Perissodus

Taxonomy & the radiation

Perissodus was erected by George Albert Boulenger in 1898 in his report on the fishes J. E. S. Moore collected during his Lake Tanganyika expedition (Proceedings of the Zoological Society of London 1898: 494–497). The type species is Perissodus microlepis, and the name is a tidy bit of descriptive Greek — perissos, "uneven" or "odd," plus odous, "tooth" — a nod to the irregular, hook-like dentition that lets these fish pry scales loose. Boulenger's original concept was broader than the genus we recognise today; names such as Perissodus burgeoni (David, 1936) and Perissodus gracilis (Myers, 1936) were later sunk into the synonymy of P. microlepis.

The genus sits in the cichlid subfamily Pseudocrenilabrinae and is the namesake of the tribe Perissodini, one of the ~12–16 endemic Tanganyikan tribes recognised by Poll (1986) and Takahashi (2003). The single most important revision is Liem & Stewart's (1976, Bulletin of the Museum of Comparative Zoology 147: 319–350), "Evolution of the scale-eating cichlid fishes of Lake Tanganyika: a generic revision." They split the scale-eaters along anatomical lines, restricting Perissodus to P. microlepis and the deepwater P. eccentricus and moving the other lepidophages — straeleni, paradoxus, elaviae, multidentatus — into the genus Plecodus. This is why hobby and even major database listings still disagree: GBIF and older aquarium literature file "Perissodus straeleni" under Perissodus, and FishBase to this day lists three valid Perissodus — microlepis, eccentricus and paradoxus — whereas Eschmeyer's Catalog of Fishes and most modern systematists keep the genus narrow (microlepis plus eccentricus) and move paradoxus, straeleni, elaviae and multidentatus into Plecodus. So the working species count is small but genuinely unsettled, depending on which authority you follow. Within the Perissodini, the tribe also contains the predatory Xenochromis and the zooplanktivorous Haplotaxodon. Molecular work by Koblmüller, Egger, Sturmbauer & Sefc (2007, Molecular Phylogenetics and Evolution 44: 1295–1305) and Takahashi, Watanabe, Nishida & Hori (2007, BMC Evolutionary Biology 7: 195) confirmed Haplotaxodon's place inside the tribe and dated the Perissodini radiation to a rapid burst roughly 1.5–2 million years ago — slightly younger than most other Tanganyikan tribes — with most lineages originating in deep water and only recently colonising the shallows. In this atlas the genus is represented by P. microlepis, the most familiar member.

Defining features

Perissodus are compact, laterally compressed, deep-bodied cichlids built entirely around a single trick: removing scales from larger fish. The defining apparatus is the mouth — small, terminal, and armed with rows of close-set, recurved, hook- or chisel-shaped teeth that work like a scraper rather than a grasping or biting jaw (Liem & Stewart 1976). The most striking feature is asymmetry: the lower jaw and entire snout are bent permanently to the left or to the right, so a "lefty" attacks prey on their right flank and a "righty" on their left (Hori 1993). Members are modest in size — P. microlepis reaches about 4.3 in (4.5 in) total length (FishBase; Maréchal & Poll 1991), and the rare, deepwater P. eccentricus is similar.

Telling Perissodus from its relatives comes down to the jaw and the teeth. The closely allied Plecodus scale-eaters (straeleni, paradoxus) tend to be larger — Plecodus straeleni runs to roughly 6 in (6.5 in) — and carry different, often broader dentition; P. straeleni in particular is famous for a dark vertical "sucker-mouth" bar and a body-colour mimicry of striped harmless cichlids. Haplotaxodon, also in the tribe, has an upturned mouth and a planktivorous build, while Xenochromis is a larger, more generalised predator. Within P. microlepis itself, colour is variable: Nshombo (1994, Journal of Ethology) documented multiple colour morphs, and a yellow-finned southern morph appears to be an aggressive mimic (Hori & Watanabe 2000), so colour alone is a poor identifier.

Range & habitat

The genus is wholly endemic to Lake Tanganyika, shared among the four riparian nations — Burundi, the Democratic Republic of the Congo, Tanzania and Zambia (IUCN 2025). Perissodus microlepis has a genuinely lake-wide distribution, recorded from the northern basin to the far south at Zambian sites such as Kasakalawe, Mbita Island, Nkamba Bay and Wonzye Point (Kovac et al. 2018, Hydrobiologia). It is most common in the shallow rocky littoral, the same scale-rich neighbourhood occupied by the Tropheus and Petrochromis it preys on, but the IUCN assessment notes it occurs across "all types of habitats." Its congener P. eccentricus, by contrast, is a deepwater specialist seldom seen by divers — consistent with the Koblmüller et al. (2007) finding that the tribe radiated mostly in deep water.

