The effects of pelagic larval duration on the geographic population structure of two sipunculan species in the Sea of Japan

Abstract

This study examines genetic diversity in two species of sipunculan worm, Phascolosoma agassizii and Themiste pyroides, in the Sea of Japan. Low sea levels of the Pleistocene era partially or completely isolated marginal seas of the northwestern Pacific, including the Sea of Japan. Themiste pyroides exhibits a larval stage of approximately 15 days, about half as long as the 31-day pelagic larval stage of P. agassizii. These differences in pelagic larval duration (PLD) may impact geographic population structure. I hypothesize that a longer PLD will result in increased inter-population gene flow and genetic homogeneity in P. agassizii. For both species, we sampled four populations within the Sea of Japan. We then compared the inter-population and intra-population genetic diversity. Analyzing sequence data from 16S ribosomal RNA (16S) and cytochrome c oxidase subunit I (COI) with the Analysis of Molecular Variance (AMOVA) statistic, we found substantially higher geographic population structure in T. pyroides than in P. agassizii. These results may support a direct correlation between PLD and gene flow, indicating a dynamic relationship between life history traits and geographic population structure.