Box Jellyfish Actively Hunt Fish, Research Finds

Some of the first research to investigate the feeding habits of Australia’s Irukandji box jellyfish (Carukia barnesi) has, interestingly, found that the jellies actively hunt fish — by utilizing twitching in their extended tentacles to move their nematocyst clusters (the parts that stings people/animals) and using them as lures, according to the research.

Considering that box jellies don’t have a centralized “brain” the fact that they are able to actively hunt is worth noting. Not surprising per se — as much research has shown that many organisms without centralized brains can still engage in complex behaviors — but interesting and worth noting nonetheless.

“This species is small, less than two centimeters (three-quarters of an inch) across the bell, they’re 96% water, they lack a defined brain or central nervous system, and yet they’re using their tentacles and nematocyst clusters like experienced fishers use their lines and lures,” stated lead author Robert Courtney.

“They’re not opportunistically grazing — they’re deliberately fishing. They’re targeting and catching fish that are at times as big as they are, and are far more complex animals. This is a really neat animal that is displaying a surprisingly complex prey capture strategy.”

The findings are the result of researchers filming the Carukia barnesi through a full 24-hour cycle — while utilizing infrared-sensitive to allow activities occurring in pitch darkness to be seen.

“We already knew what they ate, because gut contents analysis is pretty straightforward with an animal that’s transparent, but the fishing techniques we observed were a surprise,” commented senior researcher Associate Professor Jamie Seymour, from JCU’s Australian Institute of Tropical Health and Medicine.

“During the night we saw they were less active and not fishing. They contract their tentacles down to 4 to 5 centimeters (approximately 1.5 to 2 inches) long, with the nematocyst clusters all bunched up. We believe they may do this to conserve energy when visually oriented prey such as larval fish may also be less active.”

During daylight hours though, while fishing, the box jellies stretch out their tentacles by as much 1.2 meters (3’10”) — leaving the nematocyst clusters “evenly spaced along each almost-invisible thread, like a fishing line”.

“The nematocyst clusters look like a series of bright pearls, which the jellyfish twitches to attract the attention of its prey, like a series of fishing lures,” Mr Courtney noted. “It’s a very deliberate and selective form of prey capture.”

After contact is made, paralyzation and death follow shortly thereafter of course.

“It’s a highly successful fishing strategy, and the only account of a box jellyfish using aggressive mimicry to capture prey.”

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