Midwater spawning inHaplochromis chrysonotus (Boulenger) (Teleostei: Cichlidae) in Lake Malawi

Synopsis Haplochromis chrysonotus, a semipelagic shoaling cichlid from Lake Malawi, has been observed spawning in open water up to ten metres above the substratum. It is suggested that open water spawning without the establishment of a substratum-based territory eliminates competition with other species for breeding space and that it may be a stage in the evolution of a totally pelagic mode of life.     

Schistosomiasis in Lake Mala?i Villages

Historically, open shorelines of Lake Malaŵi were free from schistosome, Schistosoma haematobium, transmission, but this changed in the mid-1980s, possibly as a result of over-fishing reducing density of molluscivore fishes. Very little information is available on schistosome infections among people in lake-shore communities and therefore we decided to summarise data collected from 1998 to 2007. Detailed knowledge of the transmission patterns is essential to design a holistic approach to schistosomiasis control involving the public health, fisheries and tourism sectors. On Nankumba Peninsula, in the southern part of the lake, inhabitants of villages located along the shores of Lake Malaŵi have higher prevalence of S. haematobium infection than those living in inland villages. Overall prevalence (all age classes combined) of urinary schistosomiasis in 1998/1999 ranged from 10.2% to 26.4% in inland villages and from 21.0% to 72.7% in lakeshore villages; for school children prevalence of infection ranged from 15.3% to 57.1% in inland schools and from 56.2% to 94.0% in lakeshore schools. Inhabitants on the islands, Chizumulu and Likoma, also have lower prevalence of infection than those living in lakeshore villages on Nankumba Peninsula. This increased prevalence in lakeshore villages is not necessarily linked to transmission taking place in the lake itself, but could also be due to the presence of more numerous typical inland transmission sites (e.g., streams, ponds) being close to the lake. Temporal data witness of intense transmission in some lakeshore villages with 30–40% of children cleared from infection becoming reinfected 12 months later (also lakeshore village). The level of S. mansoni infection is low in the lakeshore communities. Findings are discussed in relation to fishing in the lake.     

The role of the littoral zone in the functioning of a shallow tropical lake ecosystem

Lake Chilwa (Malawi) is a shallow tropical lake surrounded by an extensive zone of littoral swamp mainly composed of Typha domingensis. The data produced show that the physical and chemical conditions are different and more varied in the littoral region when compared with the open lake. Primary production in the littoral is confined almost entirely to Typha domingensis, which also acts as a nutrient pump by absorbing nutrients from the soils and eventually releasing them into the lake waters. Long term and short term exchanges in organic and inorganic materials between the littoral swamps and the open lake are described. These, as well as available evidence on the distribution, reproductive habits and diet of certain faunal groups, indicate that the littoral region of Lake Chilwa has an important regulating influence on the functioning of the whole lake.     

Schistosomiasis in Lake Malawi: Relationship of Fish and Intermediate Host Density to Prevalence of Human Infection

Prior to 1985, the open waters of Lake Malawi were free from schistosome transmission. Over the past decades, however, the prevalence of urinary schistosomiasis has increased dramatically in the southern part of the lake. We found the prevalence of human schistosomiasis in school-aged children to be negatively correlated with the density of molluscivorous fishes. Specifically, the increased infection rate in southern Lake Malawi between 1978 and 1991 is coincident with the reduction in numbers of snail-eating fishes. During 2003, we determined the relative abundance of molluscivorous fishes and snail density at 18 sites throughout the lake and schistosome infection in school-aged children living in selected lakeshore communities of Lake Malawi. At the 18 sites sampled in 2003, we found that snail abundance decreased with an increase in abundance of snail-eating fishes. Furthermore, the 2003 samples showed that the abundance of snail-eating fishes increased and there was a reduction in schistosomiasis in school-aged children in Chembe Village. We believe that we will not observe a return to the 1978 infection rates until these fishes continue to increase and inhabit shallower waters.     

