Ecological studies on crater lakes in West Cameroon Lakes Kotto and Mboandong

Lake Kotto is a shallow crater lake, sometimes weakly stratified, and with a dense phytoplankton dominated by blue-green algae. The ecology of Lake Kotto and the similar, smaller lake Mboandong is described in relation to the feeding biology of their fishes. Of the five species of cichlids in Lake Kotto only one is endemic. Two are phytoplankton-feeders, one takes invertebrates as well as phytoplankton, one feeds mainly on chironomid larvae, and one preys on vertebrates as well as on invertebrates. The three phytoplanktivores are the main species eaten by man. One species of Clarias and one of Barbus also occur in Lake Kotto; and in the associated streams are four species of cyprinodont fishes. The fish fauna of Mboandong consists of three species of cichlids and two cyprinodonts, all known from the Kotto system. We contrast Lake Kotto with the oligotrophic lake Barombi Mbo, and conclude that Lake Kotto would probably be less sensitive to human interference than would Barombi Mbo.     

Kleptoparasitism of freshwater crabs by cichlid fishes endemic to Lake Barombi Mbo,Cameroon, West Africa

Synopsis Although kleptoparasitism, or food-stealing, is common in birds and some carnivorous mammals, reported instances are exceedingly rare in fishes. We studied the behaviour and ecology of the 11 endemic tilapiine cichlids of Lake Barombi Mbo, Cameroon, West Africa. Here we report that six of the Barombi cichlids (Stomatepia mariae, S. pindu, Konia eisentrauti, Pungu maclareni, Sarotherodon lohbergeri, andS. linnellii) kleptoparasitize freshwater crabsPotamon africanus.     

Ecological studies on crater lakes in West Cameroon The blood of endemic cichlids in Barombi Mbo in relation to stratification and their feeding habits

The West Cameroonian crater lake Barombi Mbo is stratified, with no detectable oxygen below 20 m. Of the endemic cichlid fishes in this lake Konia dikume is exceptional in having a mean haemoglobin concentration of 16.55 g/100 ml, while the other ten endemic cichlids range from 5.55 to 8.70 g/100 ml blood. The erythrocyte count is also higher in K. dikume than in the other species.
The phytoplankton in the lake is most abundant between 10 and 15 m from the surface.
The single species of cyclopoid copepod and the rotifers in the zooplankton remain in the top 20 m throughout the day and night, but the larvae of Chaoborus spend the daytime at depths below 20 m, mostly between 30 and 70 m. At night these larvae ascend into the upper layers, and feed on rotifers.
Konia dikume feeds on the larvae of Chaoborus , and the high concentration of haemoglobin in the blood of this fish, coupled with a large blood volume, appears to be an adaptation for the storage of oxygen. This would enable the fish to dive into the deoxygenated hypolimnion and extend the time available for feeding when the Chuoborus larvae are ascending at dusk and descending at dawn. Konia dikume appears to be a visual feeder, and in the short tropical twilight any extension of the feeding period could be of survival value.     

Visual adaptation could aid sympatric speciation in a deep crater lake

Allopatric speciation was originally suggested to be the primary mechanism of animal speciation (Mayr, 1942; Figure 1). During allopatric speciation, populations diverge when gene flow is reduced across significant biogeographic barriers. Sympatric speciation, where species diverge while inhabiting the same location, was thought to be essentially impossible. However, the advent of theoretical models followed by new experimental evidence made sympatric speciation more plausible (Via, 2001). The cichlid fishes of Barombi Mbo, a small crater lake in western Cameroon, became one of the most widely accepted examples of sympatric speciation (Schliewen, Tautz, & Paabo, 1994). Although the phylogenetic history of this clade is not quite as simple as originally thought, it remains one of the best examples of sympatric speciation (Richards, Poelstra, & Martin, 2018). However, little is known about the molecular mechanisms contributing to the splitting of these species in situ. In a From the Cover article in this issue of Molecular Ecology, Musilova et al. (2019) focus on the diversity of visual systems among these fishes. They identify genetic changes associated with several aspects of visual adaptation that may have contributed to the ecological specialization and sympatric speciation of cichlids in this lake.     

