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Bluebottle Physalia physalis

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Bluebottle Physalia physalis

The tentacles of the largest specimens of bluebottle can reach 20-30 metres.

© Bruce Hayward

New Zealanders like to pride themselves on living in a country with no harmful animals such as poisonous snakes or grizzly bears. But it is a different story in the ocean. New Zealand waters are home to many types of marine life that utilise poisons to capture prey or protect themselves from harm.

A colony of thousands

Although often thought of as one, the bluebottle is not a jellyfish, but instead consists of a colony of thousands of tiny hydroid animals. It is a common sight on New Zealand beaches and beach goers learn at an early age to avoid the vivid blue creature washed up on the sand because of its ability to deliver a nasty sting.

The bluebottle has a peacock-blue air bladder, generally 5-15 cm in length, that acts as a float and a sail. This is why it is also known as the Portuguese man o' war - the float resembles the sail of an 18th-century armed sailing ship. 

There are a variety of hanging structures below the float, including long, blue stinging tentacles which may be as much as 20-30 m long in the very largest specimens.

These tentacles partially contract when the animal is washed ashore and can deliver a powerful sting even after death.

A powerful sting

The bluebottle is carnivorous. It captures swimming or floating organisms, such as small fish, by paralysing them with powerful stinging cells (nematocysts) on the tentacles. These nematocysts are made up of a sac filled with toxin, a coiled hollow thread and a barbed head.

When small prey bumps up against the tentacles it triggers the nematocysts which fire the barbed head with enough force to penetrate the prey. Toxin is then injected through the hollow thread. The small fish is reeled in and it becomes the function of the digestive polyps to digest it and redistribute it as food.

The bluebottle is not deadly to humans although it may make you very sick. The best way to treat it is by immersing the stung area in hot water.

© Wilma Blom.

Division of labour

The bluebottle (scientific name: Physalia physalis; Māori: Ihumoana or Katiaho) belongs to a diverse group of animals called Hydrozoa. Some are soft-bodied and some have a rigid skeleton. The bluebottle is part of a sub-group of soft-bodied, colonial hydrozoans called siphonophores. These have intrigued scientists for a long time because they look like one animal but actually consist of a colony of thousands of hydroid animals (or polyps), each with its own specialised purpose.

The polyps are grouped and each group is responsible for a task - those with stinging cells (called cnematocysts) look after prey capture, some polyps look after digestion, whereas others bear the sexual cells (which look like dark-blue grapes).

Treatment for a bluebottle sting

The bluebottle has a powerful sting even after death, but it is not deadly to humans although it may make you very sick. If you are stung don't rub the skin as it may pop any stinging cells that have not yet burst. The best way to treat it is by immersing the stung area in hot water. The best temperature is about 45 degrees centigrade. It takes a little while, but the heat of the water deactivates the toxins. Get medical help if you are badly stung or if you feel unwell after being stung.

Watch Dr Wilma Blom, Auckland Museum's curator of marine invertebrates, talk about some of the poisonous polyps and toxic tinies living in New Zealand waters.

Cite this article

Blom, Wilma. Bluebottle Physalia physalis. Auckland War Memorial Museum - Tāmaki Paenga Hira. First published: 2 November 2015. Updated: 12 November 2019.

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