Species-Specific Population Structure in Rock-Specialized Sympatric Cichlid Species in Lake Tanganyika,East Africa

Species richness and geographical phenotypic variation in East African lacustrine cichlids are often correlated with ecological specializations and limited dispersal. This study compares mitochondrial and microsatellite genetic diversity and structure among three sympatric rock-dwelling cichlids of Lake Tanganyika, Eretmodus cyanostictus, Tropheus moorii, and Ophthalmotilapia ventralis. The species represent three endemic, phylogenetically distinct tribes (Eretmodini, Tropheini, and Ectodini), and display divergent ecomorphological and behavioral specialization. Sample locations span both continuous, rocky shoreline and a potential dispersal barrier in the form of a muddy bay. High genetic diversity and population differentiation were detected in T. moorii and E. cyanostictus, whereas much lower variation and structure were found in O. ventralis. In particular, while a 7-km-wide muddy bay curtails dispersal in all three species to a similar extent, gene flow along mostly continuous habitat appeared to be controlled by distance in E. cyanostictus, further restricted by site philopatry and/or minor habitat discontinuities in T. moorii, and unrestrained in O. ventralis. In contrast to the general pattern of high gene flow along continuous shorelines in rock-dwelling cichlids of Lake Malawi, our study identifies differences in population structure among stenotopic Lake Tanganyika species. The amount of genetic differentiation among populations was not related to the degree of geographical variation of body color, especially since more phenotypic variation is observed in O. ventralis than in the genetically highly structured E. cyanostictus. [Reviewing Editor: Dr. Rafael Zardoya]     

EPIPHYTES ON CLADOPHORA GLOMERATA IN THE GREAT LAKES AND ST. LAWRENCE SEAWAY WITH PARTICULAR REFERENCE TO THE RED ALGA CHROODACTYLON RAMOSUM (= ASTEROCYTIS SMARGDINA)1

The range of Cladophora glomerata² along the east and north shorelines of the Great Lakes and St. Lawrence Seaway extends form just east of Montreal to Thunder Bay on Lake Superior. However, it does not occur at sites sampled in Georgian Bay, the North Channel or eastern Lake Superior. The dominant epiphytes on Cladophora throughout this range are the blue-green algae, particularly Lyngbya diguetii, L. epiphytica and Chamaesiphon incrustans, which account for 53 to 90% of the cell density. The diatoms Cocconeis pediculus and Rhoicosphenia curvata contribute to much of the remaining density. The red alga Chroodactylon ramosum is a minor component of Cladophora epiphyton but is widespread in Lakes Ontario, Erie and Huron. Filament morphology, cell diameters and lengths of Chroodactylon are quite similar to those of marine forms. In addition, this alga has been reported to be quite tolerant of a wide range of salinities. Chroodactylon has been found in 33 freshwater sites throughout North America and 30 of these are from the Great Lakes or its drainage basin. Therefore, it appears possible that Chroodactylon, like Bangia atropurpurea, has originated in the Great Lakes by a migration from the Atlantic Ocean.     

Genetic and morphological studies of Nothobranchius (Cyprinodontiformes) from Malawi with description of Nothobranchius wattersi sp. nov.

Molecular and morphological data were used to explore evolutionary differentiation among populations of Nothobranchius in the Lake Malawi–upper Shire River and the Lakes Chilwa–Chiuta drainage systems in Malawi. The aim of the study was to test the hypothesis that Nothobranchius of the Malawi–Shire system constitute a separate evolutionary group from Nothobranchius kirki. Mitochondrial and nuclear sequence data show a strongly supported phylogenetic split into two monophyletic groups separating the Lake Malawi basin fish from N. kirki. Unlike N. kirki, Lake Malawi–Shire fish do not deviate from neutrality and express an excess of rare haplotypes and mutations in terminal branches, characteristic of recently expanded populations. Further, the two groups significantly differ in morphology. Two body characters (dorsal‐fin base length and pre‐pelvic–pre‐anal distance) are significantly different between the two species in both sexes. Several other characters are significantly different in either male or female comparisons with respect to both standard and head lengths, and robust morphological differentiation is detected by multivariate analysis. The two groups are readily distinguished on the basis of male colouration, especially in scale centres and the caudal fin. On the basis of this differentiation at the molecular and morphological levels, in addition to colouration, the Lake Malawi–Shire fish are hereby formally recognized as constituting a new species, Nothobranchius wattersi. This distinction is in agreement with the geomorphologic and recent climatic history in the region.     

