Portuguese Man o' War jellyfish on our shores

By Jim Hurley - Nature Trail

Published 22/10/2016 | 00:00

Some commentators suspect that climate change and warming sea temperatures probably have something to do with the arrival of large numbers of Portuguese Man o' War jellyfish this year.
Some commentators suspect that climate change and warming sea temperatures probably have something to do with the arrival of large numbers of Portuguese Man o' War jellyfish this year.

Unprecedented numbers of Portuguese Man o' War jellyfish with an extremely painful sting have landed on our shores in recent weeks. The dark purplish-blue tentacles can still sting when the animal is dead so remains found on the strand should not be handled.

Man-of-war was a term used during the eighteenth century for a sailing ship equipped with cannons and rigged to do battle at sea. Similarly, the jellyfish uses its floating gas-filled bladder, crimped on the upstanding, curved side like a Cornish pasty, as a sail and is equipped with highly venomous stinging tentacles that it does battle with in the never-ending quest to catch food in the vast expanses of the open ocean.

The jellyfish doesn't come from Portugal. Man-of-war ships were rigged in different ways. The jellyfish was regarded as being close to the style of rigging used by Portuguese warship, hence its common name.

To split a hair, the Portuguese Man o' War is not a jellyfish; it is a siphonophore. True jellyfishes are individual animals; siphonophores are colonies of several different kinds of highly specialised polyps living together and behaving as if they are a single animal. However, 'jellyfish' remains a long-established generic term.

There are two species: the one we get is the Atlantic Portuguese Man o' War, the other is a closely related species found in the Indian and Pacific Oceans. Our species lives at the surface in the vast expanses of the open ocean, mainly in tropical and sub-tropical regions.

The Portuguese Man o' War cannot swim. Using its gas-filled bladder as both a float and a sail, it drifts at the mercy of wind, current and tide. In the past, it was seldom recorded as far north as Ireland so the arrival of large numbers this year is indeed unusual. Some commentators suspect that climate change and warming sea temperatures probably have something to do with it.

As it drifts, the Portuguese Man o' War dangles its tentacles below waiting for prey to accidentally make contact. When contact is made it fires its stinging cells injecting venom to paralyze the unsuspecting fish or squid doomed to become the jellyfish's next meal. A big catch like a fish may be a rare event; plankton is said to make up a significant part of the jellyfish's diet.

Though venomous, the Portuguese Man o' War is sought out and eaten by the Loggerhead Turtle and is also preyed upon by some oceanic molluscs and fish.

Wexford People

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