A pedicellariae head from a collector sea urchin with its 'jaws' open.
A pedicellariae head from a collector sea urchin with its 'jaws' open. Contributed

'They resemble something out of a miniature horror movie'

WHEN under attack, the collector sea urchin has the ability to release hundreds of miniature independent 'jaws with teeth' that can latch on to predators and release venom.

The discovery from Southern Cross University marine biologist and PhD candidate Hannah Sheppard-Brennand and her research team, shows sea urchins defend themselves by counter-attacking with the 'pedicellariae' - a personal army of venomous jaws.

Her study of the phenomenon was recently published in The American Naturalist, entitled 'A Waterborne Pursuit-Deterrent Signal Deployed by a Sea Urchin' describing one of the strangest defensive structures in nature.

Ms Sheppard-Brennand said the pedicellariae function not only acts as an unpalatable defence against being eaten but also as a deterrent. These semi-autonomous structures detach from the body into the surrounding water, where they act as a cue to prevent further attack after initial contact by any overly-curious potential predators.

"When harassed, these urchins released pedicellariae into the water, producing a pursuit-deterrent halo around the urchin," Ms Sheppard-Brennand said.

"They resemble something out of a miniature horror movie."

The Collector sea urchin, Tripneustes gratilla, live on the seafloor in tropical reefs of the Indo-Pacific, eating algae and seaweed but have been documented moving into cooler subtropical waters.

Supervisor and co-researcher Associate Professor Symon Dworjanyn, from the University's National Marine Science Centre in Coffs Harbour, said this surprising discovery might explain why this sea urchin was often seen out in the open while other species tended to hide in crevices or are nocturnal.

"For other sea urchin species where regeneration has been studied it can take between 40 and 50 days to re-grow these biting appendages on their shells," he said.

"Individual urchins are capable of releasing hundreds of pedicellariae within 30 seconds, however in this study they released tens at a time".

The researchers simulated fish attacks in the laboratory by gently poking the urchins. The creatures usually responded by releasing the toothy pedicellariae.

Lab tests showed fish strongly preferred palatable diets without pedicellariae and pedicellariae without venom, and behavioural studies showed that fish were deterred from the water channel that contained a sea urchin releasing pedicellariae upstream.

Ms Sheppard-Brennand said a range of signals existed in the animal kingdom including superheated toxins sprayed by the bombardier beetle, ink and opaline secreted by the sea hare, and distasteful blood squirted by the Texas horned lizard.

"The findings of this study place the collector urchin within this group of unusual animals with unique and strange defence mechanisms," she said.


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