Melbourne beachgoers who visited the eastern suburbs coastline at the weekend might have been shocked to see an influx of red jellyfish bobbing in the water.
Former president of the Sandringham Foreshore Association Vicki Karalis said she saw them in the "thousands".
"They were in the shallow waters and the deeper waters — they were everywhere," she said.
Their unexpected arrival saw the Sandringham Lifesaving Club close the beach on Sunday, while local social media pages were awash with photos of the sea creatures.
Here's what we know about this "smack" of jellyfish.
What species are the jellyfish?
Olivia Hill, a marine biologist and education manager at SEA LIFE Melbourne Aquarium, said the creatures were lion's mane jellyfish.
Native to the bay, they are characterised by their bright red belly and long white stingers.
Ms Hill said while these jellies can be one of the largest types in the world — growing "as big as a washing machine" in Antarctica — the subspecies living in Victoria are much smaller.
"Here in the bay you won't get them much bigger than about an orange," she said.
She said the jellies in the bay could grow tentacles as long as 1.5 to 2 metres, which were very delicate and fell off easily.
"If they do fall off or get damaged, they're constantly regrowing them," she said.
"They don't run out of tentacles, unfortunately."
And these tentacles do sting, but the venom is not lethal.
"You'll get a red welt, like a red stripe, from where it stung you for maybe two or three days," she said.
"It does hurt, but it doesn't hurt as much as other sorts of jellies."
Where have they been seen, and why are there so many?
The influx of lion's mane jellyfish was witnessed along the eastern shoreline of Melbourne's Port Phillip Bay over the weekend.
Vicki Karalis said she had received messages from people seeing the stingers in large numbers from Elwood all the way down to the Mornington Peninsula.
[IMAGE: map]Ms Hill said a "perfect storm" led to large and highly visible population.
Firstly, she said the temperature had to be right for reproduction: above 18 degrees Celsius, which is common during the summer months.
Next, the baby jellyfish would have to feast on an influx of food, which was likely delivered in a bout of rain following a dry spell in recent weeks.
This pushed built-up nutrients from the Yarra River into the bay.
"When you get a big flash of rain after a long dry spell, all of these nutrients that have been sitting around in these rivers, waiting patiently to end up in the ocean, end up there all at once," she said.
"So it's a really big nutrient spike, and that all sends quite a lot of animals and small plankton into overdrive because there's so much food around."
Then, Ms Hill said, a big westerly wind on Friday pushed the large colony towards the eastern part of Port Phillip Bay.
"Generally, jellies don't swim around," she said.
"If they're going to go somewhere, they get pushed around by the water currents and the wind".
She said when lots of jellyfish were seen in one area, they would likely dissipate quickly due to the wind and currents changing direction.
Have we had this many show up before?
Ms Hill said it was "relatively common" to see something like this at this time of year, but the location varied depending on the wind patterns and water currents.
"It's absolutely natural to the ecosystem," she said.
The ABC reported a similar phenomenon happening at Melbourne's eastern beaches four years ago.
It follows another jellyfish event last month, in which multiple bluebottles were seen in the bay — but for a different reason.
Tasmania-based jellyfish researcher Lisa-Ann Gershwin said bluebottles live in the Tasman sea, between Australia and New Zealand, and in that case they happened to have been blown into the bay by the wind.
"You think about the logistics of the wind somehow doing that just at the right time — it's pretty incredible really," she said.
Is it safe to swim?
Dr Gershwin said if you swum alongside that many lion's mane jellyfish, you were likely to get stung, and that would probably hurt.
"There'll be lots of fragments of their tentacles in the water, so for people who don't want to experience those stingy feelings, give it [swimming] a break for a couple of days," she said.
Ms Hill urged beachgoers to stay away from them on the shore, because even dead ones can sting.
While children and dogs are affected by the venom in a similar way to adults, she said, they should be kept away from them.
Dr Karalis, who is also a GP, said while the sting was not fatal, anyone who had been stung in the eye or had an allergic reaction should see a doctor.
Otherwise, anyone who is stung should rinse the area with sea water to help remove the tentacles and, once home, use tweezers to remove any remaining ones.
"Don't rub it, don't use your bare hands," she said.
She said to apply hot water or a cold pack to help alleviate the pain.
Dr Karalis said if swimming among jellyfish, try to wear coverings like a stinger suit or a wetsuit — but it's best to stay out of the water if there are many of the creatures around.