Geographical ancestry of Lake Malawi's cichlid fish diversity

The Lake Malawi haplochromine cichlid flock is one of the largest vertebrate adaptive radiations. The geographical source of the radiation has been assumed to be rivers to the south and east of Lake Malawi, where extant representatives of the flock are now present. Here, we provide mitochondrial DNA evidence suggesting the sister taxon to the Lake Malawi radiation is within the Great Ruaha river in Tanzania, north of Lake Malawi. Estimates of the time of divergence between the Lake Malawi flock and this riverine sister taxon range from 2.13 to 6.76 Ma, prior to origins of the current radiation 1.20–4.06 Ma. These results are congruent with evaluations of 2–3.75 Ma fossil material that suggest past faunal connections between Lake Malawi and the Ruaha. We propose that ancestors of the Malawi radiation became isolated within the catchment during Pliocene rifting that formed both Lake Malawi and the Kipengere/Livingstone mountain range, before colonizing rivers to the south and east of the lake region and radiating within the lake basin. Identification of this sister taxon allows tests of whether standing genetic diversity has predisposed Lake Malawi cichlids to rapid speciation and adaptive radiation.     

Elevated mtDNA diversity in introduced populations of Cynotilapia afra (Günther 1894) in Lake Malawi National Park is evidence for multiple source populations and hybridization

Genetic variation in many invasive species shows little or no signs of a founder event, suggesting that high genetic diversity may facilitate establishment success. The rocky‐shore, plankton‐feeding cichlid fish Cynotilapia afra is endemic to Lake Malawi, but naturally absent from many suitable sites. In the 1960s, this species was introduced to the southern areas of the lake, presumably as a result of the aquarium fish trade. It has now become established on a number of rocky areas within the Lake Malawi National Park. Here, we analysed DNA sequence variation in the mitochondrial control region of six native and four introduced populations of C. afra, and three populations of the closely‐related and hybridizing Pseudotropheus zebra. In contrast to previous studies of Lake Malawi rock dwelling cichlids, network analyses suggested that native populations of C. afra showed high levels of lineage sorting in mtDNA. Introduced populations showed higher sequence and haplotype diversity than their native counterparts. Our analyses suggested that the elevated gene diversity was largely attributed to the fact that the introduced C. afra populations were derived from several genetically distinct and geographically separate populations, and to a lesser extent because of introgressive hybridization with native P. zebra. The establishment and spread of C. afra may be partly because of its ability to occupy a vacant ecological niche, but it may also have been facilitated by its enhanced genetic diversity.     

Development of polymorphic microsatellite DNA loci for characterizing Oreochromis shiranus subspecies in Malawi

Six sets of microsatellite primers were developed from Oreochromis shiranus nuclear DNA. These cross‐primed in species of the genera Sarotherodon and Tilapia, and were polymorphic in most of the species. The total number of alleles ranged from 18 to 30 per locus; the mean heterozygosity per population ranged from 0.51 ± 0.12 to 0.82 ± 0.03, which is higher than reported in allozyme studies. Five sets of primers were used to characterize five putative populations of O. shiranus, an indigenous mouth brooding tilapia which has been widely distributed in fish farms and reservoirs in Malawi. Two postulated subspecies of O. shiranus (subspecies shiranus and subspecies chilwae) have been difficult to distinguish morphologically. Lake Chilwa, Lake Chiuta and Bunda Reservoir populations cluster together as O. sh. chilwae, while the Lake Malombe and Bishop Reservoir populations form a second cluster as O. sh. shiranus. The assignment of the Chiuta and Chilwa populations of O. sh. chilwae to a single subspecies is consistent with the fact that the two lakes were a single open lake until about 8000–9000 _bp. There is no connection between them and the Lake Malawi‐Malombe drainage system where the O. sh. shiranus subspecies is found.     

