Plecodus

Taxonomy & the radiation

Plecodus was erected by George Albert Boulenger in 1898, with Plecodus paradoxus Boulenger 1898 as its type species (type by monotypy; holotype MRAC P-189, collected on the Belgian Lake Tanganyika expedition and still held at the Royal Museum for Central Africa). The name is a neat description of the fish: Greek plekein, 'to fold,' plus odous, 'tooth' — a reference to the bizarre, laterally folded, leaf-shaped jaw teeth that distinguish the genus. Boulenger separated it from the closely allied Perissodus (Greek perissos, 'uneven,' + odous), which he had described the same year for the same scale-eating guild.

The genus sits in family Cichlidae, subfamily Pseudocrenilabrinae, within the endemic Tanganyikan tribe Perissodini — the lake's dedicated scale-eaters, today usually counted as roughly nine species across Perissodus, Plecodus, Xenochromis and the plankton-feeding Haplotaxodon. Poll's 1986 classification of Tanganyikan cichlids and earlier work by Liem & Stewart (1976) split the scale-eaters on oral-tooth shape into a 'Plecodus group' (P. paradoxus, P. elaviae, P. straeleni, with broad, laterally bladed teeth) and a 'Perissodus group' (P. microlepis, P. eccentricus). This atlas lists Plecodus with four species — P. straeleni, P. paradoxus, P. elaviae and P. multidentatus.

Here the genus runs into a live, unresolved problem. WoRMS still lists Plecodus as an accepted genus and FishBase indexes several species under it, but Eschmeyer's Catalog of Fishes now treats Plecodus as a synonym of Perissodus, consistent with the molecular work and Konings (2015, 2019) treatments that fold these species into a single broad Perissodus. The reason is phylogenetic: AFLP and mitochondrial analyses by Takahashi, Watanabe, Nishida & Hori (2007, BMC Evolutionary Biology) and by Koblmüller et al. (2007, Molecular Phylogenetics and Evolution) recovered a Perissodini in which the traditional tooth-based 'Plecodus group' is not monophyletic — P. straeleni instead clusters with Perissodus microlepis, with P. paradoxus as their likely sister, while P. multidentatus falls out near the base. In other words, Plecodus as Boulenger conceived it does not correspond to a single branch of the tree, which is why many authorities now subsume it. FishBase indexes these fish under both names: it lists three species under the Plecodus heading and indexes paradoxus under Perissodus, so this atlas's grouping reflects the broader traditional concept rather than a settled four-species genus. We retain Plecodus as the working genus for this page while flagging that a serious keeper will see the same fish sold and published as Perissodus.

Defining features

The genus is defined from the mouth outward. Plecodus species carry a single row of enlarged, recurved oral teeth on each jaw whose crowns are widened sideways into a leaf or blade shape — in P. straeleni, P. paradoxus and P. elaviae each tooth bears a laterally widened, leaf-shaped crown with sharp lateral edges, working like a scraper to lever scales off a prey flank (Takahashi et al. 2007). This is the trait that gave the genus its name and the single most reliable way to separate it from look-alikes. P. multidentatus departs from this pattern with many more, smaller, backward-recurved teeth on slim elongated stems — fitting its habit of taking skin as well as scales.

In body these are deep-bodied, laterally compressed cichlids of modest size for the lake. P. straeleni is the one most often measured and runs to about 6.3 in (6.5 in) total length; P. paradoxus is larger, commonly cited to roughly 12 in (12 in). The genus generally shows a fairly steep forehead and a small, terminal-to-slightly-subterminal mouth that, in life, can be protruded and skewed to one side during an attack — the celebrated lateralized 'handedness' of scale-eaters, most studied in the sister Perissodus microlepis but echoed across the tribe.

Distinguishing Plecodus from other Tanganyikan cichlids is mostly a matter of that toolkit of teeth plus the deceptive color of P. straeleni. The genus is most easily confused with Perissodus proper (the tooth-shape difference is the standard separator) and, in the field, with the harmless cichlids that P. straeleni evolved to resemble. It should not be confused with the unrelated sucker-mouth catfish genus Plecostomus despite the similar-sounding name.

Range & habitat

Every species in the genus is endemic to Lake Tanganyika, the world's second-oldest and second-deepest lake, shared by Tanzania, the Democratic Republic of the Congo, Burundi and Zambia. The Catalog of Fishes records the type species P. paradoxus from Lake Tanganyika and its outflow, the Lukuga River drainage — the lake's only major outlet — so the genus's footprint is essentially the lake basin itself, with marginal occurrence in the outflowing river. GBIF's pooled museum records for P. paradoxus span collections from 1912 to 2008 around the lake's perimeter.

