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. 2025 Aug 4;35(15):3556-3569.e6.
doi: 10.1016/j.cub.2025.05.066. Epub 2025 Jun 19.

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

Samuel H Church  1 , River B Abedon  2 , Namrata Ahuja  2 , Colin J Anthony  3 , Dalila Destanović  2 , Diego A Ramirez  2 , Lourdes M Rojas  4 , Maria E Albinsson  5 , Itziar Álvarez Trasobares  6 , Reza E Bergemann  2 , Ozren Bogdanovic  7 , David R Burdick  3 , Tauana J Cunha  8 , Alejandro Damian-Serrano  9 , Guillermo D'Elía  10 , Kirstin B Dion  2 , Thomas K Doyle  11 , João M Gonçalves  12 , Alvaro Gonzalez Rajal  13 , Steven H D Haddock  14 , Rebecca R Helm  15 , Diane Le Gouvello  16 , Zachary R Lewis  17 , Bruno I M M Magalhães  12 , Maciej K Mańko  18 , C Gabriela Mayorga-Adame  19 , Alex de Mendoza  20 , Carlos J Moura  12 , Catriona Munro  21 , Ronel Nel  16 , Kohei Oguchi  22 , Jessica N Perelman  23 , Laura Prieto  6 , Kylie A Pitt  24 , Moninya Roughan  25 , Amandine Schaeffer  26 , Andrea L Schmidt  23 , Javier Sellanes  27 , Nerida G Wilson  28 , Gaku Yamamoto  29 , Eric A Lazo-Wasem  4 , Chris Simon  30 , Mary Beth Decker  31 , Jenn M Coughlan  2 , Casey W Dunn  31
Affiliations

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

Samuel H Church et al. Curr Biol. .

Abstract

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.

Keywords: biodiversity; cnidaria; hydrozoa; marine biology; morphological variation; neuston; ocean drift trajectories; ocean surface; participatory science; pelagic ecosystems.

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Conflict of interest statement

Declaration of interests The authors declare no competing interests.

Figures

Figure 1.
Figure 1.. Anatomy, distribution, and genomic variation of Physalia.
(A) Physalia colonies comprise a muscular sail attached to a gas-filled float which maintains the mature animal at the surface of the water. Colony bodies (zooids), including those specialized for feeding (gastrozooids), prey capture (palpons with tentacles), and reproduction (gonozooids) are added to the float via asexual reproduction at growth zones. Tentacles drape below the float to trap, sting, and retrieve fish using batteries of stinging capsules contained in tentilla. Photos of Physalia minuta Church and Dunn, sp. nov., specimens YPM IZ 111236 (main), YPM IZ 111237 (growth zone), and YPM IZ 111240 (tentacle). (B) Physalia are observed throughout the world, as shown by observations posted to inaturalist.org (black). Samples for genomic analysis (blue) were collected by an international collaboration of scientists. (C) The first two principal components of genomic variation reveal five clusters labeled A, B1, B2, C1, and C2. See Figure S2, Table S7.
Figure 2.
Figure 2.. Multiple lines of evidence indicate reproductive isolation between lineages.
(A) The distribution of the five clusters from Figure 1 shows some lineages span multiple ocean basins (e.g., B1, C1) and others are restricted to smaller areas (e.g., C2 observed in New Zealand and Tasmania). Labels indicate cluster present at collecting site. (B) Reciprocal fixation index (Fst) averaged across non-repeat genomic windows indicate high levels of reproductive isolation between all lineages, with the weakest between B1 and B2. (C) Phylogenetic analysis of mitochondrial genomes shows reciprocal monophyly of lineages. Bootstrap values are shown at internal Physalia nodes. (D) Shared ancestry analysis shows lineages with little evidence of mixture. See Figures S2, S7, Tables S1-S6.
Figure 3.
Figure 3.. Distinct morphologies are detectable in citizen science images.
(A) Morphological traits such as aspects of size, color, and tentacle number were scored for thousands of images on inaturalist.org. From these, four morphologies were identified, three of which correspond to historically proposed species,-,. (B) Representative photos of each morphology from iNaturalist, all made available under Creative Commons licence CC-0 or CC-BY, credited (left to right, top to bottom): Eileen Mattei, Seth Wollney, Amanda Chase, Tim (user twan3253), William Stephens, Andrew Gillespie, Manuel R. Popp, Hazel Valerie, Jacqui Geux, Emily Roberts, Arnim Littek, Arnim Littek, Jacqui Geux, Roderic Page, Arnim Littek. (C) Ranges of positively identified iNaturalist records for each morphology, using a rule-based analysis of morphological traits (Figure S4). (D) Morphologies were assigned to a genomic cluster by scoring the same traits of genomic specimens. Cluster B2 could not be definitively assigned due to lack of images of specimens upon collection. See Figure S4.
Figure 4.
Figure 4.. Drift trajectories and PCA show subpopulations have distinct regions of origin, aligned with predominant currents and winds.
(A-D) 1000 hindcast drift trajectories were hindcast for each specimen using an ocean circulation model,, starting from the location and date of collection to the predicted position nine months prior to collection. Shapes indicate sampling location (e.g., filled circle for Florida) and colors indicate regions of the ocean (e.g., Northwest Atlantic). Trajectories were not hindcast for specimens collected in 2024 (e.g., P. megalista from New Zealand). (E-H) Principal component analyses of genomic variation within species revealed samples clustered by oceanic region. Exceptions to this pattern are marked with black arrows; these individuals suggest long-distance dispersal events across regions. See Figures S5-S7, Tables S5-S6.
Figure 5.
Figure 5.. Type series of Physalia minuta Church and Dunn, sp. nov.
Dorsal and ventral images of specimens post-fixation in 95% ethanol, scale bar = 5 mm. Gonodendra images taken upon collection, scale bar = 1 mm, asterisks indicate scale bar was not recorded. Black arrows indicate example gonophores used in determining sex.
See this image and copyright information in PMC

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