On the Age and Origin of Lake Ejagham, Cameroon, and Its Endemic Fishes

Abstract

Lake Ejagham is a small, shallow lake in Cameroon, West Africa, which supports five endemic species of cichlid fishes in two distinct lineages. Genetic evidence suggests a relatively young age for the species flocks, but supporting geologic evidence has thus far been unavailable. Here we present diatom, geochemical, mineralogical, and radiocarbon data from two sediment cores that provide new insights into the age and origin of Lake Ejagham and its endemic fishes. Radiocarbon ages at the base of the longer core indicate that the lake formed approximately 9 ka ago, and the diatom record of the shorter core suggests that hydroclimate variability during the last 3 millennia was similar to that of other lakes in Cameroon and Ghana. These findings establish a maximum age of ca. 9 cal ka BP for the lake and its endemic species and suggest that repeated cichlid speciation in two distinct lineages occurred rapidly within the lake. Local geology and West African paleoclimate records argue against a volcanic, chemical, or climatic origin for Lake Ejagham. Although not conclusive, the morphometry of the lake and possible signs of impact-induced effects on quartz grains are instead more suggestive of a bolide impact.

Speciation in sympatry with ongoing secondary gene flow and a potential olfactory trigger in a radiation of Cameroon cichlids

“…Our estimate of the timing of colonization of Lake Ejagham by the Coptodon lineage (9.76 ka ago, Figure a) was similar to the estimated age of the lake itself (9 ka years ago, Stager et al., ), suggesting that the lake was rapidly colonized by the ancestral lineage. It should be noted that this estimate in turn relies on an estimate of the mutation rate.…”

“…One possibility is that earlier speciation events did occur, but were followed by extinction. While we cannot exclude this scenario, there are no indications for environmental disruptions such as major changes in water chemistry or depth during the history of Lake Ejagham (Stager et al., ).…”

“…Despite being the second smallest (0.49 km 2 : Seehausen, ) and one of the youngest lakes (ca. 9 kya, Stager et al., ) containing endemic cichlids, Lake Ejagham contains two independent endemic cichlid radiations: two species of Sarotherodon cichlids (Neumann, ) and potentially four species of Coptodon cichlids (Dunz & Schliewen, ).…”

“…Despite being the second smallest (0.49 km 2 : Seehausen, ) and one of the youngest lakes (ca. 9 kya, Stager et al., ) containing endemic cichlids, Lake Ejagham contains two independent endemic cichlid radiations: two species of Sarotherodon cichlids (Neumann, ) and potentially four species of Coptodon cichlids (Dunz & Schliewen, ). The existence of these radiations is even more remarkable given that they are an exception to the two best predictors of endemic radiation in African cichlids: overall lake depth and sexual dichromatism (Wagner, Harmon, & Seehausen, ).…”

In search of the ice age tropics, a tribute to Prof. Daniel Livingstone and Prof. Paul Colinvaux

“…Subsequent oscillations in forest cover reflect a combination of change in precipitation and human land-use practices. Stager et al (2018) report a 14 C-dated paleoecological record from Lake Ejagham in Cameroon. Originally cored by Dan Livingstone, the sediments from this lake define the age of the lake and constrain the evolutionary window that allowed five cichlid fish to speciate.…”