During this time of year, jellyfish are often driven ashore by winds and currents, and occasionally our beaches are littered with the iridescent blue floats of the Portuguese Man-O’-War (Physalia physalis).

By Tom Baird


During this time of year, jellyfish are often driven ashore by winds and currents, and occasionally our beaches are littered with the iridescent blue floats of the Portuguese Man-O’-War (Physalia physalis). Normally a tropical species of the open ocean, changes in the Florida Current, the northern curving loop of water that moves along the coast of Central America, through the Yucatan Straits and into the northern Gulf of Mexico, can bring Portuguese Man-O’-War and other tropical seeds and animals onto our shores. Loops and eddies of the Florida Current, combined with storms at sea, will cast hundreds of Portuguese Man-O’-War onto the beach on a high tide.

Few sea creatures are prettier to look at in the sunlight, with their bluish-lavender floats tinged with pink, yet the Portuguese Man-O’-War can exact a price if you handle it. In life, the gas filled float sits atop the water while the tentacles trail beneath it. The outer layer of the tentacles are covered with nematocysts that when touched spring out of their capsule. These are filled with venom.  An unwary fish swims into the tentacles, becomes paralyzed by the nematocysts, and the tentacles then move the fish to the cells that will do the work of digestion.

The nematocysts can also penetrate human skin, as many a swimmer can attest. Since the tentacles may be up to fifty feet long trailing beneath the float and almost invisible in the water, it is easy for a diver not to realize they are near a Portuguese Man-O’-War. The realization comes when they brush against the tentacles and immediately feel the pain. In this author’s experience, the sensation is like being burned, decidedly uncomfortable, but not too long lasting. However, swimmers with multiple stings may exhibit a variety of symptoms, including rash, blisters, and swelling, and in severe cases respiratory difficulties, convulsions, and in rare instances, death has occurred.

For those finding Portuguese Man-O’-War washed on the beach, touching the gas-filled float is harmless. The tentacles may be almost invisible however, and some may still be washing in the surf. While the animals on the beach are dead or dying, the nematocysts on the tentacles can still fire, so handle carefully.

But here’s the surprise – the Portuguese Man-O’-War is not a single animal. It’s a colony of animals. Not only that but it is not even a true jellyfish. They are hydroids – more closely related to Fire Coral than jellyfish.  Of all the amazing adaptations found in plants and animals in the sea, the Portuguese Man-O’-War is surely one of the most amazing. The colony is composed of smaller animals called zooids or polyps. These are in turn highly specialized, so that some zooids form the nematocysts, others digest food, others perform the reproductive function, and others form the float. It’s as if nature were experimenting with forming tissues – groups of cells that perform a function within a single organism – like skin, nerves, food absorption, etc.

So what kind of gas is inside the float? Zooids form an oval disc at the base of the float that secretes a mixture of gases, somewhat similar to air, although up to 14% can be carbon monoxide.  Atop the float is a small “sail” with crenulations in it. The “sail” is set diagonally on the top of the float. There are two types of Portuguese Men-O’-War based on this characteristic – the ‘left sailing’ form, where the sail extends from the upper left corner to the lower right corner when viewed from above the float, and a ‘right sailing’ form, where the sail extends from the upper right corner to the lower left corner. Those Portuguese Men-O’-War found in the Gulf of Mexico are predominantly the ‘left-sailing’ form.

The float even has a siphon that allows gas to be vented out in case of attack, which allows the colony to briefly submerge. What would attack it? Loggerhead sea turtles eat Portuguese Men-O’-War; their skin is too thick for the nematocysts to penetrate. Purple sea snails (Janthina spp.) drift in the open ocean using a bubble raft of secreted mucous. These beautiful little snails drift along with and prey upon the Portuguese Men-O’-War.

Although Physalia physalis is predominantly a tropical Atlantic species, the Gulf Stream may carry groups as far north as the Bay of Fundy, and storms and winds can deposit them on the beaches of Atlantic seaboard states. In some instances, they are so numerous that officials close public beaches. There is a closely related species in the Pacific and Indian Oceans.

If you see what appears to be bright blue toys washing in the surf, take some photos of this beautiful colony of animals, but tread carefully around them.

Tom Baird has been a fisheries biologist, high school and community college teacher (oceanography and microbiology), director of a science and environmental center, teacher of science and principal in Pinellas County as well as an educational consultant. He retired from the Florida Department of Education and he and his wife divide their time between Tallahassee and Cape San Blas.