Another Life: Intricate manoeuvres of the man-of-war

Rachel Carson, whose great honour for her book Silent Spring somewhat overshadowed her gifts as a writer about the sea, once found a Portuguese man-of-war stranded on a beach in South Carolina. She gathered it – don't ask me how – into a bucket of salt water for study overnight. Next morning, respectful as always in these things, she waded into a chilly March sea to hurl the creature as far as she could into the waves.

Writing about this in The Edge of the Sea (published in 1955 and still worth chasing online), she was struck by the creature's behaviour as it fought to leave the shallows. Swept back repeatedly by a difficult wind, "it would manage to take off again, visibly adjusting the shape and position of the sail as it scudded along before the wind . . . But whether in difficulty, or enjoying momentary success, there was nothing passive in the attitude of the creature. There was, instead, a strong illusion of sentience. This was no helpless bit of flotsam, but a living creature exerting every means at its disposal to control its fate."

Few, I would think, of the hundreds of these animals washed up on our shores this autumn have received any similar courtesy. This is as well, given the risk of contact with the venomous threads of Physalia physalis. But Carson's vivid, even moving, observation has stayed with me. Are these vagrant, wind-blown predators just the passive drifters they have seemed?

The apparent increase in "jellyfish" of all kinds, including the commune of specialised zooids, or polyps, that live beneath Physalia's exotic balloon, generally coincides with unprecedented swarms of humans dangling their limbs in the shallows. The potential threat of stings, or worse, to the tourism industry, has intensified study of the animal's casual congregations and their armoury of venomous harpoons. In 2010, for example, the Mediterranean Sea experienced an "outburst" of the Portuguese man-of-war, with impacts that included the region's first recorded human death from a sting.

Given speculation on the role of climate change, EU Commission researchers last year published a study, Portuguese Man-of-war (Physalia physalis) in the Mediterranean: A Permanent Invasion or a Casual Appearance? (see nature.com/srep/2015/150625/srep11545). It analysed meteorological and oceanographic data in the north Atlantic in the months before the men-of-war began sailing past Gibraltar and concluded that the invasion resulted from "an unusual combination of . . . conditions" rather than a local population explosion.

As Ireland's big armadas began beaching themselves last month, from Cork to Donegal, citizens were urged through a Facebook page to join a "Big Jellyfish Hunt". The initiative came from Dr Tom Doyle and his team at NUI Galway's Ryan Institute. His research into the economic cost of jellyfish blooms, funded by the Environmental Protection Agency (EPA), extends to seeking new treatments for stings.

But back to that behaviour described by Rachel Carson. In Physalia's intertwining colony of organisms, each with its specialised job (hunting, killing, digesting, reproducing, or whatever), what manipulations between them might help to control its collective fate? She described the man-of-war "visibly adjusting the position and shape of the sail as it scudded along before the wind" like a yachtsman trimming his canvas for maximum stability and speed. Among scientists intrigued by the workings of the animal's intricate behaviour are Gil Iosilevskii and Danny Weihs, whose work on Israeli aerospace engineering has gained from investigation of "speed limits on swimming of fishes and cetaceans" and the predatory dives of sharks. Their paper Hydrodynamics of Sailing of the Portuguese Man-of-war Physalia physalis, published by the Royal Society in 2008, is offered online at DOI: 10.1098/rsif.2008.0457.

Its approach will appeal to any technically minded yachtsman, but I must warn of its intimidating density of formulae. Picking my slow, landlubber's way between them, I discover that Physalia's gas-filled balloon, with its corrugated crest, can be twisted this way or that by muscular contractions of the basal float. These affect both angle and shape of the sail, the centre of buoyancy and degree of list. "The trim of the sail is very delicate," say the authors, "and a minute increase in camber may luff the sail."

The animal's hunting and stinging tentacles, which can grow to 15 metres long, can be raised or lowered to control their drag and spread in light winds to float horizontally for maximum encounter with prey. For a yachtsman, a big factor missing from all this is compass direction – how does the man-of-war get where it's going? But this is to miss the point. Physalia is not going anywhere in particular. It just wants to stay afloat and keep sailing to the next feed of fish.

Strong winds and currents will herd it into fleets, spread it across the Gulf Stream, wash it ashore in lesser or greater numbers. Climate change is reshaping major forces in the sea: expect anything.

Michael Viney's Reflections on Another Life, a selection of columns from the past four decades, is available from irishtimes.com/irishtimesbooks