Fig. 1: Talorchestia martensii sampling locations, population dynamics, and dispersal simulation models.

a Collection sites included in this study. Colored dots correspond to T. martensii populations: Vietnam-Thailand (VT, red), North Borneo (NB, blue), South Borneo (SB, purple), East Borneo (EB, orange), and South Sulawesi (SS, green). Light green dots correspond to other talitrid species. The right pictures show driftwood in the coastal area and T. martensii on wood. b Unrooted phylogeny reconstructed using RAxML. c Population structure analysis of T. martensii from K = 3–6, in which K = 5 was the best-fit model. d Effective population size fluctuations of the five geographical populations estimated by PSMC. e Gene flow between populations estimated by G-PhoCS suggest northward gene flow (n = 11) is significantly greater than the southward (n = 9). Statistical significance was assessed using the two-tailed Mann-Whitney U test. f Two migration events estimated by TreeMix; arrows indicate northward migration, and are colored according to the migration weight. g–i Alternative hypotheses of dispersal and genetic simulations. The inset top-right plots represent the genetic simulations of each migration model. Representative random dispersal from one location to surrounding places; the inset bottom-left plot shows that the Mantel test rejected that gene flow increased with geographic proximity, and Spearman’s P-value used. Shading indicates the 95% confidence intervals. A linear regression model with a two-sided test was used for the statistical analysis, and the multiple R-squared was used. g Representative directional dispersal, including northward (h) and southward dispersal (i) from source to destination. All genetic simulations are available in Supplementary Figs. 5 and 6, and Supplementary Table 4. The color gradients of oceans indicate ocean current velocity, and arrows show the directions in summer (h) and winter (i). Source data are provided as a Source Data file.