoxus

About this species