Genetic homogeneity in soldier crab <i>Mictyris guinotae</i> populations across their geographic distribution in the Ryukyu Islands, Japan

The Japanese Archipelago comprises a chain of islands; therefore, gaps between islands can represent potential dispersal barriers for coastal species, especially those inhabiting tidal flats. This study investigates the population genetic structure of the soldier crab <i>Mictyris guinotae</i> (Brachyura: Thoracotremata: Mictyridae) using mitochondrial COI gene sequences. Specimens were collected from Tanegashima Island and five Ryukyu Islands, covering the species’ known geographic distribution. Although the Mantel test identified a moderate trend (r² = 0.41, <i>P</i> &lt; 0.01) for isolation by distance, all populations exhibited high haplotype and nucleotide diversities, with neither analysis of molecular variance nor Φ_ST detecting genetic differences among populations. Notably, the northernmost population (Tanegashima Island), approximately 260 km from the nearest known geographic distribution on Amami-Oshima Island, shared haplotypes with other populations, although the number of specimens from Tanegashima Island is only four due to its small population size. This absence of genetic differences among populations of <i>M. guinotae</i> differs from other intertidal organisms in the region examined in previous studies. Mismatch distributions suggest contrasting demographic history in <i>M</i>. <i>guinotae</i> and the sand bubbler crab <i>Scopimera ryukyuensis</i>, which has been revealed to show population genetic structuring despite its similar geographic distribution to <i>M</i>. <i>guinotae</i>. The present study provides another case of genetic diversity among intertidal organisms in the Ryukyu Islands.