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Jelly Fish: Portuguese Man O’ War

Updated: Jul 30, 2022

When it comes to introducing Physalia physalis, one must first talk about taxonomic classification. The genus Physalia is the sole representative of the Physaliidae family, and includes a single described species: Physalia physalis, according to the World Register of Marine Species. This species is also commonly referred by the public as the ‘Portuguese man o’ war’ or ‘blue bottle jellyfish’.

Amazing colouration

It is important to note that Physalia physalis is not a jellyfish but a siphonophore, an order of Hydrozoans, somewhat related to jellyfishes but also, and maybe more surprising, to corals. Altogether they form a larger phylum known as Cnidaria, rich with about 11,000 species.

This species is easily identifiable from its overall blue color and the gas-filled translucent float that can have blue, purple or pink coloration. This coloration frequently disappears in museum preservation, a difficult process for soft-bodied invertebrates, such as for the specimen at the HKBM.

Loss of colouration in specimen

Physalia physalis is thus not a “true jellyfish”, but a floating colonial hydroid made up of many individual zooids, each characterized by specialized functions – catching prey, reproduction, feeding, or floating. These individuals act in concert to perform their functions and to ensure the survival of the colonial organism, while unable to survive individually.

Hydroid made up of zooids

This siphonophore feeds on fishes, larvae, crustaceans, and marine invertebrates, catching them using its feeding tentacles which can be as long as 50 meters! It extends its tentacles through the water like a large net, sweeping up prey into its grasp, where its stinging cells inject their toxins. The stinging cells can cause a lot of damage to human beings as well, and large swarms of P. physalis have caused beaches to close across the world, particularly in the Mediterranean Sea and around the Australian coasts. Research has shown that commercially available vinegars can be effective at neutralizing their stinging cells. Besides these thousands of nematocysts (stinging cells), P. physalis possesses another defensive strategy of briefly deflating the float and then submerging when threatened.

But the nematocysts of P. physalis can also be advantageous in developing symbiotic relationships with other marine organisms. Several species of fishes live within the tentacles and feed upon the siphonophore itself or the scraps of its prey. One species in particular, Nomeus gronovii, is partially immune to the stings, but can still be overcome and eaten if not careful. It is also thought that the presence of Nomeus gronovii can act as a lure, convincing other more susceptible fish to come into the tentacles to be caught and consumed. Additionally, two types of nudibranchs (a group of marine gastropods) use its nematocysts as part of their personal defense strategies.

Nomeus gronovii circling around P. physalis

Found in tropical to subtropical marine waters worldwide, P. physalis is considered as an uncommon visitor to Hong Kong. It should be noted, however, that P. physalis is considered to be a pleustonic organism, meaning that it is a macroscopic organism that float or swim on the surface of a body of water, drifting with the ocean currents or being pushed by the wind, and unable to propel itself independently.

Because of their pleustonic lifestyle, P. physalia occurrences have been shown to correlate with variable environmental conditions (wind, currents, waves) depending on location. This has led people to set up monitoring groups in different areas of the world to detect the presence of the stinging siphonophores and other jellyfish. Here in Hong Kong, a citizen science project called the Hong Kong Jellyfish Project is collecting information about the presence, abundance and distribution of jellyfishes and other related organisms in Hong Kong’s waters. Learn more at and let them know if you see any jellies you may observe in the water or on the beaches.

Text: John Terenzini


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