Taxonomy & naming
Coptodon thysi was described in 1992 by Melanie L. J. Stiassny, Ulrich K. Schliewen and Wallace J. Dominey in their landmark paper announcing 'a new species flock of cichlid fishes from Lake Bermin, Cameroon', which erected eight new tilapias in a single stroke (Ichthyological Exploration of Freshwaters 3(4):311-346). The original combination was Tilapia thysi; the holotype (ZSM 28390, in the Zoologische Staatssammlung München) and its paratype series were collected from Lake Bermin at roughly 5°09'30"N, 9°38'E in the Nguti subdivision of what was then South West Province. For most of its published life the fish sat in the catch-all genus Tilapia, the bin into which generations of workers had swept the African substrate-spawning tilapiines. That changed when Dunz & Schliewen (2013) produced a molecular phylogeny that dismantled the old 'Tilapia' and resurrected several long-available names for its component lineages; the Lake Bermin radiation, together with the widespread redbelly tilapias of the Tilapia zillii group, was placed in the genus Coptodon Gervais, 1848, and the species became Coptodon thysi. Within Pseudocrenilabrinae it belongs to the tribe Coptodonini. The nine Bermin Coptodon — thysi, bemini, bythobates, gutturosa, imbriferna, snyderae, spongotroktis, flava and bakossiorum — form a monophyletic, endemic assemblage that has become one of the most-cited African examples of speciation within a single small lake.
Morphology
Coptodon thysi is a small cichlid: FishBase gives a maximum of about 2.5 in standard length, and even the largest members of the Bermin flock top out near 7 in, with most species running considerably smaller in life. The body is the typical compact, laterally compressed tilapiine oval. Meristics from the original description and FishBase are diagnostic at the genus and species level rather than flashy: 16 dorsal-fin spines with 10-11 soft rays, 3 anal spines with 7-8 soft rays, and 11-13 gill rakers on the first arch. The mouth is not squared or obtuse, the ventral margin of the lower lip is smooth, and the oral teeth are slender and movably implanted with spatulate (flattened, spoon-shaped) crowns — the bicuspid grazing dentition expected of an algae-and-detritus feeder rather than a predator. The lower pharyngeal jaw is robust and of moderate size, carrying a large heart-shaped tooth plate paved with numerous fine, regularly arranged teeth. Because the nine Bermin species are so closely related and overlap heavily in body shape and colour, individual identification in the lake rests largely on this kind of subtle jaw, tooth and proportion data; the species are far easier to tell apart as a flock than one at a time. Sexual dimorphism follows the tilapiine norm — males average a little larger and become more intensely coloured and territorial when breeding, while ripe females are rounder-bodied — but it is modest and not dramatically marked.
Habitat
Coptodon thysi is endemic to Lake Bermin and is found nowhere else on Earth. Bermin is a small volcanic crater lake on the 'Cameroon Line', the chain of extinct volcanoes that runs southwest-to-northeast across southwestern Cameroon; it sits in the forested Southwest Region near Nguti. The lake is tiny and roughly circular — most sources put its diameter at about 2297 ft and its surface area at a little over 0 mi² (a regional fish inventory gives about 60 ha) — and it is shallow for a crater lake, with a maximum depth on the order of 46–49 ft. It has no inflowing streams and only a single small outlet, which keeps the basin biologically isolated and is a large part of why an endemic flock could evolve there at all. The water is warm, soft and only weakly mineralised: hobby and field accounts describe surface temperatures around 75 °F and a near-neutral to mildly alkaline pH of roughly 7.5 with low alkalinity, while the genus tolerates a broader West-African lowland envelope of about pH 6.5-8.0 and 75–86 °F. The fish themselves are benthopelagic, working the shallow vegetated and rocky-to-soft margins where algae, detritus and aufwuchs accumulate. Bermin belongs to the Western Equatorial Crater Lakes freshwater ecoregion (FEOW 519), the same volcanic-lake province that produced the better-known endemic cichlid flock of nearby Lake Barombi Mbo.
Feeding
Coptodon thysi is a grazer and generalist detritivore rather than a predator. FishBase records it feeding primarily on plants — especially algae — together with diatoms and insects, and within the Bermin flock it is one of the eight 'generalist' species that share a broad diet of algae, aufwuchs, detritus, phytoplankton and small invertebrates. Its slender, spatulate-crowned oral teeth and finely toothed pharyngeal plate are the equipment of an animal that rasps and processes soft plant and organic matter off surfaces. This dietary sameness is precisely what makes the Bermin radiation so puzzling: most of the nine species overlap heavily in what they eat and where they live, so classic resource partitioning cannot, on its own, explain how they diverged. Only two of the lake's species are clear trophic specialists — Coptodon imbriferna, a phytoplankton feeder, and the remarkable Coptodon spongotroktis, which has evolved to eat the lake's freshwater sponges, glassy silica spicules and all. Coptodon thysi sits firmly with the generalist majority, occupying a trophic level estimated near 2.8 and functioning as one of the lake's many overlapping algae-and-detritus grazers.