Within the lake the genus partitions habitat strongly by depth, and this is where the species diverge most. Takahashi et al. (2007) found P. straeleni mainly on shallow rocky regions shallower than about 230 ft (230 ft); P. paradoxus across an unusually wide band from roughly 6 to 530 ft (7–531 ft), bridging shallow rocky shores and deep water; and the remaining species — including P. elaviae and P. multidentatus — taken only deeper than about 130 ft (131 ft). The shallow-water members are reef-and-rubble fish, hunting along the rocky littoral where their cichlid prey are densest, while the deepwater members occupy the dim, sediment-dusted slopes below the recreational dive zone. P. paradoxus is also reported to gather in large schools, sometimes exceeding 500 individuals. In-situ Tanganyikan water is alkaline and stable: pH about 8.6–9.2, conductivity near 600–620 µS/cm, and a warm surface around 75–82 °F (75–82 °F); the genus's deepwater members live at the cooler, oxygen-limited margins of that envelope rather than in the well-mixed surface.

Ecology & diet

Plecodus is built around lepidophagy — eating the scales of other living fish — one of the most specialized and rarest trophic niches among the world's cichlids, and the defining habit of the tribe Perissodini. In a stomach-content survey across the tribe, Takahashi et al. (2007) found that in the specialized scale-eaters scales (mostly of cichlids) made up roughly 90% of the diet; the prey fish survive the attack, which makes these cichlids functional ectoparasites of the lake's other fishes rather than conventional predators. The genus's leaf-bladed teeth are the working tool: the fish darts at a victim's flank, twists its skewed jaws against the body, and scrapes or wrenches loose a mouthful of scales.

The genus also preserves the evolutionary gradient out of which scale-eating arose, which is its real scientific value. Takahashi et al. reconstructed scale-eating as having evolved from general carnivory, and — strikingly — as having originated in deepwater habitats before radiating shallow. P. multidentatus sits near that transition: its diet is roughly half fish skin and half scales (largely of clupeids, the lake's sardine-like Limnothrissa and Stolothrissa), and because the skin is always found with scales it appears to bite whole pieces from a prey's flank rather than neatly lifting scales — a 'skin-eater' more than a pure scale-eater. P. straeleni is the outlier at the other end: scales make up less of its diet than in its congeners, because much of its foraging is bound up with a deceptive, stalking attack on a wide range of species. Across the genus, then, the niche shades from skin-and-scale biting in the deep to refined, mimicry-assisted scale-scraping on the reef — a textbook case of trophic divergence within a single small lineage.

Behaviour & breeding

Behaviorally the headline is aggressive mimicry, best documented in P. straeleni. Boileau et al. (2015, Biology Letters), working in southern Lake Tanganyika, showed that P. straeleni wears a characteristic blue-striped pattern closely resembling harmless model cichlids — long associated in the hobby with Neolamprologus sexfasciatus and the bulky Cyphotilapia frontosa — and uses that resemblance to approach prey before lunging for a bite. Their key, and slightly subversive, finding from sequencing the actual scales in the gut was that P. straeleni does not preferentially bite the species it resembles; instead it attacks a broad variety of dissimilar-looking fish, suggesting the mimicry works by lowering the guard of many bystander species rather than parasitizing one model. Field observation by Nshombo and others (foraging study published 1994) tracked individual P. straeleni by SCUBA and recorded solitary, opportunistic hunting along rocky slopes. The genus's scale-eaters are loners outside breeding: cichlid-forum.com and Cichlid Room Companion notes both describe P. straeleni as hunting alone and refusing to tolerate conspecifics, while P. paradoxus by contrast aggregates in large schools.

Reproduction in the tribe is maternal mouthbrooding, the dominant breeding mode among Tanganyika's open-water and rocky cichlids — eggs are fertilized and then carried and incubated in the female's buccal cavity, with no substrate nest or cave-spawning of the kind seen in the lake's Lamprologus-group cichlids. Detailed in-situ breeding accounts for Plecodus specifically are thin in the literature, a gap worth stating honestly rather than papering over; the best-studied reproductive and developmental biology in the tribe comes from the sister Perissodus microlepis, where mouthbrooding and biparental care of fry are documented. Spawning is presumed to track the lake's stable thermal regime rather than a strong season.

In the aquarium

Be honest: this is a genus for the specialist, and only one species, P. straeleni, turns up in the hobby with any regularity — and even that one is uncommon. Cichlid Room Companion's own notes call P. straeleni rarely seen, capable of exceeding 6 in (6.5 in), and naturally detrimental to tankmates, because its whole biology is built around biting chunks out of other fish. That is the central, unavoidable problem of keeping the genus: a healthy Plecodus will harass and scale-bite its tankmates, and there is no training that out. Most aquarists who keep one do so as a curiosity in a species-style setup or alongside fast, large, thick-scaled robust Tanganyikans that can shrug off the occasional nip.

