Extreme population subdivision despite high colonization ability: contrasting regional patterns in intertidal tardigrades from the west coast of North America

Aim To determine whether microscopic marine organisms experience more extreme population subdivision than co‐distributed macroscopic organisms. Location The intertidal zone of the west coast of North America. Methods We used phylogenetic methods, haplotype networks and analyses of molecular variance (AMOVAs) to examine geographical subdivision in tardigrades (Echiniscoides) between the northern tip of Vancouver Island and San Diego. Results Analyses of 330 individuals from 19 populations revealed a genetically diverse set of tardigrades probably comprising between four and fifteen species. Southern populations displayed extreme population subdivision and contained many deeply divergent lineages that are fully or nearly restricted to single sampling sites. In contrast, northern populations exhibited lower levels of phylogeographical structure and few geographically restricted lineages. AMOVAs showed that inter‐population variation was maximized when nearly all southern populations were kept distinct and all northern populations were treated as a single population. Main conclusions Strong patterns of population subdivision on fine spatial scales in tardigrades stand in sharp contrast to previous studies of macroscopic organisms across this same range, suggesting that geographical subdivision in microscopic organisms is stronger than in co‐occurring macroscopic organisms.