Irukandji jellyfish actively hunt prey researchers find
Despite being made of more than 90 per cent water and lacking a recognisable brain, irukandji jellyfish are able to actively hunt their prey far north Queensland researchers have discovered.
Researchers at James Cook University's (JCU) Australian Institute of Tropical Health and Medicine in Cairns made the discovery while studying the properties of the irukandji's deadly venom.
The lead author of the study, Robert Courtney said it was previously thought the feeding habits of irukandji were no different to any other species of jellyfish.
"Most jellyfish just swim around or are driven by the currents of the ocean ... they're basically considered an opportunistic grazer [because] anything that runs into the tentacles then gets consumed," he said.
"It seems that [irukandji jellyfish] actively lure fish with a set of nematocyst clusters [stinging structures] that are found along their tentacles.
"These clusters are spaced out along the tentacles in a very visible pattern."
Like an angler flicking a lure to catch a fish, the jellyfish twitches these clusters to attract the attention of small fish.
"The fish swim over and basically get envenomed or stung right in the mouth region," Mr Courtney said.
"They're actually trying to eat these nematocyst clusters and end up being consumed themselves."
Mr Courtney said it was remarkable for a jellyfish to have such a highly developed method of feeding.
"This is an animal without a defined brain, is 96 per cent water and not much structure to it," he said.
"But we're seeing some really complex behaviour in a very deliberate and selective form of prey capture."
Researchers have also observed that irukandji only hunt fish during daylight hours.
When it is dark the jellyfish retract their tentacles, allowing them to conserve energy when the fish it hunts are less active.
"When it's light [the irukandji] extend their tentacles out quite long ... over 1.2 metres," Mr Courtney said.
"What they're feeding on are obviously visually oriented fish - ones that can see the lure clusters - and ones that predominantly feed during the day."
Mr Courtney said he hoped the discovery would pave the way for more research into the deadly jellyfish.
"This has just given us a little bit more of a handle on what's going on with a venomous animal that we don't really know too much about," he said.
"There's heaps of other questions about their lifecycle and all sorts of things we don't know about this species that we're going to keep pursuing."
Various irukandji species have previously been found in waters off the Queensland, Northern Territory and West Australian coastlines.
Learn about what to do if you are stung by an irukandji jellyfish with the ABC's Be prepared: irukandji first aid article.
Mr Courtney said the stinging structures the irukandji uses to lure fish are highly visible, even to the human eye. "The nematocyst clusters look like a series of bright pearls," he said. "They're using them like experienced fishers use their lines and lures." ()
