Just when Sydneysiders thought it was safe to go back in the water, a potential deadly jellyfish has been found thousands of km south of its usual home in the tropics.

Over the last two weekends, the Shark Island Psycho Swimmers (SIPS) ocean swimming group out of Cronulla has encountered a suspected Box Jellyfish while on their daily swims out of Cronulla Beach. 

Previously, Box Jellyfish have only been sighted as far south as the Gold Coast, and even then, that has been extremely rare. 

SIPS ocean swimmer Scotty Belcher first spotted the ocean swimmer’s nightmare after splitting off from the group for a longer swim one morning.

“After myself and a couple of other swimmers came around from Shark Island, we split off and went swimming to Shelly Beach,” said Mr Belcher. 

“We stopped at the back of the surf there and that is when we ran across it.

“A few days later, we then saw another one out behind Shark Island.”

Initially thinking it was a Jimble, Mr Belcher looked a little closer to find out it was something he hadn’t seen before. 

“When I first saw it, I thought it was a Jimble, but then I had a closer look and started to think, that’s not a Jimble and that is when I started filming,” said Mr Belcher. 

“It was quite amazing that I happened to have my camera at that time because I swim with it most days but there have been plenty of times where the camera has failed or I haven’t charged it when I’ve come across marine life.

“So, I was just lucky that it pieced together and I could capture it to send to Dr Lisa-Ann Gershwin to analyse.

“It was a decent size, the head was about 10cm body and tentacles about 20-30cm.” 

Mr Belcher sent away the photos and videos to Dr Lisa-Ann Gershwin, a biologist and Jellyfish expert via the Jellyfish app, who was very interested in the find.

Dr Gershwin said the jellyfish was definitely not the northern box jellyfish (commonly known as the Australian box jellyfish), that reigns as the world’s most venomous animal. But until the jellyfish filmed by Mr Belcher is officially identified, she is holding “equal space” for the possibility the jellyfish is dangerous or harmless.

“After looking at the images and videos, I’ve seen it’s definitely not a Jimble,” said Dr Gershwin.

“It looks like a Chironex, which is a part of the Chirodropida family and this group contains the deadly Box Jellyfish.”

“Without actually seeing it in person it looks like a new species of Box Jellyfish or a cross between Chironex fleckeri and Chirosella saxony.” 

Treating tropical vs non-tropical jellyfish stings

After the Cronulla sighting, Gershwin warned that pouring hot water on a jellyfish wound – a common treatment for bluebottle stings – can aggravate venomous stings.

“When someone is stung by a box jellyfish, only about 10 to 20 per cent of the stinging cells fire,” she said. “Fresh water triggers all the stinging cells that haven’t fired so the remaining 80 to 90 per cent of venom floods in.

“It’s the worst conceivable thing to do for a potentially dangerous species.”

The Australian Resuscitation Council recommends the application of vinegar in tropical areas because it freezes box jellyfish stinging cells that haven’t delivered venom. But vinegar could make bluebottle stings worse.

That’s why swimmers who are stung outside the tropics, if uncertain about which species they encountered, should wash sting sites thoroughly with seawater before applying hot or cold water.

“That will rinse away any remaining stinging cells remaining on the skin,” Gershwin said.

She speculated the floods could have attracted an open-water species closer to the coast due to recent record rain sweeping extra nutrients into the ocean. The specimen could also be a “supercharged” pygmy box jellyfish, a “tiny guy the size of your thumb” found on the Gold Coast that Gershwin identified and described in 2015.