Western Equatorial Crater Lakes

The Western Equatorial Crater Lakes are a scatter of small, deep, mostly circular lakes in the rainforest hills of southwestern Cameroon — droplets of water sitting in the throats of old volcanoes along the Cameroon Volcanic Line. None of them is large. The biggest, Barombi Mbo, covers barely 2–3 mi²; most of the rest are smaller than a square kilometre, and one of the famous flocks lives in a crater only about 2297 ft across. Yet for evolutionary biology these ponds punch far above their size. Several of them hold their own private radiation of cichlid fishes — a whole cluster of closely related species found in that one lake and absolutely nowhere else — and in each case the fish appear to have multiplied in place, inside a basin too small and too uniform to have ever split a population in two. Barombi Mbo has eleven endemics, Lake Bermin nine, Lake Ejagham five, and the smaller lakes Barombi ba Kotto and Dissoni a few more. This is the textbook ground for sympatric speciation, the controversial idea that new species can bud apart while living side by side. It is also a conservation emergency: the lakes are tiny and closed, the endemics are mostly Critically Endangered, and the same volcanic plumbing that built the crater lakes can, in the worst cases nearby, turn them lethal.

Geography & hydrology

The lakes belong to the Cameroon Volcanic Line, a chain of volcanic centres running southwest-to-northeast from the Atlantic island of Bioko inland across Cameroon, anchored at its coastal end by the active volcano Mount Cameroon. The line has been producing craters and calderas for roughly 25 million years, and around three dozen crater lakes — about 36 are now catalogued — dot the highlands and lowlands of the country's Southwest and Northwest regions. The freshwater ecoregion that ichthyologists call the Western Equatorial Crater Lakes gathers the cichlid-bearing members of this set: Barombi Mbo, Bermin (Beme), Dissoni (also called Soden), Barombi ba Kotto, Mboandong, and the geological oddity Ejagham, among others. All lie wholly within Cameroon and drain, where they drain at all, toward the Gulf of Guinea.

Most of these basins are maars or explosion craters — holes blasted out by violent steam-driven eruptions and then filled with rainwater and seepage. The defining trait is isolation. Most have no outflow, or an outflow so short and steep that fish cannot use it, which seals each lake off from the surrounding river network. Barombi Mbo, the largest and best studied, sits at about 984 ft above sea level near the town of Kumba; it spans roughly 2–3 mi², plunges to about 361 ft, and sediment cores date it to roughly one million years old — among the oldest crater lakes in West and Central Africa. Lake Bermin lies further north and is far smaller, little more than 0 mi² and only about 2297 ft across. Barombi ba Kotto, south of Barombi Mbo, covers around 2 mi² at just 361 ft elevation, and Lake Dissoni in the Rumpi Hills about 2 mi².

Lake Ejagham is the misfit of the group and a useful one. It is not, strictly, a volcanic crater lake: it sits on Cretaceous sedimentary rock at the edge of the Mamfe basin, oval rather than circular (about 1,050 by 2297 ft), shallow at roughly 56–59 ft, and ringed by only a low rim. Earlier work guessed it was a solution basin dissolved out of soluble rock, but sediment-core study (Stager et al. 2017) found no karst and instead radiocarbon-dated the lake's formation to about 9,000 years ago, tentatively favouring a small bolide (meteorite) impact as the likeliest origin. Its outlet is cut off from the nearby Munaya River by a waterfall that cichlids cannot climb — so, isolated like the true craters, it grew its own endemic fauna in a fraction of the time. The whole region is drenched: equatorial monsoon rains of roughly 3,0.5 in a year fall between about April and October, with a short drier spell from roughly November to February.

Water & habitat

Chemically these are soft, dilute, near-neutral freshwaters — nothing like the hard, alkaline Rift lakes of East Africa. Surface pH runs circumneutral, generally in the 7s, and conductivity is low: Barombi Mbo measures around 49 microsiemens per centimetre at the surface; Ejagham runs a little higher, roughly 86 to 120 µS/cm, possibly because saline springs leak into the Mamfe basin around it. The water that the lakes do carry — modest bicarbonate, calcium and magnesium — comes mostly from streams draining the forested catchment rather than from the rock of the crater itself. Tannins from rainforest leaf-fall tint the inshore margins tea-brown, which is why aquarists recreating a Cameroon crater-lake biotope reach for sand, leaf litter and driftwood under dim, slightly acidic water rather than the bright rockwork of a Malawi tank.

The deep lakes share a defining physical trait that shapes everything living in them: strong, year-round stratification. Warm, light surface water floats on cold, dense deep water and the two almost never mix, so oxygen is confined to a thin sunlit upper layer while the great volume of deep water below the oxycline is permanently anoxic — a dark, lifeless monimolimnion. In Barombi Mbo the habitable lake is essentially the top 66–131 ft; everything beneath is off-limits to fish. The shallow lakes such as Ejagham and Barombi ba Kotto mix more freely and lack so severe a dead zone, which partly explains why the deep-water specialists evolved only in the deep crater. Within that habitable shell, the inshore zone offers a surprising spread of microhabitats — rocky shelves near the crater walls, submerged wood and leaf litter, sandy and silty bottoms, and open water near the surface — and the endemic cichlids partition it finely. Several of the lakes also carry their own endemic invertebrates, including freshwater sponges and shrimps that some of the cichlids have evolved to eat.

