On the Indonesian island of Sulawesi, formerly known as Celebes, two unconnected systems of ancient lakes, i.e. Lake Poso and the Malili lake system, harbour several endemic species and species flocks that apparently evolved as a consequence of adaptive radiation (von Rintelen et al., 2004; Roy et al., 2004, 2007; Herder et al., 2006). The Malili lake system consists of the three major lakes, Matano, Mahalona and Towuti, and two additional smaller ones. Overall, the water of these lakes is clear and oligotroph with very limited biological production (Haffner et al., 2006). Lake Matano is especially remarkable by being a graben lake of approximately 590m depth (Haffner et al., 2001) with an age of about 2-4 Myr (Brooks, 1959; Hamilton, 1997).
A variety of animal taxa evolved species flocks within the Malili lake system, e. g. gastropods (von Rintelen et al., 2004), fish (Roy et al., 2004, 2007; Herder et al., 2006) and shrimps (von Rintelen et al., in preparation), of which the phylogenetic relationships and adaptive radiations are being studied. Within the lakes, there are also at least five endemic species of freshwater crabs (Chia and Ng, 2006) belonging to three different ecotypes (Schubart and Koller, 2006; Schubart and Ng, in press). The latter authors have shown that both Lake Poso and the Malili lake system have been colonized independently at least twice. Part of the ecological and taxonomic diversity of these crabs can thus not be the consequence of a single adaptive radiation. The previous studies also showed that two species of freshwater crabs from the Malili lakes, namely Parathelphusa ferruginea and Nautilothelphusa zimmeri (from lakes Matano and Mahalona), share identical haplotypes of the mitochondrial 16S rRNA gene. In contrast, the Lake Matano population of N. zimmeri forms the outgroup to this clade of sympatrically occurring populations of N. zimmeri and P. ferruginea from lakes Mahalona and Towuti, despite the fact, that the monotypic genus Nautilothelphusa appears morphologically and ecologically well characterised and differentiated.
The aim of the present study was to investigate the phylogenetic mitochondrial history of Nautilothephusa zimmeri and Parathelphusa ferruginea with more detail. A more variable genetic marker and a larger sample size was therefore used to help distinguish consistently between representatives of both species in Lake Mahalona and Lake Towuti and to test, whether P. ferruginea mtDNAmaintains its intermediate phylogenetic position between two populations of N. zimmeri, or whether this pattern would change with a better resolved phylogeny. Compared to the previous qualitative work, the larger sample size of this study allows application of quantitative population genetic methods in order to determine whether there is ongoing gene flow among the sympatric species.