Field sampling for the diet and behaviour studies was done by SCUBA at depths of roughly 16–66 ft (16–66 ft), which is the practical band where P. microlepis overlaps its littoral prey, though the species ranges deeper. The lake's surface waters where it lives are warm and chemically stable: about 75–82°F (75–82 °F), highly alkaline with a pH near 8.6–9.0 and hard, mineral-rich, conductive water (Coulter 1991, Lake Tanganyika and its life). These are the in-situ conditions any keeper must reproduce.

Ecology & diet

Perissodus is the archetype of lepidophagy — eating the scales of other fish, a foraging mode so rare that it has evolved only a handful of times across all teleosts, and nowhere more elaborately than in the Tanganyikan Perissodini (Sazima 1983; Martin & Wainwright 2013). The hunt is an ambush: the scale-eater stalks or shadows a larger fish, darts in from the rear quarter, and rasps off a single scale or a small cluster together with a bit of epidermis before fleeing. Scales are nutritionally rich in protein, minerals and mucus, and they regenerate, making the prey a renewable resource rather than a kill.

Far from being a fussy specialist, P. microlepis is an opportunist within that niche. Kovac, Boileau, Muschick & Salzburger (2018, Hydrobiologia) DNA-barcoded more than 300 scales from the guts of southern-lake fish and identified 39 different prey species across eight cichlid tribes; the five commonest — Petrochromis polyodon, Pe. ephippium, Eretmodus cyanostictus, Tropheus moorii and Simochromis diagramma — were all herbivores and together made up over half the diet. Diet shifts with age: juveniles feed on copepods and other zooplankton and only "scale up" to lepidophagy once they reach about 1.6–2 in (1.5–2 in) (Nshombo, Yanagisawa & Nagoshi 1985; Konings 2019). The two best-known Perissodus diverge ecologically by depth — P. microlepis works the shallow rocky reefs while P. eccentricus operates in deep water — and the broader tribe spreads the niche further still, with sister Plecodus species taking different prey (Plecodus straeleni even feeds heavily on mastacembelid eels; Boileau et al. 2015) and Haplotaxodon abandoning scale-eating altogether for plankton.

Behaviour & breeding

The signature behaviour of the genus is mouth-handedness and the evolutionary dynamic it drives. Because lefties strike one flank and righties the other, prey fish learn to guard whichever side is being attacked more often, which hands a survival advantage to whichever scale-eater morph is currently rarer. Hori's landmark 1993 Science paper showed that this negative frequency-dependent selection holds the population near a 1:1 ratio of left- and right-mouthed fish over time — one of the cleanest natural demonstrations of balancing selection known. The detail has been refined since: Kusche, Lee & Meyer (2012) argued the asymmetry is a continuous distribution rather than two crisp morphs, while Takeuchi & Oda (2017, Scientific Reports) showed the lateralized attack is partly learned, fish improving by using their naturally stronger side.

Breeding is where Perissodus is genuinely interesting taxonomically, because it sits on the evolutionary boundary between substrate-spawning and mouthbrooding. P. microlepis forms a monogamous pair bond, yet it is a maternal mouthbrooder: the female lays more than 200 tiny eggs on the substrate, the male fertilises them there, and the female then takes them into her mouth (IUCN 2025; Konings 2019). Eggs hatch in about two days and fry are released roughly a week later, but parental care continues — the male holds and defends the territory and starts collecting and sheltering the free-swimming fry around day ten or eleven, once they reach about 0.5 in. Large schools of hundreds of juveniles are often seen near brooding pairs. This biparental, transitional care style is shared across the Perissodini and once prompted Fryer & Iles (1972) to suggest the tribe was ancestral to Tanganyika's mouthbrooders, an idea later phylogenetic work rejected (Salzburger et al. 2002).

In the aquarium

Perissodus microlepis turns up in the specialist Tanganyikan trade, but it is honestly a connoisseur's fish rather than a beginner's, and P. eccentricus is essentially never available. The novelty is obvious — people want to watch a fish eat scales — and that is also the first disappointment: a healthy, well-fed P. microlepis in a community of robust tankmates will mostly leave them alone, so you do not get a nightly scale-stripping show. Where it can become a problem is harassment of slow, deep-bodied, or thin-finned fish, and especially newcomers. The veteran observation, echoed by Pam Chin of the Cichlid Room Companion, is telling: once a Perissodus is settled with its tankmates it tends to ignore them, but it recognises and attacks any fish added later. The practical takeaway is to buy them young and stock the tank as a cohort, so everyone grows up together.

