Multiple genetic lineages and inter-island gene flow in <i>Nemadactylus monodactylus</i> at Tristan da Cunha: implications for phylogeography and taxonomy

<jats:p>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 (&lt;i&gt;Nemadactylus monodactylus&lt;/i&gt;), a dominant 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 &lt;i&gt;N. macropterus&lt;/i&gt; or &lt;i&gt;N. bergi&lt;/i&gt;, whereas one was newly described and may be endemic to TDC. The overall phylogeographic patterns add to evidence of a complex evolutionary history within &lt;i&gt;Nemadactylus&lt;/i&gt; and likely inadequacy of current taxonomic designations. Among the TDC samples, mtDNA and microsatellite data reported no structuring among islands, including between the distinct Gough morphotype and the northern islands. This lack of genetic structure, despite environmental contrasts among islands, supports treatment of &lt;i&gt;N. monodactylus&lt;/i&gt; as a single genetic population and management unit across the archipelago. Broader geographic sampling and genome-wide nuclear data are recommended to test species limits and reconstruct the evolutionary history across the genus.</jats:p>