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.     

Mate Choice and Hybridization in Lake Malawi Cichlids, Sciaenochromis fryeri and Cynotilapia afra

Lake Malawi is home to hundreds of cichlid species, most of which are endemic to the lake. Several of these species exhibit a remarkably similar behavioural repertoire including territorial and reproductive behaviours. Despite their diverse anatomy and body colouration, hybridization among Malawi cichlids has been observed in captivity and has also been reported to occur in nature. Here, possible factors underlying hybridization between two species, Sciaenochromis fryeri, a large piscivore, and Cynotilapia afra (Jalo reef), a small planktivore biocover grazer, were investigated in captivity. In a series of experiments the size of the territory, the size of the males and the availability of conspecific mates were manipulated. It was found that mate selection in these two species under semi‐natural conditions is dependent not only upon species‐specific cues but also upon the status of the male. Larger male body size and larger territory size was generally preferred by females and when these attributes characterized a heterospecific male they also increased the probability of heterospecific courtship. Given that spawning between the above‐mentioned species resulted in fertile offspring and that their geographical location and habitat overlap in nature, the effect of their mate‐choice strategies on hybridization and speciation is considered.     

Evolution of a cichlid fish in a Lake Malawi satellite lake

Allopatric divergence in peripheral habitats may lead to rapid evolution of populations with novel phenotypes. In this study we provide the first evidence that isolation in peripheral habitats may have played a critical role in generation of Lake Malawi's cichlid fish diversity. We show that Lake Chilingali, a satellite lake 11.5 km from the shore of Lake Malawi, contains a breeding population of Rhamphochromis, a predatory genus previously thought to be restricted to Lake Malawi and permanently connected water bodies. The Lake Chilingali population is the smallest known Rhamphochromis, has a unique male nuptial colour pattern and has significant differentiation in mitochondrial DNA from Lake Malawi species. In laboratory mate choice trials with a candidate sister population from Lake Malawi, females showed a strong tendency to mate assortatively indicating that they are incipient biological species. These data support the hypothesis that isolation and reconnection of peripheral habitats due to lake level changes have contributed to the generation of cichlid diversity within African lakes. Such cycles of habitat isolation and reconnection may also have been important in evolutionary diversification of numerous other abundant and wide-ranging aquatic organisms, such as marine fishes and invertebrates.     

Evidence for lacustrine breeding by sanjika Opsaridium microcephalum (Teleostei: Cyprinidae) in Lake Malawi

Three species of Opsaridium occur in the Lake Malawi catchment. The kabyabya Opsaridium tweddleorum is a small species restricted to inflowing rivers and streams, the mpasa Opsaridium microlepis is a large lake-dwelling species that ascends the lower reaches of inflowing rivers during the rains to spawn, and the sanjika Opsaridium microcephalum has riverine, potamodromous and lake spawning populations. Evidence for sanjika spawning in the lake itself, rather than just in rivers, is presented here in the form of observations of courting behaviour by fully mature males and females on exposed rocky shores, together with the presence of pin-head fry in areas remote from any inflowing streams during the dry season.     