Notes on the biology of the mayfly Povilla adusta in West Africa

A study of the mayfly Povilla adusta Navas in five West African lakes showed at least one population (in Barombi Mbo) with a clear lunar periodicity of emergence, and at least one without, at Kainji Lake. The seasonal distribution of Povilla eggs in fish stomachs at Kainji implies a delayed effect of lake level on adult numbers. The relatively small size and short generation time (probably three months) of Povilla in Barombi Mbo, by comparison with the same species in Lake Victoria, may be related to the higher temperature of Barombi Mbo. The eggs of bdelloid rotifers were found attached to Povilla larvae from some of the lakes.     

Ecological studies on crater lakes in West Cameroon Zooplankton of Barombi Mbo, Mboandong, Lake Kotto and Lake Soden

The zooplankton of four crater lakes in North-west Cameroon has been studied by means of vertical hauls from the bottom to the surface. Two of the lakes are deep, with sparse phytoplankton, while the other two are shallow with dense blooms of blue-green algae. All the lakes are without planktonic Cladocera. In Lake Kotto, Mboandong and Barombi Mbo the dominant zooplankter is Mesocyclops (Thermocyclops) hyalinus , which is replaced in Lake Soden by Mesocyclops leuckarti. Larvae of Chaoborus are present in all the lakes. In the two deep lakes, Soden and Barombi Mbo, the dominant rotifer is Polyarthra dolichoptera , but in the two shallow lakes, Kotto and Mboandong, three species, Hexarthra mira, Brachionus fakatus and B. caudatus , make up the bulk of the rotifers.
In Barombi Mbo during the daytime most of the zooplankton was in the top 20 m, but most of the Chaoborus larvae were below 20m.
The standing crops of zooplankton in these lakes are low compared with those of lakes in the Kigezi District of Uganda.
The most remarkable feature of the zooplankton in these lakes is the occurrence of dwarfed forms. Both species of cyclopoid copepods and the eight identifiable species of Rotifera are smaller here than in most other localities. Factors which may be associated with this dwarfing are low latitude with constant high temperature, low phytoplankton availability, and the low ratio of drainage area to lake area.     

Weak disruptive selection and incomplete phenotypic divergence in two classic examples of sympatric speciation: cameroon crater lake cichlids

Abstract Recent documentation of a few compelling examples of sympatric speciation led to a proliferation of theoretical models. Unfortunately, plausible examples from nature have rarely been used to test model predictions, such as the initial presence of strong disruptive selection. Here I estimated the form and strength of selection in two classic examples of sympatric speciation: radiations of Cameroon cichlids restricted to Lakes Barombi Mbo and Ejagham. I measured five functional traits and relative growth rates in over 500 individuals within incipient species complexes from each lake. Disruptive selection was prevalent in both groups on single and multivariate trait axes but weak relative to stabilizing selection on other traits and most published estimates of disruptive selection. Furthermore, despite genetic structure, assortative mating, and bimodal species-diagnostic coloration, trait distributions were unimodal in both species complexes, indicating the earliest stages of speciation. Long waiting times or incomplete sympatric speciation may result when disruptive selection is initially weak. Alternatively, I present evidence of additional constraints in both species complexes, including weak linkage between coloration and morphology, reduced morphological variance aligned with nonlinear selection surfaces, and minimal ecological divergence. While other species within these radiations show complete phenotypic separation, morphological and ecological divergence in these species complexes may be slow or incomplete outside optimal parameter ranges, in contrast to rapid divergence of their sexual coloration.     

The oldest fossil cichlids (Teleostei: Perciformes): indication of a 45 million-year-old species flock

Five closely related species of fossil cichlids collected from an Eocene site in Tanzania, East Africa, represent the oldest known cichlids. The specimens are whole-body, articulated fishes that are extremely well preserved and, therefore, have the potential to add to our knowledge of the history of this family. Modern cichlids are particularly well known for the numerous species flocks of the East African Great Lakes. A great deal of research is ongoing regarding all aspects of the fishes in these flocks, including their evolutionary history The new collection of fossils reported here is interpreted as representing a species flock that arose in a small crater lake. These fossils indicate that cichlids' ability to form species flocks evolved early in the history of this family.     

