The Nactus pelagicus complex consists of a unisexual lineage, N. pelagicus, as well as several genetically distinct bisexual lineages. The unisexual and bisexual lineages are rarely sympatric but do co-occur on two islands in southern Vanuatu. Nactus pelagicus and the bisexual N. multicarinatus co-occur on Aneityum and Tanna Islands, although the distribution of N. multicarinatus may be limited on Aneityum and may be limited to a single locality (Port Resolution) on the eastern coast of Tanna. Previous analyses suggested that these species have different chin scalation patterns. However, species identifications were assigned solely on presence or absence of males from localities. Moreover, because these species occupy different regions, morphological analyses can be confounded by interisland variation. To evaluate whether chin scalation patterns differ consistently between N. pelagicus and N. multicarinatus as previously suggested, we used molecular sequence data to delimit species from nine populations on a single island in Vanuatu. All N. multicarinatus examined had a single chin scalation pattern whereas variation in this trait was observed in N. pelagicus; there was no overlap in this trait between the species. We hypothesize that the variation in this trait in the unisexual species N. pelagicus could result from developmental instability as a result of perturbations during development caused by incompatibility between the two parental genomes that contributed to the formation of this unisexual lineage.
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Vol. 69 • No. 2