Molecular phylogenetic investigations of the Viviparidae (Gastropoda: Caenogastropoda) in the lakes of the Rift Valley area of Africa

The freshwater gastropod family Viviparidae is nearly cosmopolitan, but absent from South America. On the African continent, two genera are recognized; the widespread Bellamya and the monotypic Neothauma, which is confined to Lake Tanganyika. Most of the African Bellamya species are confined to the major lakes of the Rift Valley area in Africa, i.e. Lake Albert, Lake Malawi, Lake Mweru, and Lake Victoria. The phylogenetic analyses of mitochondrial (COI and 16S) and nuclear (H3, 18S and 28S) DNA inferred three major lake-clades; i.e. Lake Victoria/Kyoga/Albert, Lake Malawi and Lake Mweru/Bangweulu. The endemic B. rubicunda from Lake Albert and B. unicolor from Lake Kyoga were inferred to be part of the Lake Victoria clade. Bellamya capillata as identified by shell characters was polyphyletic in gene trees. The monophyletic Bellamya species radiation in Lake Malawi was most nearly related to the Lake Victoria/Kyoga/Albert-clade. Taxa from the Zambian lakes, Mweru and Bangweulu, were inferred together and placed ancestral to the other lakes. Neothauma tanganyicense was inferred as the sister-group to the Zambian Bellamya. Within the lake-clades the endemic radiations show very low genetic diversities (0–4.1% in COI), suggesting much faster morphological divergence than molecular divergence. Alternatively, Bellamya in Africa constitutes only a few species with several sub-species or eco-phenotypic morphs. The African viviparids were inferred to be the sister-group to a clade comprising Asian species, and the relatively low genetic diversity between the clades (12.6–15.5% in COI) makes a recent Miocene dispersal event from Asia to Africa much more likely than an ancient Gondwana vicarience distribution.     

Microsatellites reveal high levels of population substructuring in the species-poor Eretmodine cichlid lineage from Lake Tanganyika

This study investigated fine-scale population substructuring in an apparently monogamous, biparental mouth-brooding cichlid. Microsatellite allele frequencies were determined at four polymorphic loci for nine populations of Eretmodus cyanostictus. We provide empirical support for the hypothesis that a species employing this breeding strategy should exhibit high levels of population substructuring. Stretches of sand represent considerable barriers to dispersal and, in contrast to the rock-dwelling cichlids of Lake Malawi, distance alone, along a continuous rocky shoreline, is sufficient to reduce gene flow significantly There was a significant pattern of isolation by distance both along the whole study area and over the stretch of continuous shoreline, suggesting that this species has poor dispersal capabilities and that juveniles establish territories close to their natal site. Despite limited dispersal, E. cyanostictus populations are not significantly more inbred than a more-widely dispersing rock-dwelling cichlid from Lake Malawi. This finding may cast doubt on the hypothesis that polyandry has evolved as a mechanism for maintaining genetic diversity in Lake Malawi cichlids. High levels of substructuring may not always promote high levels of speciation, and other factors, such as the intensity of sexual selection, may be more important in determining the speciation potential of a lineage.     

Adult plasticity in African cichlids: Rapid changes in opsin expression in response to environmental light differences

Phenotypic plasticity allows organisms to adapt quickly to local environmental conditions and could facilitate adaptive radiations. Cichlids have recently undergone an adaptive radiation in Lake Malawi where they inhabit diverse light environments and tune their visual sensitivity through differences in cone opsin expression. While cichlid opsin expression is known to be plastic over development, whether adults remain plastic is unknown. Adult plasticity in visual tuning could play a role in cichlid radiations by enabling survival in changing environments and facilitating invasion into novel environments. Here we examine the existence of and temporal changes in adult visual plasticity of two closely related species. In complementary experiments, wild adult Metriaclima mbenji from Lake Malawi were moved to the lab under UV‐deficient fluorescent lighting; while lab raised M. benetos were placed under UV‐rich lighting designed to mimic light conditions in the wild. Surprisingly, adult cichlids in both experiments showed significant changes in the expression of the UV‐sensitive single cone opsin, SWS1, in only 3 days. Modeling quantum catches in the light environments revealed a possible link between the light available to the SWS1 visual pigment and SWS1 expression. We conclude that adult cichlids can undergo rapid and significant changes in opsin expression in response to environmental light shifts that are relevant to their habitat and evolutionary history in Lake Malawi. This could have contributed to the rapid divergence characteristic of these fantastic fishes.     

