23-06-2025
The Portuguese Man Of War Is Actually Four Distinct Species
After more than 200 years of controversy, genomics has finally started to address the mystery of whether the Portuguese man o' war is one species or many.
A beached Portuguese Man Of War (Physalia species).
A large international team of scientists sequenced the genomes of 151 Portuguese men o' war from across their range throughout the Pacific, Atlantic and Indian Oceans (Figure 1b) and discovered these animals comprise at least four distinct species, each with its own unique morphology, genetics, and distribution. Additionally, the researchers also think there are several distinct subpopulations, likely shaped by regional winds and ocean currents.
The name, man o' war, is derived from the animal's resemblance to the Portuguese sailing warship, the caravel or man of war, in full sail. The man o' war is also known as the bluebottle. This animal is part of a poorly known flotilla of peculiar creatures living on or at the surface of the water known as the neuston.
The man o' war is a siphonophore, a carnivorous colonial animal comprised of many smaller units called zooids that are genetically identical but specialized for different functions, such as hunting, digestion, floating or reproduction, thereby allowing the colony to operate as a single individual (Figure 1a). A colorful gas-filled float, the pneumatophore, provides buoyancy so the animal stays afloat whilst its tentacles, which can be up to 30 m (100 ft) long, hang below the surface, containing venomous cnidocytes that use powerful stings to capture and kill prey – usually fish. A sail atop the float, which may be left or right-handed, allows the animal to drift freely across the surface of open ocean, often in groups.
As the result of their study, the researchers found strong evidence of reproductive isolation amongst the five genetic lineages, challenging the long-held assumption that the open ocean supported a single, well-mixed population (Figure 1c).
F I G U R E 1 : Anatomy, distribution, and genomic variation of Physalia (A) Physalia colonies ... More 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 the 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 (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.
The team sequenced the genome of each bluebottle, then grouped them together in clusters based on their geographic range in the oceans (Figure 1c). What they found was distinct genetic differences between groups of men o' war from different regions, despite some overlap between the populations.
'We were shocked, because we assumed they were all the same species,' said one of the study's co-authors, Kylie Pitt in a statement provided by Griffith University. 'But the genetic data clearly show they're not only different, they're not even interbreeding despite overlapping ranges. The bluebottle is uniquely suited to long-distance travel, using its gas-filled float and muscular crest to catch the wind and sail the sea surface.'
Men o' war (Physalia physalis) often sail around on the ocean's surface in large clusters and can ... More even wash up on beaches in large groups. Hamilton Parish, Bermuda. Credit: Miguel A Mejias, PhD., CC-BY-NC)
'These animals have been fascinating to swimmers, sailors, and scientists for many centuries, in part because they are stunningly beautiful, but also because they have a powerful sting they use to catch fish,' said the study's lead author, evolutionary biologist Samuel Church, an Associate Research Scientist at Yale University. Dr Church, who is affiliated with both Yale University and the Yale Peabody Museum, uses a combination of field work, lab work, and computer modelling to explore the evolutionary tree of life, and is poised to start a new position as an assistant professor at New York University this autumn.
'In our case, we were aware of the scientific debate that has been going on for 200+ years about how many species there are around the world,' Dr Church elaborated in email. 'By sampling globally and using modern genomic technologies, we hoped to settle that debate.'
Man o' war (Physalia megalista), still alive on the shore at Anawhata Beach, Aotearoa New Zealand. ... More (Credit: Jacqui Geux, iNaturalist, some rights reserved CC-BY)
The study describes these species: Physalia physalis, P. utriculus, and P. megalista, alongside a newly identified species, Physalia minuta, a small man o' war found near New Zealand and Australia. Each species is further subdivided into genetically distinct subpopulations shaped by regional winds and ocean currents, according to advanced ocean circulation modelling.
Dr Church and collaborators then matched the genomic lineages to four distinct physical forms identified from more than 4,000 citizen science images shared on Dr Church and collaborators compared differences in tentacle length, color and other physical characteristics to previous records of men o' war found around the world, including descriptions in centuries-old publications that argued there are some number of different species. These distinct animals – originally proposed as separate species in the 18th and 19th centuries but later disregarded – have now been verified by modern genomic evidence. Which of course, raises the question: Why did people think there was just one species of Physalia?