Isoenzyme analysis of Schistosoma haematobium,S. intercalatum and their hybrids and occurrences of natural hybridization in Cameroon

Isoelectric focusing of glucose-6-phosphate dehydrogenase (G6PD) produced clearly identifiable profiles for S. haematobium and S. intercalatum and their hybrids. To provide a more detailed analysis of the interactions of S. haematobium and S. intercalatum in South West Cameroon over the last 12 years, G6PD analyses were carried out on individual schistosomes collected in Kumba in 1990, Loum in 1990, 1999 and 2000 and Barombi Mbo and Barombi Kotto in 1999. Studies were also carried out on the two parental species S. haematobium Barombi Mbo, S. intercalatum Edea and subsequent generations of hybrids resulting from laboratory crosses of the two parental species. The isoenzyme analysis demonstrated that the 1990 isolate from Kumba, was a recombinant of S. intercalatum x S. haematobium, and that 30% of individual schistosomes collected in 1990 in Loum were also recombinants. The remainder gave data indicative of S. haematobium. In 1999, 12.5% of individuals from Loum showed recombination and 10% in 2000. Results from the most recent parasitological survey in October 2000 showed the persistence of the recombinant population in addition to that of S. haematobium. There was also evidence of recombination having taken place in Barombi Kotto but not Barombi Mbo. This study demonstrates how the situation has changed over the last 12 years, and emphasizes the importance of assessing morphological, biological and molecular data together to gain a true picture of the rapidly evolving situation.     

Age of cichlids: new dates for ancient lake fish radiations

Timing divergence events allow us to infer the conditions under which biodiversity has evolved and gain important insights into the mechanisms driving evolution. Cichlid fishes are a model system for studying speciation and adaptive radiation, yet, we have lacked reliable timescales for their evolution. Phylogenetic reconstructions are consistent with cichlid origins prior to Gondwanan landmass fragmentation 121-165 MYA, considerably earlier than the first known fossil cichlids (Eocene). We examined the timing of cichlid evolution using a relaxed molecular clock calibrated with geological estimates for the ages of 1) Gondwanan fragmentation and 2) cichlid fossils. Timescales of cichlid evolution derived from fossil-dated phylogenies of other bony fishes most closely matched those suggested by Gondwanan breakup calibrations, suggesting the Eocene origins and marine dispersal implied by the cichlid fossil record may be due to its incompleteness. Using Gondwanan calibrations, we found accumulation of genetic diversity within the radiating lineages of the African Lakes Malawi, Victoria and Barombi Mbo, and Palaeolake Makgadikgadi began around or after the time of lake basin formation. These calibrations also suggest Lake Tanganyika was colonized independently by the major radiating cichlid tribes that then began to accumulate genetic diversity thereafter. These results contrast with the widely accepted theory that diversification into major lineages took place within the Tanganyika basin. Together, this evidence suggests that ancient lake habitats have played a key role in generating and maintaining diversity within radiating lineages and also that lakes may have captured preexisting cichlid diversity from multiple sources from which adaptive radiations have evolved.     

Determination of water quality,and trace metals in endemic Sarotherodon linellii,Pungu maclareni and Clarias maclareni,in crater Lake Barombi Mbo,Cameroon

Considering water pollution as a potential threat to some endemic cichlids of Lake Barombi Mbo, Cameroon, an investigation was done in 2011 to determine trace metals in its water, linking their uptake in gills and liver of fish to water chemistry. ICP-MS and ICP-OES analyses of trace metals based on total concentration of unfiltered lake water samples showed the presence of trace metals. All fish species accumulated Al, Mn and Sr in the highest concentrations in their gills, with Cu, Cd, Co, Cr, Pb and U highest in the liver. Pungu maclareni accumulated Al, Cr, Co, Sr and Pb in the highest concentrations. The highest mean gill Al concentration of 140 µg g^?1 dry weight was measured in P. maclareni gills, this being one of the critically endangered cichlids of the lake. Stable isotope analyses of carbon δ¹³C and nitrogen δ¹⁵N showed that P. maclareni had the highest mean δ¹³C (?30.2‰) and highest concentrations of Cr, Co, Pb and U in liver, probably linking the carbon source to the accumulation of metals. Though trace metal levels in the lake water were low, their presence in fish tissues suggest they are bioavailable, bioaccumulate and may pose a threat to the aquatic biota, and therefore should be monitored.     

