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Irukandji Jellyfish Go Fishing

The first feeding study of the Irukandji box jellyfish (Carukia barnesi) has found that they actively fish. They attract larval fish by twitching their extended tentacles, highlighting their stinging nematocyst clusters and using them as lures.

“This species is small, less than 2 cm 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,” said Robert Courtney of James Cook University, who was lead author of the study published in PLoS ONE.

“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.”

Carukia barnesi attracted to light. By submerging high-powered lights in the waters north of Cairns, the researchers were able to trap the jellyfish as they approached the lights.

The jellyfish were then observed in a large, temperature-controlled tank in which the water rotates vertically, simulating local oceanic conditions. They were filmed through a complete night–day cycle using infrared-sensitive equipment to record behaviour during times of complete darkness.

“During the night we saw they were less active and not fishing,” said A/Prof Jamie Seymour. “They contract their tentacles down to 4–5 cm long, with the nematocyst clusters all bunched up. We believe they may do this to conserve energy when visually oriented prey (larval fish) may also be less active.”

In daylight the tiny jellyfish went fishing, stretching their tentacles out as long as 1.2 metres with 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,” Courtney said. “It’s a very deliberate and selective form of prey capture.”

Once a fish makes contact with the nematocyst clusters it is quickly paralysed by Carukia barnesi’s powerful venom. “It’s a highly successful fishing strategy, and the only account of a box jellyfish using aggressive mimicry to capture prey,” Courtney said.