Jellyfish nap in the afternoon

Box jellyfish take a rest in the middle of the afternoon, according to Australian research.

Marine biologist and PhD student Matthew Gordon says scientists at James Cook University in Cairns have found that the animals are extremely active most of the day.

But in the afternoon they "tilt their bodies down and rest on the bottom of the sea with their tentacles spread out".

"They are really good swimmers," Gordon says. "They have a high metabolic rate. They run around all day at a high speed. But they can't swim 24 hours a day. They catch the food and need to digest it ... at around 3 to 4pm they sink to the bottom of the sea and stay motionless for hours at a time."

Box jellyfish or Chironex fleckeri live in northern Australian tropical waters.

They are made up of 97% water, are virtually invisible and have no brain. But with around 60 tentacles and five billion stinging cells or nematocysts, they can kill an adult human in up to 4 minutes.

As part of a three-year study that could help to manage the country's northern coastline, the university's Tropical Australian Stinger Research Centre has been tracking the movements of box jellyfish using ultrasound tags, for 24 hours at a time.

Gordon says while researchers don't know much about the animals' movements. But the centre has tracked one in particular that swam up and down 5 kilometres of the coast, for around 20 hours continuously.

It was also thought that box jellyfish swim up creeks towards the end of the season to reproduce but there was no concrete evidence of this until late April when one jellyfish was followed up an estuary.

The centre plans to build a computer model to identify their movements, and the information would then be used to warn councils, lifesavers and beach-goers. This could also help plan resorts and tourist attractions, and even where jet skis are located.

"The aim is to make the coastline safer," Gordon says. "[Box jellyfish are] part of the ecosystem. I'd like to develop a way to live with them."

Gordon, who presented his information at the recent Fresh Science forum in Melbourne, says his team plans to submit the research for publication.

Source: abc.net.au