Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

Species: The collector sea urchin (Tripneustes gratilla) – also called the cake or Parson’s hat sea urchin

Habitat: Coral reefs, seagrass meadows and algae forests in the tropical seas of the Indo-Pacific

The collector sea urchin looks like a pretty pincushion lying on the ocean floor, going about its business of munching on algae and seaweed.


But when threatened, this sedate pincushion has a most extraordinary defence. It releases a cloud of semiautonomous weapons: hundreds of tiny jaws that are still capable of biting and releasing venom even when separated from the sea urchin’s body.

Sea urchins have a hard, chalky shell covered in long spikes. Nestled among the spikes are tubular stalks topped with biting jaws known as pedicellariae. One type of these appendages even has venom as well.

“The globiferous pedicellariae are minute and terrifying,” says Hannah Sheppard-Brennand at Southern Cross University in New South Wales, Australia.

The three-jawed, pincer-like heads bite attacking predators, releasing venom as they do so. They can get torn from the sea urchin but stay attached to the predator.

“When they were first observed, they were thought to be parasites, as they give the appearance of independent action from the main animal,” says Sheppard-Brennand.

Defensive cloud

In sea urchins, the pedicellarie usually remain attached to the body unless ripped off. But when the collector sea urchin is attacked, it releases the biting heads, forming a defensive cloud in the water around it, Sheppard-Brennand’s team has shown.

It can take 40 to 50 days for the pedicellariae to regenerate. But the urchins have a high density of these appendages and use only a small proportion of them per attack, says Sheppard-Brennand.

When the team simulated fish attacks in the lab by gently poking the urchins, they usually responded by releasing tens of the appendages. Individuals can emit hundreds in just 30 seconds, says Sheppard-Brennand.

Fish given food that contained pedicellariae rejected it, both in the lab and in experiments in the field. The venom seems to be crucial: when it was removed by soaking the pedicellariae in alcohol, there was far less of a deterrent effect.

Journal reference: The American Naturalist, DOI: 10.1086/691437