Comparative ecology and phylogeography in east African cichild fishes

Since almost two decades speciation outbursts, so-called adaptive radiations, have been put forward as a major reason for a large portion of the bio-diversity we see today. Adaptive radiations are indisputably very complex processes with many factors to consider. They are, however, not separable from the concept of ecological speciation by the means of natural selection. Hence the concept of convergent evolution, which states that different organisms independently evolve similar morphological or behavioral traits as a result of similar ecological selection regimes, was put forward as an essential indicator of the ‘adaptiveness’ of respective species differences and/or similarities. In the exceptionally species rich and eco-morphologically highly diverse assemblages of the East African Rift lakes, the paradox was put forward that competitive ecological exclusion of converging species seems to require a temporal and special separation (allopatry) of the different lineages in order for them to coexist. Recent phylogenetic framework and molecular dating seem to indicate though that in fact many lineages formed very rapidly thus not allowing the avoidance of competitive exclusion. These are questions addressed in the first part of my thesis (“Comparative ecology”) consisting of two Chapters. 1: “Convergent evolution within an adaptive radiation of cichlid fishes” where we investigated ecologically based convergence within the Lake Tanganyika cichlid radiation and 2. “The ecological and genetic basis of convergent thick-lipped phenotypes in cichlid fishes” where we investigated the convergent occurrence of a conspicuous trait which is thought to be highly adaptive, the thick lipped phenotype of cichlid fishes. Considering the strong connectivity of convergent evolution with the ecological properties of a habitat mediated by natural selection it is apparently crucial to study ecological parameters of habitats connected by convergent phenotypes which we did in Chapter 3: "Depth-dependent abundance of Midas Cichlid fish (Amphilophus spp.) in two Nicaraguan crater lakes". Here we characterised effective population sizes by means of transect methods in order to compare two lakes exhibiting convergent phenotypes. In a second part (“Phylogeography”) I combine different studies dealing with a combination of distributional patterns, patterns of phylogenetic relationships and ecological factors of east African riverine cichlids since they have become increasingly important in the understanding of large-scale relations of African cichlid fishes. In Chapter 4: “Back to Tanganyika: a case of a recent immigration into a species flock of East African cichlid fishes” we investigate a recently discovered dispersal event of a modern cichlid lineage (Haplochromis spp.) across major watershed barriers in Eastern Africa. In Chapter 5: “Divergence between lake and stream habitats in an East African cichlid fish” we investigate the degree of ecological divergence of a riverine cichlid species, which also occurs in pure lake habitats (Astatotilapia burtoni). In Chapter 6: “Admixture between divergent mitochondrial lineages and greater phenotypic variation in a basal haplochromine cichlid fish from Lake Chila, Zambia” we investigate the phylogeographic history of a basal haplochormine clade (genus: Pseudocrenilabrus). Morphological diversity within natural populations is a crucial prerequisite for natural selection to act on and to enable ecological adaptive evolution. A special case of such morphological variation, the mouth asymmetry of scale eating cichlid fish of Lake Tanganyika, was the main topic of the third part of my thesis (“Asymmetrical Polymorphism”) and Chapter 7: “A field based assessment of attack strategies and feeding success in the scale eating cichlid fish Perissodus microlepis (Perciformes)”

Where Am I? Niche constraints due to morphological specialization in two Tanganyikan cichlid fish species

Food resource specialization within novel environments is considered a common axis of diversification in adaptive radiations. Feeding specializations are often coupled with striking morphological adaptations and exemplify the relation between morphology and diet (phenotype-environment correlations), as seen in, for example, Darwin finches, Hawaiian spiders, and the cichlid fish radiations in East African lakes. The cichlids' potential to rapidly exploit and occupy a variety of different habitats has previously been attributed to the variability and adaptability of their trophic structures including the pharyngeal jaw apparatus. Here we report a reciprocal transplant experiment designed to explore the adaptability of the trophic structures in highly specialized cichlid fish species. More specifically, we forced two common but ecologically distinct cichlid species from Lake Tanganyika,; Tropheus moorii; (rock-dweller), and; Xenotilapia boulengeri; (sand-dweller), to live on their preferred as well as on an unpreferred habitat (sand and rock, respectively). We measured their overall performance on the different habitat types and explored whether adaptive phenotypic plasticity is involved in adaptation. We found that, while habitat had no effect on the performance of; X. boulengeri; ,; T. moorii; performed significantly better in its preferred habitat. Despite an experimental duration of several months, we did not find a shift in the morphology of the lower pharyngeal jaw bone that would be indicative of adaptive phenotypic plasticity in this trait

The taxonomic diversity of the cichlid fish fauna of ancient Lake Tanganyika, East Africa

