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Bizarre 'blue fleet' blows onto Australia's east coast

14 February 21

They're normally found floating around in the middle of the ocean, but are dubbed the blue "fleet" because they catch a ride to shore with the wind.

A bunch of bizarre-looking blue-coloured sea creatures are washing up on Australia's east coast.

Lately they've been hitching a ride on strong north-easterly winds, said final year marine biology student Lawrence Scheele, who has been tracking and photographing the blue fleet all summer.

Normally based in north Queensland, Mr Scheele was down in Sydney this week "escaping the cyclones, crocodiles and box jellyfish", where he captured the fleet as it landed on the northern beaches.

"I was really lucky to have caught them all at Long Reef," he said.

While you might be familiar with one member of the fleet — the stinging bluebottle — you're less likely to see the tiny blue dragon.

This is actually a unique nudibranch that floats on the water, upside down.

One blue dragon (Glaucus atlanticus) is about 3 centimetres long and has a blue and silver "foot" on its underside, that can be mistaken for its topside.

The other (Glaucilla marginate) is only about 1.3cm long and has a light and dark blue foot and a shorter tail than the bigger dragon.

Another member of the blue fleet is the blue button (Porpita porpita).

Like coral, this creature is made up of a bunch of tiny animals called polyps — in this case they're all connected to a central disc of keratin that enables them to keep afloat.

The more familiar bluebottle (Physalia utriculus) is actually a colony polyps with varying functions that make up the different parts of the animal.

One part secretes the gas-filled sail that is responsible for catching the wind, other polyps form the tentacles that capture and immobilise prey with their venom, others digest food, and the rest are responsible for reproduction.

Mr Scheele said you have to be in the right place at the right time to catch the blue fleet, but he loves posting about them on Instagram when he gets the chance.

"I think they're just fascinating, and whenever I post about them fans want to know more," he said.

Among the other creatures he snapped in Sydney this week was the violet snail (Janthina janthina), which relies on secreting a mucus-covered bunch of bubbles to keep it afloat.

It's a dog-eat-dog world for the blue fleet

Blue dragons and violet snails feed on bluebottles, blue buttons and another member of the blue fleet, called by-the-wind sailor (Velella velella). Bluebottles themselves eat fish!

Like the blue button, the by-the-wind sailor is a raft of polyps, but with an added sail.

Some have a right-sided sail, while others have a left-sided sail — which means they come to shore on different winds, said marine scientist Sarah-Jo Lobwein of the Australian Environmental Educators Association.

The blue fleet are part of what scientists call the pleuston community, which lives partly in the water and partly in the air and relies on the winds to carry them places.

But sometimes you can see blue dragons move themselves.

"They can't swim again currents but sometimes you'll see them suck in air, twirl around and do somersaults," Ms Lobwein said.

"This helps them get a bubble of air inside them so they can keep afloat and upside down."

Relying on the wind to push you in the right direction for food is hardly something you imagine a predator relying on.

Yet, said Ms Lobwein: "In their own small world, they are predators."

Soon after a blue dragon eats a bluebottle it absorbs its prey's stinging cells, which then makes it dangerous for humans to touch.

"So, proceed with caution if you're checking them out on the beach," Mr Scheele said.

The violet snail isn't venomous while the by-the-wind sailor and blue button only give a mild irritation.

Why are they all blue?

Scientists suspect it comes down to the fact that lying exposed on the surface of the ocean makes the blue fleet vulnerable to being eaten by others and being blue helps them blend in.

"It's great camouflage," Ms Lobwein said.

Like Mr Scheele, Ms Lobwein has also been tracking blue dragons.

"I notice that in the past few years we're having a lot more of those nudibranchs than we have before," she said.

"I think it is the combination of warming or changing seas … possibly leading to a 'trigger' in the explosion of the animals at certain times but reliant on that perfect mix of the effect of the Moon on tides, wind direction, water temperature and the currents etc."

And Ms Lobwein's data of when the sea dragons arrive appear to suggest something interesting.

"They seem to arrive a few days after a full Moon," she said.

She's seeing if she can confirm the observation and meanwhile speculates why this could be the case.

"The Moon phases affect many marine organisms' reproduction, and at different times of the year [such as coral spawning]," Ms Lobwein said.

Independent marine researcher and author Lisa Gershwin is intrigued.

"Some of this totally rings true to me," said Dr Gershwin, an expert in the blue fleet and jellyfish.

"Warming positively affects the population of many marine organisms."

But she wondered how the Moon might be related to the wind, which needs to be in the right direction to bring the animals onshore.




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