The cichlid fauna

What makes this ecoregion famous is that several lakes each hold a monophyletic flock — a cluster of endemic cichlids all descended from a single colonising ancestor, all of them found in that one lake and nowhere else. Barombi Mbo is the flagship, with eleven endemic species in five genera, four of which (Konia, Stomatepia, Pungu, Myaka) exist only in that crater, plus an endemic radiation of Sarotherodon. Its specialists are extreme for so small a place: Pungu maclareni eats freshwater sponge; the deep-water Konia dikume carries an unusually high blood-haemoglobin load that lets it forage at the very edge of the anoxic zone; others take fish, insect larvae or open-water plankton. Lake Bermin holds a separate radiation of nine endemic cichlids, all in the genus Coptodon and only distantly related to the Barombi Mbo fish — among them the sponge-eater Coptodon spongotroktis and the phytoplankton-feeder Coptodon imbriferna. Most of the Bermin species were described only in 1992 (Stiassny et al.). Smaller lakes carry their own endemics too: Coptodon kottae of Barombi ba Kotto (which also occurs in Mboandong), and the Coptodon and Sarotherodon lineages of Dissoni.

Lake Ejagham is the most peculiar of all. In a basin only about 9,000 years old it grew not one but two separate endemic flocks side by side — a Coptodon group (including the large predator Coptodon ejagham, the benthivore C. deckerti and the zooplanktivore C. fusiforme) and a Sarotherodon pair — making it the only crater-lake lake known to host multiple speciating cichlid lineages at once. That a tiny, shallow, geologically young pond produced this much divergence so fast is part of what made these lakes a landmark.

The scientific story is genuinely contested, and the honest version matters. Schliewen, Tautz and Pääbo's 1994 Nature paper showed the Barombi Mbo flock to be monophyletic, and because the crater is too small and uniform to plausibly split a fish population, they argued for sympatric speciation — divergence without geographic isolation. Schliewen and Klee (2004) added that one Barombi Mbo species, Pungu maclareni, looks like a hybrid of two of the lake's own lineages. But more recent genome-scale work (Martin et al. 2015; and see Stager et al. 2017) found signs that several of these lakes were colonised by riverine cichlids more than once, with gene flow continuing after the first arrival — which would complicate, and in places undercut, the clean sympatric story. The current picture is messier and more interesting than the textbook: some splits probably did happen in sympatry, while others may have been seeded or assisted by repeated colonisation and hybridisation. Either way, the radiations are real, fast, and found nowhere else.

Conservation

Every one of these flocks faces the same brutal arithmetic: a single small lake, no second range, and nowhere to retreat. The Barombi Mbo cichlids are all listed by the IUCN as Critically Endangered, and several Bermin and other crater-lake endemics carry CR or EN assessments; many are CARES conservation-priority species and the focus of European zoo and aquarium breeding programmes precisely because the wild populations are so precarious. Stomatepia mongo, the rarest of the Barombi Mbo fish, has been repeatedly feared extinct since the 1980s and confirmed to survive only through careful fieldwork (Musilová et al. 2014).

The pressures are local and direct. The lakes sit in a populous, fast-developing corner of Cameroon, and most double as drinking-water sources and fisheries for the villages on their rims and the towns nearby. Deforestation and farming — cocoa, oil palm, slash-and-burn plots — strip the catchment slopes and feed sediment and nutrients into closed basins that cannot flush them out; documented threats to Coptodon kottae and its neighbours include sedimentation, pollution and deoxygenation driven by agriculture and plantations. Village runoff and sewage have driven measurable eutrophication and bacterial contamination in Barombi Mbo. Introduced fish are a growing danger: a breeding population of an introduced catfish has established in Lake Ejagham, and any introduction of a hardy generalist such as Nile tilapia into one of these closed lakes could overwhelm a small endemic flock outright.

Overlaid on all of this is the volcanic hazard that built the lakes in the first place. The Western Equatorial Crater Lakes share their province with Lakes Nyos and Monoun, the two Cameroonian crater lakes whose deep water saturated with carbon dioxide and then burst to the surface in sudden 'limnic eruptions' in the 1980s, killing nearly 1,800 people. The deep, permanently stratified lakes of this ecoregion store dissolved gas in the same way, and fishermen at Barombi Mbo have reported episodes of cloudy water and dead fish consistent with gas release or upwelling of anoxic bottom water — a reminder that the very stratification that made these lakes evolutionary cradles also makes them, in the wrong circumstances, dangerous. For a fauna found nowhere else on Earth, that combination of tiny range, mounting human pressure and geological risk is exactly what makes these small lakes both a marvel and an emergency.

Where these cichlids live

Geography & hydrology

Water & habitat

The cichlid fauna

Conservation

Sources

Cichlids recorded here

Coptodon · 11

Konia · 2

Myaka · 1

Pungu · 1

Sarotherodon · 3

Stomatepia · 2