Plan on a tank of at least 55 gallons (around 55 US gal) with a long footprint, rockwork for cover and sightline breaks, and the hard, alkaline water the lake demands — roughly pH 8.0–9.0, high mineral hardness, and 76–82°F (75–82 °F). Feeding is the catch that trips people up: although Pam Chin and others report that captive fish convert readily to prepared foods and frozen items, this is a fish whose whole gut and dentition are tuned to scales and mucus, so a varied, high-protein diet matters and aufwuchs-style flake alone is not enough. They are bi-parental mouthbrooders that, given a settled pair, can produce big broods — but documented aquarium spawnings are scarce, which fairly marks the genus as advanced. Two further cautions apply to the whole Tanganyikan context: do not mix Perissodus with the Tropheus and Petrochromis it would attack in the wild and that are themselves bloat-prone if their diet is mishandled, and avoid keeping multiple superficially similar scale-eaters or congeners that could hybridise. This is a fish for someone who already keeps Tanganyikans well, not a first rift-lake cichlid.

Conservation

At the species level the news is reassuring: Perissodus microlepis was reassessed as Least Concern on the IUCN Red List in March 2025 (Sibomana 2025), on the grounds that it is widely distributed across the entire lake with no known major threats. It is described as very common throughout its habitat though never abundant, and there are no targeted collection or trade pressures of concern — it is a minor incidental fishery catch and only a trickle reaches the aquarium hobby. Its deepwater congener P. eccentricus is poorly studied but faces no documented species-specific threat. So the honest summary is that the genus itself is not in trouble.

The caveat is the lake. Lake Tanganyika is under real and measurable strain, and a scale-eater is only as secure as the prey community it depends on. Climate-driven warming has strengthened thermal stratification and reduced the deep mixing that fertilises the surface waters; O'Reilly et al. (2003, Nature) estimated this has cut primary productivity by around 20% over the past century, and Cohen et al. (2016, PNAS) inferred a roughly 38% loss of oxygenated benthic habitat as the oxygenated layer shoals — a direct squeeze on the deepwater zone where much of the Perissodini, including P. eccentricus, evolved. Sedimentation from deforestation and shoreline development smothers the rocky littoral that P. microlepis and its Tropheus and Petrochromis prey occupy, and the pelagic clupeid-and-Lates fishery that feeds four nations places its own pressure on the wider ecosystem. Governance is coordinated through the Lake Tanganyika Authority under the regional Convention, but enforcement is uneven. None of this currently registers as a threat to Perissodus specifically; the responsible reading is that a Least Concern species sits inside a stressed lake whose long-term trajectory bears watching.

Sources

1. FishBase — Perissodus microlepis (species summary, size, IUCN, etymology)
2. Eschmeyer's Catalog of Fishes — genus Perissodus (Boulenger 1898)
3. GBIF — Perissodus (genus, species list, Boulenger 1898 original publication)
4. IUCN Red List — Perissodus microlepis (Sibomana 2025; Least Concern, breeding biology)
5. Koblmüller, Egger, Sturmbauer & Sefc 2007 — Evolutionary history of Lake Tanganyika's scale-eating cichlid fishes (Mol. Phylogenet. Evol.)
6. Takahashi, Watanabe, Nishida & Hori 2007 — Evolution of feeding specialization in Tanganyikan scale-eating cichlids (BMC Evol. Biol.)
7. Kovac, Boileau, Muschick & Salzburger 2018 — The diverse prey spectrum of Perissodus microlepis (Hydrobiologia)
8. Hori 1993 — Frequency-dependent natural selection in the handedness of scale-eating cichlid fish (Science)
9. Takeuchi & Oda 2017 — Lateralized scale-eating behaviour acquired by learning to use the naturally stronger side (Scientific Reports 7: 8984)
10. Takeuchi, Hori, Tada & Oda 2016 — Acquisition of lateralized predation behaviour associated with development of mouth asymmetry (PLoS ONE 11: e0147476)
11. Stewart & Albertson 2010 — Evolution of a unique predatory feeding apparatus: a genetic locus for jaw laterality (BMC Biology 8: 8)
12. MIT Press Reader — Lessons from Lake Tanganyika's scale-eating fish (frequency dependence explainer)
13. Practical Fishkeeping — Tanganyikan scale-eating cichlids studied (Koblmüller et al. summary)
14. Cichlid Room Companion — Perissodus eccentricus species profile
15. Cichlid Room Companion (Ask Pam) — keeping & breeding Perissodus microlepis (Pam Chin) — community/anecdotal
16. The Cichlid Stage — the scale-eating cichlid (hobby blog overview)
17. tanganyika.si — Perissodus / Plecodus species gallery & checklist
18. PlanetCatfish Cat-eLog — Perissodus straeleni (size, husbandry notes)
19. ResearchGate — Nshombo et al. 1985, Scale-eating in Perissodus microlepis and change of food habits with growth