Mating
Like the rest of its genus, Coptodon thysi is a monogamous, pair-bonding substrate spawner, and the social action centres on a defended breeding territory rather than a lek or a mouthbrooding female. As fish approach spawning they pair off, the male intensifying in colour, and the pair jointly clears and guards a patch of substrate. In the wild these are open-substrate spawners — pairs select and clean an exposed surface rather than a hidden cave. Outside of breeding the Bermin Coptodon are not strongly aggressive toward one another and live more or less communally, which is part of how nine species coexist in so small a basin; aggression flares mainly around active nests. Maturity comes early and at a small size — within the flock, females of the dwarf species Coptodon snyderae can ripen at around 1 in, and hobbyists report Coptodon thysi-type Bermin tilapias spawning while still only a couple of inches long — so the population turns over its breeding territories quickly. The precise courtship sequence of Coptodon thysi has not been described in isolation from its congeners, but it conforms to the well-documented tilapiine pattern of mutual substrate-cleaning, lateral displays and joint territory defence.
Breeding
Coptodon thysi is a biparental open-substrate brooder, the ancestral and defining reproductive mode of the genus — both sexes guard the eggs and the resulting brood, in sharp contrast to the maternal mouthbrooding of Sarotherodon and Oreochromis tilapias. In the wild the pair deposits a clutch of adhesive eggs on a cleaned, exposed surface; both parents fan and defend the eggs, then shepherd the free-swimming fry as a school. A precise fecundity figure for Coptodon thysi has not been published, but the mode is well documented and the genus is typically modest in clutch size: a close Bermin congener, Coptodon bakossiorum, lays clusters of only about 30 eggs, and Bermin Coptodon as a group are noted as devoted, capable parents that raise broods successfully even when the female is barely mature. Parental care is biparental and prolonged through the guarding stage. In captivity the species shifts its behaviour slightly: although wild pairs spawn in the open, aquarium females often choose more sheltered sites — inside flowerpots, ceramic caves or among hardscape — and will defend their fry fiercely, sometimes to the point of dominant females stealing and adopting the broods of subordinate ones. Fry are easy to start on newly hatched brine shrimp, microworms and powdered fry foods and grow quickly, reaching breeding size themselves within roughly half a year.
In the aquarium
The Lake Bermin Coptodon, including Coptodon thysi, are kept by a small circle of dedicated hobbyists — largely through conservation-breeding programmes such as C.A.R.E.S. — and they are surprisingly undemanding for fish with such a precarious wild status. They are medium-small, generally peaceful toward other species, and adaptable to ordinary tap water. A tank of about 35.5 in length is a sensible minimum even for the smaller species, because the fish can be a little scrappy among themselves; the usual advice is to start with a group of six to eight juveniles and let pairs form. The single big behavioural caveat is spawning: Bermin Coptodon are amiable until a pair sets up a nest, at which point it is best to move out the other fish (or have a tank divider ready), as a defending pair will otherwise clear the tank on its own terms. Decorate with driftwood, rock piles, ceramic caves and upturned flowerpots to break sightlines and give subordinate fish refuge — and note that live plants are treated as salad, not scenery, with even Anubias, Java fern and Java moss eaten over time. Water parameters need not be extreme: wild Bermin water is around 75 °F, near pH 7.5 and low in hardness, and the fish thrive across a moderate range so long as conditions are stable and clean. Large, regular water changes (around 50% weekly) and strong biological filtration are appreciated; sponge filters are a poor choice here, because these tilapias — relatives of the sponge-eating Coptodon spongotroktis — will graze synthetic sponge. Feeding is easy: a quality herbivore or mbuna-type pellet rich in vegetable matter, supplemented with blanched greens, peas and the occasional small offering of live or frozen food, suits their algae-and-detritus diet, and an algae film left to grow on the back and side glass gives them something to graze. Heavy, protein-rich carnivore diets should be avoided as a long-term staple. These are good aquarium fish kept for the right reasons; the main argument for keeping them is that maintaining captive populations is, for a single-lake endemic like this, a genuine conservation hedge.
Conservation
Coptodon thysi is assessed by the IUCN Red List as Critically Endangered, under criteria B1ab(iii)+2ab(iii) (assessed 16 February 2009 by T. Moelants; published 2010). The reasoning is geographic above all: the species' entire range is one tiny crater lake of well under one square kilometre, so any single event that degrades Lake Bermin threatens the whole population at once. The documented and potential pressures are real. Oil-palm plantations and slash-and-burn agriculture in the surrounding catchment drive sedimentation and pollution into the lake. Beyond that lies the distinctive hazard of the Cameroon crater lakes themselves: deep, gas-charged lakes in this region — most notoriously Lake Nyos in 1986 — can undergo catastrophic limnic overturns that release dissolved CO2 and asphyxiate everything in and around them. Bermin's shallowness makes a Nyos-style gas burst less likely than in the deep lakes, but deforestation of the crater rim could increase wind and wind-driven mixing, potentially turning over the lake's stratified layers and bringing deoxygenated, organic-rich bottom water up into the zone where the fish live — a mixing event that could cause a mass die-off. Because all nine endemic Coptodon share this one basin, the conservation case for ex-situ breeding is unusually strong, and the Bermin tilapias are actively maintained as CARES priority species in the hobby precisely so that a single bad year in Cameroon does not erase the entire flock.