Ancient Lake Tanganyika in East Africa houses the world's ecologically and morphologically most diverse assemblage of cichlid fishes, and the third most species-rich after lakes Malawi and Victoria. Despite long-lasting scientific interest in the cichlid species flocks of the East African Great Lakes, for example in the context of adaptive radiation and explosive diversification, their taxonomy and systematics are only partially explored; and many cichlid species still await their formal description. Here, we provide a current inventory of the cichlid fish fauna of Lake Tanganyika, providing a complete list of all valid 208 Tanganyikan cichlid species, and discuss the taxonomic status of more than 50 undescribed taxa on the basis of the available literature as well as our own observations and collections around the lake. This leads us to conclude that there are at least 241 cichlid species present in Lake Tanganyika, all but two are endemic to the basin. We finally summarize some of the major taxonomic challenges regarding Lake Tanganyika's cichlid fauna. The taxonomic inventory of the cichlid fauna of Lake Tanganyika presented here will facilitate future research on the taxonomy and systematics and the ecology and evolution of the species flock, as well as its conservation

Back to Tanganyika: a case of recent trans-species-flock dispersal in East African haplochromine cichlid fishes

The species flocks of cichlid fishes in the East African Great Lakes are the largest vertebrate adaptive radiations in the world and illustrious textbook examples of convergent evolution between independent species assemblages. Although recent studies suggest some degrees of genetic exchange between riverine taxa and the lake faunas, not a single cichlid species is known from Lakes Tanganyika, Malawi and Victoria that is derived from the radiation associated with another of these lakes. Here, we report the discovery of a haplochromine cichlid species in Lake Tanganyika, which belongs genetically to the species flock of haplochromines of the Lake Victoria region. The new species colonized Lake Tanganyika only recently, suggesting that faunal exchange across watersheds and, hence, between isolated ichthyofaunas, is more common than previously thought

Structural manipulations of a shelter resource reveal underlying preference functions in a shell-dwelling cichlid fish

Many animals can modify the environments in which they live, thereby changing the selection pressures they experience. A common example of such niche construction is the use, creation or modification of environmental resources for use as nests or shelters. Because these resources often have correlated structural elements, it can be difficult to disentangle the relative contribution of these elements to resource choice, and the preference functions underlying niche-construction behaviour remain hidden. Here, we present an experimental paradigm that uses 3D scanning, modelling and printing to create replicas of structures that differ with respect to key structural attributes. We show that a niche-constructing, shell-dwelling cichlid fish,; Neolamprologus multifasciatus; , has strong open-ended preference functions for exaggerated shell replicas. Fish preferred shells that were fully intact and either enlarged, lengthened or had widened apertures. Shell intactness was the most important structural attribute, followed by shell length, then aperture width. We disentangle the relative roles of different shell attributes, which are tightly correlated in the wild, but nevertheless differentially influence shelter choice and therefore niche construction in this species. We highlight the broad utility of our approach when compared with more traditional methods (e.g. two-choice tasks) for studying animal decision-making in a range of contexts

Diet and habitat as determinants of intestine length in fishes

Fish biologists have long assumed a link between intestinal length and diet, and relative gut length or Zihler’s index are often used to classify species into trophic groups. This has been done for specific fish taxa or specific ecosystems, but not for a global fish dataset. Here, we assess these relationships across a dataset of 468 fish species (254 marine, 191 freshwater, and 23 occupy both habitats) in relation to body mass and fish length. Herbivores had significantly relatively stouter bodies and longer intestines than omni- and faunivores. Among faunivores, corallivores had longer intestines than invertivores, with piscivores having the shortest. There were no detectable differences between herbivore groups, possibly due to insufficient understanding of herbivorous fish diets. We propose that reasons for long intestines in fish include (i) difficult-to-digest items that require a symbiotic microbiome, and (ii) the dilution of easily digestible compounds with indigestible material (e.g., sand, wood, exoskeleton). Intestinal indices differed significantly between dietary groups, but there was substantial group overlap. Counter-intuitively, in the largest dataset, marine species had significantly shorter intestines than freshwater fish. These results put fish together with mammals as vertebrate taxa with clear convergence in intestine length in association with trophic level, in contrast to reptiles and birds, even if the peculiar feeding ecology of herbivorous fish is probably more varied than that of mammalian herbivores

Depth-dependent abundance of Midas Cichlid fish ( Amphilophus spp: ) in two Nicaraguan crater lakes