Planktonic nitrogen fixation in Lake Malawi/Nyasa

Nitrogen (N₂) fixation has been identified as possibly an important source of “new” nitrogen (N) to the epilimnion of Lake Malawi but studies in 1999–2000 and 2002 (September–December) estimated that the contribution of N₂-fixation by heterocystous Anabaena filaments to the N budget of the lake’s epilimnion is only 3–4% of total N input. N₂-fixation rates in Lake Malawi, as estimated by the acetylene reduction assay were higher during the stratified season (September–March) than during the rest of the year. Planktonic N₂-fixation in Lake Malawi can be monitored by measuring heterocyst biovolume concentrations because a significant correlation (r ² = 0.945, P < 0.0001) was observed between the two parameters. Heterocyst density cannot be used to estimate N₂-fixation because heterocyst sizes in the lake change continuously as at least two Anabaena species with different heterocyst dimensions are present. During September–October 2002, a species similar to A. maxima, contained larger heterocysts (16.34 ± 2.46 and 13.25 ± 1.89 μm in cross and apical section dimensions, respectively). In November–December of 2002, A. discoidea dominated and had smaller sized heterocysts (8.92 ± 1.13 and 7.25 ± 0.95 μm in cross and apical section dimensions). Since planktonic N₂-fixation is higher near shore than offshore, its contribution to the N budget in the littoral zone where high densities of grazing benthic fish occur may be more critical to maintaining the high rates of benthic algal productivity observed.     

Mimosa pigra in eastern and southern Africa: Distribution and socio‐ecological impacts

The semiaquatic weed Mimosa pigra has negative impacts on biodiversity, fishing, crop and livestock production, and tourism in most places where it has been introduced, established and proliferated. Many of the ecological impacts are well known, but its impacts on rural livelihoods are less well documented, especially in Africa. We mapped the distribution of M. pigra in eastern and southern Africa, and then compared that with its potential distribution based on an ecoclimatic niche model. Household interviews were conducted to assess the impacts of this weed on local livelihoods. Mimosa pigra was found to be invasive in western Ethiopia, around the shores of Lake Victoria and Lake Tanganyika, and along the Tanzanian coastline, northern Malawi, parts of Mozambique and along the Kafue River and in the Barotse floodplain on the Zambezi River in Zambia. According to respondents living along the Kafue River floodplains in Zambia, it has a negative impact on biodiversity, wildlife, livestock, crop production, fishing and mobility. Dense stands prevented the movement of people and livestock, limiting access to croplands, grazing lands and fishing areas. Fish catches have been reduced and fishing equipment damaged. All respondents agreed that their livelihood options would be considerably enhanced if M. pigra was removed from the landscape. Based on its current and potential impact, we therefore recommend that an integrated management plan be developed and implemented, including the appropriate use of biological control agents to reduce the negative impacts of the weed.     

Plasticity of opsin gene expression in cichlids from Lake Malawi

Sensory systems play crucial roles in survival and reproduction. Therefore, sensory plasticity has important evolutionary implications. In this study, we examined retinal plasticity in five species of cichlid fish from Lake Malawi. We compared the cone opsin expression profiles of wild‐caught fish to lab‐reared F₁ that had been raised in a UV minus, reduced intensity light environment. All of the opsin genes that were expressed in wild‐caught fish were also expressed in lab‐reared individuals. However, we found statistically significant differences in relative opsin expression among all five species. The most consistent difference was in the SWS2B (violet) opsin, which was always expressed at higher levels in lab‐reared individuals. Estimates of visual pigment quantum catch suggest that this change in expression would increase retinal sensitivity in the light environment of the lab. We also found that the magnitude of plasticity varied across species. These findings have important implications for understanding the genetic regulation of opsin expression and raise many interesting questions about how the cichlid visual system develops. They also suggest that sensory plasticity may have facilitated the ecological diversification of cichlids in Lake Malawi.