The first feeding study of the tiny but deadly Irukandji box jellyfish has found that they actively fish. It's a particularly impressive feat for an animal that doesn't have a defined brain.
The jellies attract larval fish by twitching their extended tentacles, highlighting their nematocyst clusters (stinging structures) and using them as lures.
The laboratory-based study was conducted at James Cook University.
"This species is small, less than two centimetres 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," the study's lead author Robert Courtney said.
"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 researchers were able to catch Carukia barnesi in the act by filming them through a full day and night cycle, using infrared-sensitive equipment to record behaviour in times of complete darkness.
"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," senior researcher Associate Professor Jamie Seymour said.
"During the night we saw they were less active and not fishing. They contract their tentacles down to 4-5cm clong, 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 1.2m 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," Mr 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," Mr Courtney said.
Video of an Irukandji jellyfish capturing a fish can be seen below:
'Prey Capture Ecology of the Cubozoan Carukia barnesi' is published in PLOS One.
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