Global genomics of the man-o'-war (Physalia) reveal biodiversity at the ocean surface

The open ocean is a vast, highly connected environment, and the organisms found there have been hypothesized to represent massive, well-mixed populations. Of these, the Portuguese man-o'-war (Physalia) is uniquely suited to dispersal, sailing the ocean surface with a muscular crest. We tested the hypothesis of a single, panmictic Physalia population by sequencing 133 genomes, and found five distinct lineages, with multiple lines of evidence showing strong reproductive isolation despite range overlap. We then scored thousands of citizen-science photos and identified four recognizable morphologies linked to these lineages. Within lineages, we detected regionally endemic subpopulations, connected by winds and currents, and identified individual long-distance dispersal events. We find that, even in these sailing species, genetic variation is highly partitioned geographically across the open ocean.