Population genomics of a sailing siphonophore reveals genetic structure in the open ocean.

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 man-o'-war or bluebottle (Physalia) is uniquely suited to long-distance travel, using its gas-filled float and muscular crest to catch the wind and sail the sea surface. We tested the hypothesis of a global, panmictic Physalia population by sequencing whole genomes of 151 samples and found five distinct lineages, with multiple lines of evidence indicating strong reproductive isolation, despite range overlap. We then scored thousands of images of Physalia uploaded to the citizen-science website inaturalist.org and identified four recognizable morphologies, described their geographical distribution, and linked them to four of the lineages that were identified with genomic data. We conclude there are at least four species, three of which correspond to species proposed by scientists in the 18th and 19th centuries, along with one newly named species, Physalia minuta Church and Dunn, sp. nov. Within each species, we observe significant population structure, with evidence of persistent subpopulations at a regional scale. We used ocean circulation modeling to show that these subpopulations align with predominant currents and winds. Our findings indicate that, even in these sailing species, genetic variation is highly partitioned across the open ocean.