'Over the past 200+ years there have been dozens of species of Physalia described, but in the 20th century people came to believe there was only one. This is in part because the open ocean is often treated as a single environment, that is basically the same across the world's oceans,' Dr Church explained in email. But the genomic data reveal this is not the case. Further, some parts of the world, such as Eastern Australia, are home to multiple men o' war species that remain distinct despite co-existing alongside each other.
Man o' war (P. utriculus) from Hawai'i. (Credit: Casey W. Dunn, Namrata Ahuja, Dalila Destanović, ... More image appears here via Samual Church with kind permission.)
'It was exciting to show that was not the case, and that some of the species that were proposed hundreds of years ago were actually valid,' Dr Church observed in email. 'Even within species we see strong regional structure, so that populations, for example, from one side of the Atlantic are distinct from those on the other side.'
How could they evolve separate species when they all live side-by-side in 'the same' environment? Why don't they interbreed?
'We still don't know,' Dr Church replied in email. 'We can see that there are multiple species with overlapping ranges, but we don't yet know what keeps them from interbreeding. There is a lot more to learn, especially about their reproduction.'
Do you have any clues what the selection pressures might have been that led to the evolution of these distinct species?
'Again, we have a lot more to learn about these animals,' Dr Church responded in email. 'We don't know if they may eat different fish species, or if they all have the same potent sting. What we can say is that they are very different sizes – our new species, Physalia minuta, is much smaller than the others even at maturity' – thus earning that new species its name, 'minuta,' meaning small.
A new species of man o' war (Physalia minuta), washed up on Foxton Beach, Aotearoa New Zealand. ... More (Credit: Arnim Littek, CC-BY, iNaturalist)
Are there any other regional species, other than Physalia minuta?
'We found evidence suggesting that within species there are strong signatures of different subpopulations. More research needs to be done to test how different these might be,' Dr Church proposed in email. 'Our new species, Physalia minuta, is a regional endemic, found only around New Zealand and Australia. We tried to get samples from all over the world, but it's still possible there are other such regional species we haven't yet observed.'
Investigating the diversity of the man o' war is just the beginning of the many questions that will be inspired by this knowledge.
'[T]he ocean surface ecosystem has more biodiversity than previously assumed,' Dr Church explained in email. 'This unique ecosystem is sometimes characterized as having only a few species, each with huge populations stretching thousands of miles. Our study is a part of a growing body of work that shows instead there are multiple species at the ocean surface, with distinct but overlapping ranges, some of which are local only to certain parts of the world.'
Dr Church and collaborators point out that this important study was made possible by a large collaborative team effort.
'It's important to highlight the amazing international contribution of this work. We received samples from dozens of scientists across countries and continents,' Dr Church stated in email. 'This was truly a global team effort, with our co-authors collecting specimens from their local beaches and sending them to us at the Yale Peabody Museum.'
Future investigations of the physical, environmental, and biological processes that generate and maintain this genetic and morphological variation in men o' war will be crucial in recalibrating our understanding of open-ocean biodiversity.
Man o' war (Physalia physalis) photographed at sunset when washed up on the beach, Straits of ... More Florida, Hollywood, Florida. (Credit: Keara R. Giannotti, iNaturalist, CC-BY-NC)
Source:
Samuel H. Church, River B. Abedon, Namrata Ahuja, Colin J. Anthony, Dalila Destanović, Diego A. Ramirez, Lourdes M. Rojas, Maria E. Albinsson, Itziar Álvarez Trasobares, Reza E. Bergemann, Ozren Bogdanovic, David R. Burdick, Tauana J. Cunha, Alejandro Damian-Serrano, Guillermo D'Elía, Kirstin B. Dion, Thomas K. Doyle, João M. Gonçalves, Alvaro Gonzalez Rajal, Steven H.D. Haddock, Rebecca R. Helm, Diane Le Gouvello, Zachary R. Lewis, Bruno I.M.M. Magalhães, Maciej K. Mańko, C. Gabriela Mayorga-Adame, Alex de Mendoza, Carlos J. Moura, Catriona Munro, Ronel Nel, Kohei Oguchi, Jessica N. Perelman, Laura Prieto, Kylie A. Pitt, Moninya Roughan, Amandine Schaeffer, Andrea L. Schmidt, Javier Sellanes, Nerida G. Wilson, Gaku Yamamoto Eric A. Lazo-Wasem, Chris Simon, Mary Beth Decker Jenn M. Coughlan and Casey W. Dunn (2025). Population genomics of a sailing siphonophore reveals genetic structure in the open ocean, Current Biology | doi:10.1016/
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