Multiple Genetic Lineages and Inter-Island Gene Flow in Nemadactylus monodactylus at Tristan da Cunha: Implications for Phylogeography and Taxonomy.

Tristan da Cunha (TDC) is a remote archipelago of four South Atlantic islands whose nearshore fish assemblages remain comparatively poorly studied. Genetic studies of such disjunct groups can provide valuable information on evolutionary history, taxonomy, and stock structure relevant for conservation and sustainable fishery management. Here, we address such topics in the Five-finger (Nemadactylus monodactylus), an abundant nearshore fish harvested for subsistence at TDC, using a combination of mitochondrial control region sequencing and microsatellite analysis. Mitochondrial data, including publicly available data for Southern Hemisphere congeners, recovered four well-supported clades. Three clades were polyphyletic, containing sequences previously assigned to N. macropterus or N. bergi, whereas one was newly described and may be endemic to TDC. These patterns add to evidence for a complex evolutionary history within Nemadactylus and suggest that current taxonomic designations warrant re-evaluation. mtDNA also confirms colonisation of TDC by multiple lineages. Neither mtDNA nor microsatellites provided consistent evidence of genetic differentiation among islands, including between the distinctive Gough morphotype and the northern islands. These results are consistent with substantial archipelago-wide connectivity, but the limited nuclear marker set means that weak structure cannot be excluded. We therefore regard a single archipelago-wide population as a provisional working hypothesis rather than a confirmed management unit. Broader geographic sampling and genome-wide nuclear data are needed to test species limits, reconstruct evolutionary history, and resolve fine-scale population structure relevant to management.