There are currently 13 described mastacembelid species endemic to LT [25], as opposed to a single (possibly two) endemic species within Lake Malawi [26, 27]. This asymmetry is also seen in other groups that form radiations in Tanganyika but not Malawi (e.g. Synodontis catfish), although notably Lake Malawi Bathyclarias catfish form a small species flock [28]. Despite the age and size of Lake Malawi, and the fact that, like LT, it supports a large-scale radiation of cichlids [29], this asymmetry between the two lakes in species diversity of Mastacembelus and Synodontis is noteworthy. Potential factors, such as the repeated periods of desiccation experienced in Lake Malawi [30, 31], or niche availability with the presence of an extensive cichlid radiation, may have impinged on the abilities of other faunas to diversify.
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Diversification of the main Mastacembelus lineages occurred contemporaneously at 6.2-7.2 Myr, soon after their initial seeding of the lake. This initial diversification upon colonising LT is also apparent in Tropheus cichlids [44], and represents a short lag time, or phylogenetic 'tail' [45]. Following the initial post-colonisation divergence, there are further contemporaneous speciation events 3-4 Myr. The split of M. platysoma and M. aff. platysoma occurs around 3.5 Myr, coinciding with the estimated formation of the southern basin of LT 2-4 Myr [1], and a period of lower lake-levels, caused by an episode of aridification [46]. The clades (M. micropectus, M. sp. nov. 1) and (M. ellipsifer (M. flavidus (M. zebratus, M. plagiostomus))) also arose during this time of lake-level change, as do internal lineages within LT Synodontis [7, 15] and Platythelphusa [6] radiations. The coinciding of speciation events in unrelated taxa with an extrinsic event, indicates that this period of lower lake-level is likely to have been a key factor responsible for promoting speciation conditions; for example, repeated segmentation and recombination of habitats along the rocky shorelines, caused by these fluctuations in water level, is likely to have resulted in the allopatric speciation of these LT radiations.
The endemic LT Mastacembelus eel radiation is an important assemblage for studying comparative lacustrine systems, as it is divergent in life history to those already studied within the Great Lakes. The use of molecular phylogenetic techniques has revealed as yet undescribed diversity, with our data providing evidence for two potentially new LT species (M. aff. platysoma and M. sp. nov. 1). The LT Mastacembelus demonstrates both similarities and differences in patterns of speciation when compared to other LT radiations. For example, the origination of LT Mastacembelus via a single colonisation event is also demonstrated by Platythelphusa crabs [6] and Cyprichromis cichlids [12]. Using fossil calibrations from a related family, our results indicate Mastacembelus colonised the lake 7.9 Myr, and is therefore an older radiation than Synodontis catfish, Platythelphusa crabs, and many cichlid tribes (e.g. Cyprichromini, Tropheini, Ectodini) if fossil dates are assumed [12]. This puts the origin of LT Mastacembelus within the age of the LT basin, but prior to the onset of full lacustrine conditions. Their radiation within lacustrine conditions does however further demonstrate LT as a hotspot of diversification, as opposed to an 'ancient evolutionary reservoir.' As demonstrated by other ichthyological faunas with lacustrine and fluviatile distributions (e.g. Synodontis catfish and cichlid fishes), our data also highlights evidence of distinct biogeographic clades. At a deeper phylogenetic level, we find evidence for an Africa-Asia split of mastacembelid eels (19 Myr) occurring long after the divergence of the associated continents (121-165 Myr). This divergence coincides with the closure of the Tethys Sea and we therefore suggest a dispersal scenario for this group, which should be validated in the future with increased taxon sampling.
Previous studies have shown that environmental influences, mainly climatically driven lake level fluctuations (Cohen et al., 1997; Scholz et al., 2007; McGlue et al., 2008), have synchronized population divergence and patterns of past population size changes of stenotopic rock-dwelling cichlids within and across Lake Tanganyika and Malawi (Sturmbauer et al., 2001; Genner et al., 2010a; Koblmüller et al., 2011; Nevado et al., 2013). Whether population histories of highly mobile benthopelagic or truly pelagic species have been equally affected by large lake level fluctuations remains largely unknown.
Boulengerochromis microlepis is one of the top-predators in Lake Tanganyika. As such, B. microlepis is highly mobile and not restricted to a particular type of habitat or depth and apparently disperses over long distances without habitat-imposed restrictions. Similar to B. microlepis, phylogeographic structure is also lacking in benthopelagic cichlids in the genera Rhamphochromis and Diplotaxodon from Lake Malawi (Shaw et al., 2000; Genner et al., 2008, 2010b). Both genera comprise medium-sized to large offshore predators that feed in the water column, with large fish being almost exclusively piscivorous (Turner et al., 2002). One Rhamphochromis species studied in detail, R. longiceps, uses lagoons and satellite lakes as nursery areas, which might promote population differentiation if fish remained close to their breeding grounds. Yet, no geographic population structure has been detected, which has been explained by an apparent lack of homing behavior, and dispersal mainly determined by prey availability and opportunistic selection of suitable nursery grounds (Genner et al., 2008). Similarly, four of the eight Diplotaxodon species studied so far show no spatial population subdivision, whereas the other four species show slight but significant spatial genetic differences among breeding grounds, indicating natal homing to breeding grounds in these species (Genner et al., 2010b). Fish in our sample were mainly caught by hook and line (by artisanal fishermen) and are therefore unlikely to comprise a large portion of breeding and fry-guarding individuals, which purportedly do not feed at all (Poll, 1956; Kuwamura, 1986; Fohrmann, 1994) and hence cannot be caught with bait. Consequently, our sample does not allow inferences on natal homing, as these large and mobile fish may have been collected far from their breeding grounds.
Another puzzle, given the age of the lineage, is its failure to seed more than a single species. The other lineages of predatory benthopelagic cichlids in Lakes Malawi and Tanganyika are typically species poor as well, but nevertheless radiated into at few distinct taxa (Turner, 1996; Genner et al., 2008; Koblmüller et al., 2008). A meta-analysis of lacustrine cichlid assemblages suggested that the major factors that predispose cichlids to adaptive radiations are ecological opportunity and intrinsic lineage-specific traits related to sexual selection (Wagner et al., 2012). Compared to the benthopelagic, polygynous mouthbrooders in Lakes Tanganyika and Malawi, sexual selection has probably been weak in B. microlepis, judging by its monogamous mating system and lack of sexual dimorphism. Moreover, the community structure in Lake Tanganyika may also have played a significant role in preventing a radiation of the Boulengerochromini. In Lake Tanganyika, numerous piscivorous fish species from other (cichlid and non-cichlid) lineages coexist with B. microlepis, including the smaller piscivores in the genera Lepidiolamprologus and Bathybates as well as Hemibates stenosoma, and several endemic species of clariid catfish and lates perches that grow considerably larger than B. microlepis. Hence, available niche space of B. microlepis, as determined by body size, is probably restricted by heterospecific trophic competition. The extant piscivorous cichlid lineages colonized Lake Tanganyika roughly simultaneously, shortly after the establishment of deep-water conditions (Koblmüller et al., 2008; no data are available for late perches and clariid catfish). Sexual selection in the mouthbrooding piscivores and geographic isolation among the littoral substrate breeders may have given these lineages a head start in speciation, such that the Boulengerochromini ancestor quickly found the neighboring niches occupied. Contrary to restricted diversification, extinction may have eliminated traces of past speciation in the Boulengerochromini. Unfortunately, no means exist to reconstruct historic diversity beyond the coalescence of the extant mitochondrial haplotypes. 2ff7e9595c
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