Blue Fleet: Bluebottles

Australia's curious and exquisitely beautiful ocean nomads

By Underwater Earth

Part I of II in a story about the Blue Fleet. Title image by Matty Smith.

Blue Fleet at Malabar Beach (2021-03-05) by Vanessa Torres MachoUnderwater Earth

Introducing the bluebottle and the interconnected relationships and similarities it has with the other members of Australia’s Blue Fleet: blue dragons, the by-the-wind sailor, the violet sea snail, and the blue button.

Members of this specialised ocean community are all perpetual ocean castaways. They are all part of the pleuston food web - organisms living in the thin surface layer existing at the air-water interface of the ocean.

Beached Blue Button by Brett LobweinUnderwater Earth

Blue Fleet creatures are nomadic, rely on the winds and currents to carry them around and normally go unnoticed out in the open ocean. During certain conditions however, they are swept into coastal waters or washed up on Australian beaches allowing for more frequent encounters of these curious creatures.

Blue Fleet II at Malabar Beach (2021-03-05) by Vanessa Torres MachoUnderwater Earth

Each creature has specific adaptations such as air-filled sacs, rigid sail-like structures and flattened disc-shaped bodies making them excellent sailors. Some have stinging cells for hunting and defense, some feed on others in this specialised ocean community.

Bluebottle II by Matty SmithUnderwater Earth


The bluebottle (Physalia utriculus) is also known as the Pacific man o' war, not to be confused with its larger more venomous Atlantic cousin, the Portuguese man o’ war. 

Although it superficially resembles a jellyfish, it is in fact a siphonophore. A colonial organism made up of many smaller polyps called zooids, all genetically identical yet highly modified to fulfill specialised functions, all operating as a single individual, working in harmony, all dependent on one another for survival.

Bluebottle III by Matty SmithUnderwater Earth

The bluebottle has four kinds of zooids. The exquisitely blue float (pneumatophore) remains at the surface and supports the rest of the colony submerged. The tentacles (dactylozooids) detect and capture food underwater and convey their prey to the digestive polyps (gastrozooids). Bluebottles are hermaphroditic (each individual produces both eggs and sperm) and reproduction is carried out by the gonozooids.

Bluebottle Sails by Brett LobweinUnderwater Earth

Bluebottles have no means of propulsion. They move passively, driven by the winds, currents, and tides, in directions also influenced by the curve of their floats. They are either left-crested or right-crested meaning that the same wind will push the two variations in different directions, avoiding all the colonies becoming washed up on the beach and dying at the same time.

Bluebottle Tentacles by Sheree Marris: www.shereemarris.comUnderwater Earth

The bluebottle's long tentacles continuously fish the water looking for larval fish, molluscs and small crustaceans. Each tentacle bears tiny, coiled, thread-like structures called nematocysts that trigger and inject venom on contact, stinging, paralyzing, and killing their prey. Once captured, muscles in the tentacle contract and drag the prey into range of the digestive polyps.

Glaucus Party by Talia GreisUnderwater Earth

It is the bluebottle’s tentacles that many Australian beachgoers fear most. Thousands of bluebottle stings are reported each summer when the NE winds and warmer currents bring swarms of bluebottles close to the coast on the incoming tides, stranding many on the sands to die.

The bluebottle has a number of predators including loggerhead turtles, sunfish (Mola mola) and other members of the Blue Fleet such as blue dragons.

Blue Button by Matty SmithUnderwater Earth

To learn about the other members of the Blue Fleet — blue dragons, the by-the-wind sailor, the violet sea snail, and the blue button — read on in Part II.

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The story featured may in some cases have been created by an independent third party and may not always represent the views of the institutions, listed below, who have supplied the content.
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