Realistically, plan on a long tank of at least 75 gallons (about 75 US gal) for a single P. straeleni, with hard, alkaline water (pH around 8.5–9.0), warm stable temperatures near 77–80 °F (77–81 °F), strong filtration and rockwork that breaks sightlines. They are solitary by nature and do not tolerate conspecifics, so the usual cichlid trick of a crowd to spread aggression backfires. Feeding is the other honest hurdle: you cannot supply live scales, so keepers wean them onto meaty frozen and prepared foods (mysis, krill, brine shrimp, quality carnivore pellets), accepting that captive specimens are eating a substitute for their natural prey. Two recurring mistakes deserve flagging — first, treating a scale-eater as a peaceful community fish because it 'looks' like a harmless Cyphotilapia or Neolamprologus, which is exactly the deception the fish evolved; second, the broader Tanganyikan pitfall of an over-rich, over-fed diet driving bloat in alkaline-water cichlids. None of the deepwater members (P. elaviae, P. multidentatus, P. paradoxus in its deep form) are realistic aquarium subjects. There is no beginner species here; the whole genus is advanced, more interesting to read about than easy to keep.

Conservation

Every Plecodus species is a Lake Tanganyika endemic, so the genus's fate is bound to the lake's. On the IUCN Red List the assessed members sit at Least Concern — P. straeleni and P. paradoxus are evaluated as widespread, lake-wide fishes without evidence of major decline — and there is no significant targeted collection or aquarium-trade pressure on the genus, since these scale-eaters are scarce in the hobby and not a food fishery. That is the accurate headline: the species themselves are not currently flagged as threatened.

But Least Concern at the species level does not mean the lake is healthy. Tanganyika is warming and stratifying more strongly, which suppresses the deep mixing that lifts nutrients to the surface; O'Reilly et al. (2003, Nature) linked that warming to an estimated ~20% decline in primary productivity, the base of the food web these fish ultimately sit atop. Cohen et al. (2016, PNAS) found a ~38% loss of oxygenated benthic habitat as the oxygenated layer thins — a direct squeeze on exactly the deepwater rocky and slope habitats where the genus's deep specialists (P. elaviae, P. multidentatus, and the deep form of P. paradoxus) live and hunt. Sedimentation from deforestation and shoreline development continues to smother the rocky littoral that the shallow scale-eaters and their cichlid prey depend on. Overlaid on all this is the lake's pelagic clupeid-and-Lates fishery, which feeds millions across four nations and removes the very clupeid fry that appear in P. multidentatus stomachs. Governance is coordinated through the Lake Tanganyika Authority under the Convention on the Sustainable Management of Lake Tanganyika. The honest summary: the genus is not endangered today, but it is a deepwater-leaning, single-lake lineage exposed to precisely the warming-driven loss of oxygenated deep habitat that the lake is now experiencing.

Sources

1. Eschmeyer's Catalog of Fishes — Plecodus paradoxus (type species, authority, status)
2. World Register of Marine Species — Plecodus Boulenger, 1898
3. FishBase — Plecodus species list
4. FishBase — Plecodus straeleni summary (etymology, size, endemism)
5. GBIF — Plecodus paradoxus (holotype MRAC P-189, occurrences, range)
6. Encyclopedia of Life — Plecodus paradoxus (size to 30 cm)
7. iNaturalist — Perissodus straeleni (endemism, 16 cm TL)
8. Takahashi, Watanabe, Nishida & Hori 2007 — Evolution of feeding specialization in Tanganyikan scale-eating cichlids (BMC Evol. Biol.)
9. Koblmüller et al. 2007 — Evolutionary history of Lake Tanganyika's scale-eating cichlid fishes (Mol. Phylogenet. Evol.)
10. Boileau et al. 2015 — A complex mode of aggressive mimicry in a scale-eating cichlid fish (Biology Letters)
11. Boileau et al. 2015 — mimicry paper, full text (blue-striped morph, model resemblance)
12. Foraging behavior of the scale-eater Plecodus straeleni in Lake Tanganyika (1994, SCUBA field study)
13. Scale-eating Habits of African Cichlid Fishes (Nature, note on genus Plecodus)
14. Lessons From Lake Tanganyika's Scale-Eating Fish (MIT Press Reader — frequency dependence in scale-eaters)
15. Cichlid Room Companion / cichlidae.com — feeding behavior of P. straeleni (mimicry, model species)
16. tanganyika.si — Perissodus/Plecodus multidentatus & straeleni profiles (biotope, coloration)
17. Cichlid-Forum.com — Plecodus straeleni keeping thread (rarity, temperament, size) — community/anecdotal
18. O'Reilly et al. 2003 — Climate change decreases aquatic ecosystem productivity of Lake Tanganyika (Nature)
19. Cohen et al. 2016 — Climate warming reduces fish production and benthic habitat in Lake Tanganyika (PNAS)
20. Lake Tanganyika Authority — Convention on the Sustainable Management of Lake Tanganyika