The Midas Cichlid species complex (Amphilophus spp.) in Central America serves as a prominent model system to study sympatric speciation and parallel adaptive radiation, since small arrays of equivalent ecotype morphs have evolved independently in different crater lakes. While the taxonomy and evolutionary history of the different species are well resolved, little is known about basic ecological parameters of Midas Cichlid assemblages. Here, we use a line transect survey to investigate the depth-dependent abundance of Amphilophus spp. along the shores of two Nicaraguan crater lakes, Apoyo and Xiloá. We find a considerable higher density of Midas cichlids in Lake Xiloá as compared to Lake Apoyo, especially at the shallowest depth level. This might be due to the higher eutrophication level of Lake Xiloá and associated differences in food availability, and/or the presence of a greater diversity of niches in that lake. In any case, convergent forms evolved despite noticeable differences in size, age, eutrophication level, and carrying capacity. Further, our data provide abundance and density estimates for Midas Cichlid fish, which serve as baseline for future surveys of these ecosystems and are also relevant to past and future modeling of ecological speciatio

Drivers and dynamics of a massive adaptive radiation in cichlid fishes

Adaptive radiation is the likely source of much of the ecological and morphological diversity of life; 1-4; . How adaptive radiations proceed and what determines their extent remains unclear in most cases; 1,4; . Here we report the in-depth examination of the spectacular adaptive radiation of cichlid fishes in Lake Tanganyika. On the basis of whole-genome phylogenetic analyses, multivariate morphological measurements of three ecologically relevant trait complexes (body shape, upper oral jaw morphology and lower pharyngeal jaw shape), scoring of pigmentation patterns and approximations of the ecology of nearly all of the approximately 240 cichlid species endemic to Lake Tanganyika, we show that the radiation occurred within the confines of the lake and that morphological diversification proceeded in consecutive trait-specific pulses of rapid morphospace expansion. We provide empirical support for two theoretical predictions of how adaptive radiations proceed, the 'early-burst' scenario; 1,5; (for body shape) and the stages model; 1,6,7; (for all traits investigated). Through the analysis of two genomes per species and by taking advantage of the uneven distribution of species in subclades of the radiation, we further show that species richness scales positively with per-individual heterozygosity, but is not correlated with transposable element content, number of gene duplications or genome-wide levels of selection in coding sequences

Mouth dimorphism in scale-eating cichlid fish from Lake Tanganyika advances individual fitness

Random asymmetry, that is the coexistence of left- and right-sided (or -handed) individuals within a population, is a particular case of natural variation; what triggers and maintains such dimorphisms remains unknown in most cases. Here, we report a field-based cage experiment in the scale-eating Tanganyikan cichlid Perissodus microlepis, which occurs in two morphs in nature: left-skewed and right-skewed individuals with respect to mouth orientation. Using underwater cages stocked with scale-eaters and natural prey fish, we first confirm that, under semi-natural conditions, left-skewed scale-eaters preferentially attack the right flank of their prey, whereas right-skewed individuals feed predominantly from the left side. We then demonstrate that scale-eaters have a higher probability for successful attacks when kept in dimorphic experimental populations (left- and right-skewed morphs together) as compared to monomorphic populations (left- or right-skewed morphs), most likely because prey fishes fail to accustom to strikes from both sides. The significantly increased probability for attacks appears to be the selective agent responsible for the evolution and maintenance of mouth dimorphism in P. microlepis, lending further support to the hypothesis that negative frequency-dependent selection is the stabilizing force balancing the mouth dimorphism at quasi-equal ratios in scale-eating cichlids

Data from: Mouth dimorphism in scale-eating cichlid fish from Lake Tanganyika advances individual fitness

Random asymmetry, that is the co-existence of left- and right-sided (or -handed) individuals within a population, is a particular case of natural variation; what triggers and maintains such dimorphisms remains unknown in most cases. Here, we report a field-based cage experiment in the scale-eating Tanganyikan cichlid Perissodus microlepis (Boulenger, 1898), which occurs in two morphs in nature: left-skewed and right-skewed individuals with respect to mouth orientation. Using underwater cages stocked with scale-eaters and natural prey fish, we first confirm that, under semi-natural conditions, left-skewed scale-eaters preferentially attack the right flank of their prey, whereas right-skewed individuals feed predominantly from the left side. We then demonstrate that scale-eaters have a higher probability for successful attacks when kept in dimorphic experimental populations (left- AND right-skewed morphs together) as compared to monomorphic populations (left- OR right-skewed morphs), most likely because prey fishes fail to accustom to strikes from both sides. The significantly increased probability for attacks appears to be the selective agent responsible for the evolution and maintenance of mouth dimorphism in P. microlepis, lending further support to the hypothesis that negative frequency-dependent selection is the stabilizing force balancing the mouth dimorphism at quasi-equal ratios in scale-eating cichlids