[9] The Portuguese man o' war is a conspicuous member of the neuston, the community of organisms that live at the surface of the ocean.

It has numerous microscopic venomous cnidocytes which deliver a painful sting powerful enough to kill fish, and even, in some cases, humans.

These different types of zooids fulfill specialized functions, such as hunting, digestion and reproduction, and together they allow the colony to operate as a single individual.

[16] Both of these body plans comprise entire individuals in non-colonial cnidarians (for example, a jellyfish is a medusa, while a sea anemone is a polyp).

Like all siphonophores, P. physalis is a colonial organism: each animal is composed of many smaller units (zooids) that hang in clusters from under a large, gas-filled structure called the pneumatophore.

[10][32] The gas in the pneumatophore is mostly air which diffuses in from the surrounding atmosphere, but it also contains as much as 13% carbon monoxide, which is actively produced by the animal.

Long tentacles hang below the float as the animal drifts, fishing for prey to sting and drag up to its digestive zooids.

[10] A fertilized man o' war egg develops into a planula that buds off new zooids as it grows, gradually forming a new colony.

This development initially occurs under the water, and has been reconstructed by comparing different stages of planulae collected at sea.

[39] The organism has few predators; one example is the loggerhead sea turtle, which feeds on the Portuguese man o' war as a common part of its diet.

Rather than using mucus to prevent nematocysts from firing, as is seen in some of the clownfish sheltering among sea anemones, the man-of-war fish appears to use highly agile swimming to physically avoid tentacles.

Individuals have been observed to carry broken man o' war tentacles,[48] which males and immature females rip off and use for offensive and defensive purposes.

[49] The stinging, venom-filled nematocysts in the tentacles of the Portuguese man o' war can paralyze small fish and other prey.

In some cases, the venom may travel to the lymph nodes and may cause symptoms that mimic an allergic reaction, including swelling of the larynx, airway blockage, cardiac distress and shortness of breath.

[53] Stings from a Portuguese man o' war can result in severe dermatitis characterized by long, thin, open wounds that resemble those caused by a whip.

[60][61] P. physalis uses a float filled with carbon monoxide and air as a sail to travel by wind for thousands of miles, dragging behind long tentacles that deliver a deadly venomous sting to fish.

[62] This sailing ability, combined with a painful sting and a life cycle with seasonal blooms, results in periodic mass beach strandings and occasional human envenomations, making P. physalis the most infamous of the siphonophores.

[10] Despite being a common occurrence, the origin of the man o' war or bluebottle before reaching the coastline is not well understood, and neither is the way it drifts at the surface of the ocean.

[36] The Portuguese man o' war is asymmetrically shaped: the zooids hang down from either the right or left side of the midline of the pneumatophore or bladder.

In comparison, the typical man o' war has a float of around 15 to 30 centimetres (5.9 to 12 in), and several hunting tentacles that can reach 30 metres (100 ft) in mature colonies when fully extended.

[10][36] When combined with the trailing action of the tentacles, this left- or right-handedness makes the colony sail sideways relative to the wind, by about 45° in either direction.

Earlier studies modelled the movement of the man o' war with Lagrangian particle tracking to explain major beaching events.

In 2017, Ferrer and Pastor were able to estimate the region of origin of a significant beaching event on the southeastern Bay of Biscay.

[66] In 2015 Prieto et al. included both the effect of the surface currents and wind to predict the initial colony position prior to major beaching events in the Mediterranean.