PHENOTYPIC PLASTICITY AND HETEROCHRONY IN CICHLASOMA MANAGUENSE (PISCES,CICHLIDAE) AND THEIR IMPLICATIONS FOR SPECIATION IN CICHLID FISHES

Cichlid fishes in African rift lakes have undergone rapid speciation, resulting in “species flocks” with more than 300 endemic species in some of the lakes. Most researchers assume that there is little phenotypic variation in cichlid fishes. I report here extensive phenotypic plasticity in a Neotropical cichlid species. I examined the influence of diet on trophic morphology during ontogeny in Cichlasoma managuense. Two groups of full siblings were fed two different diets for eight months after the onset of feeding; thereafter both groups were fed a common diet. Phenotypes that differed significantly at 8.5 months converged almost completely at 16.5 months. If feeding on two different diets is continued after 8.5 months, the phenotypes remain distinct. Differences in diet and possibly in feeding mode are believed to have caused these phenotypic changes. Phenotypic plasticity is described in terms of a qualitative model of heterochrony in which phenotypic change in morphology is explained as retardation of the normal developmental rate. If phenotypic expression of morphology is equally plastic in African cichlid species as it may be in the American cichlids, as exemplified by C. managuense, then taxonomic, ecological, and evolutionary analyses of “species flocks” may be in need of revision. However, Old World cichlids may be less phenotypically plastic than New World cichlids, and this may contribute to the observed differences in speciation rate and degree of endemism.     

Reticulate sympatric speciation in Cameroonian crater lake cichlids

BACKGROUND: Traditionally the rapid origin of megadiverse species flocks of extremely closely related species is explained by the combinatory action of three factors: Disruptive natural selection, disruptive sexual selection and partial isolation by distance. However, recent empirical data and theoretical advances suggest that the diversity of complex species assemblages is based at least partially on the hybridization of numerous ancestral allopatric lineages that formed hybrids upon invasion of new environments. That reticulate speciation within species flocks may occur under sympatric conditions after the primary formation of species has been proposed but not been tested critically. RESULTS: We reconstructed the phylogeny of a complex cichlid species flock confined to the tiny Cameroonian crater lake Barombi Mbo using both mitochondrial and nuclear (AFLP) data. The nuclear phylogeny confirms previous findings which suggested the monophyly and sympatric origin of the flock. However, discordant intra-flock phylogenies reconstructed from mitochondrial and nuclear data suggest strongly that secondary hybridization among lineages that primarily diverged under sympatric conditions had occurred. Using canonical phylogenetic ordination and tree-based tests we infer that hybridization of two ancient lineages resulted in the formation of a new and ecologically highly distinct species, Pungu maclareni. CONCLUSIONS: Our findings show that sympatric hybrid speciation is able to contribute significantly to the evolution of complex species assemblages even without the prior formation of hybrids derived from allopatrically differentiated lineages.     

Evolution of the visual sensory system in cichlid fishes from crater lake Barombi Mbo in Cameroon

In deep‐water animals, the visual sensory system is often challenged by the dim‐light environment. Here, we focus on the molecular mechanisms involved in rapid deep‐water adaptations. We examined visual system evolution in a small‐scale yet phenotypically and ecologically diverse adaptive radiation, the species flock of cichlid fishes in deep crater lake Barombi Mbo in Cameroon, West Africa. We show that rapid adaptations of the visual system to the novel deep‐water habitat primarily occurred at the level of gene expression changes rather than through nucleotide mutations, which is compatible with the young age of the radiation. Based on retinal bulk RNA sequencing of all eleven species, we found that the opsin gene expression pattern was substantially different for the deep‐water species. The nine shallow‐water species feature an opsin palette dominated by the red‐sensitive (LWS) opsin, whereas the two unrelated deep‐water species lack expression of LWS and the violet‐sensitive (SWS2B) opsin, thereby shifting the cone sensitivity to the centre of the light spectrum. Deep‐water species further predominantly express the green‐sensitive RH2Aα over RH2Aβ. We identified one amino acid substitution in the RH2Aα opsin specific to the deep‐water species. We finally performed a comparative gene expression analysis in retinal tissue of deep‐ vs. shallow‐water species. We thus identified 46 differentially expressed genes, many of which are associated with functions in vision, hypoxia management or circadian clock regulation, with some of them being associated with human eye diseases.     

Replicated divergence in cichlid radiations mirrors a major vertebrate innovation

Decoupling of the upper jaw bones—jaw kinesis—is a distinctive feature of the ray-finned fishes, but it is not clear how the innovation is related to the extraordinary diversity of feeding behaviours and feeding ecology in this group. We address this issue in a lineage of ray-finned fishes that is well known for its ecological and functional diversity—African rift lake cichlids. We sequenced ultraconserved elements to generate a phylogenomic tree of the Lake Tanganyika and Lake Malawi cichlid radiations. We filmed a diverse array of over 50 cichlid species capturing live prey and quantified the extent of jaw kinesis in the premaxillary and maxillary bones. Our combination of phylogenomic and kinematic data reveals a strong association between biting modes of feeding and reduced jaw kinesis, suggesting that the contrasting demands of biting and suction feeding have strongly influenced cranial evolution in both cichlid radiations.     

Lake Malawi cichlid evolution along a benthic/limnetic axis

Divergence along a benthic to limnetic habitat axis is ubiquitous in aquatic systems. However, this type of habitat divergence has largely been examined in low diversity, high latitude lake systems. In this study, we examined the importance of benthic and limnetic divergence within the incredibly species‐rich radiation of Lake Malawi cichlid fishes. Using novel phylogenetic reconstructions, we provided a series of hypotheses regarding the evolutionary relationships among 24 benthic and limnetic species that suggests divergence along this axis has occurred multiple times within Lake Malawi cichlids. Because pectoral fin morphology is often associated with divergence along this habitat axis in other fish groups, we investigated divergence in pectoral fin muscles in these benthic and limnetic cichlid species. We showed that the eight pectoral fin muscles and fin area generally tended to evolve in a tightly correlated manner in the Lake Malawi cichlids. Additionally, we found that larger pectoral fin muscles are strongly associated with the independent evolution of the benthic feeding habit across this group of fish. Evolutionary specialization along a benthic/limnetic axis has occurred multiple times within this tropical lake radiation and has produced repeated convergent matching between exploitation of water column habitats and locomotory morphology.     

Genetic signatures of a demographic collapse in a large-bodied forest dwelling primate (Mandrillus leucophaeus)

It is difficult to predict how current climate change will affect wildlife species adapted to a tropical rainforest environment. Understanding how population dynamics fluctuated in such species throughout periods of past climatic change can provide insight into this issue. The drill (Mandrillus leucophaeus) is a large-bodied rainforest adapted mammal found in West Central Africa. In the middle of this endangered monkey's geographic range is Lake Barombi Mbo, which has a well-documented palynological record of environmental change that dates to the Late Pleistocene. We used a Bayesian coalescent-based framework to analyze 2,076 base pairs of mitochondrial DNA across wild drill populations to infer past changes in female effective population size since the Late Pleistocene. Our results suggest that the drill underwent a nearly 15-fold demographic collapse in female effective population size that was most prominent during the Mid Holocene (approximately 3-5 Ka). This time period coincides with a period of increased dryness and seasonality across Africa and a dramatic reduction in forest coverage at Lake Barombi Mbo. We believe that these changes in climate and forest coverage were the driving forces behind the drill population decline. Furthermore, the warm temperatures and increased aridity of the Mid Holocene are potentially analogous to current and future conditions faced by many tropical rainforest communities. In order to prevent future declines in population size in rainforest-adapted species such as the drill, large tracts of forest should be protected to both preserve habitat and prevent forest loss through aridification.     

Interspecific relationships of aufwuchs-eating fishes in Lake Tanganyika

Synopsis On a rocky shore of Lake Tanganyika aufwuchs-eating is practiced by 18 fish species: 17 cichlids and 1 cyprinid. The majority takes mostly either filamentous or unicellular algae.Tropheus moorei and its taxonomically related species most closely resemble one another in diet among the species taking mostly filamentous algae, and thePetrochromis species do so among the species taking mostly unicellular algae.Petrochromis polyodon andT. moorei severely interfere with the feeding activities of related species. These two dominant species share both intensive grazing sites and temporal grazing patterns, and there seems to be a symbiotic relationship between them. Symbiotic relationships are also seen in seven cases of associated feeding by carnivorous fishes. Differences in diet or feeding behavior are found between these symbionts. Small but clear differences are found betweenP. polyodon andT. moorei and between the associating and the carnivorous host fishes in the four cases of associated feeding. These small differences seem attributable to morphological differences and may have been effective for the evolution of symbiotic relationships.Contribution from the Laboratory of Animal Ecology, Kyoto University, No. 461.