Quirks and Quarks

Normally herbivorous sea urchins turn the tables on a predatory sun star

Fisheries scientists see surprising predator-prey role reversal in an unintentional experiment

Fisheries scientists see surprising predator-prey role reversal in an unintentional experiment

This sun star has had several of it's limbs partially eaten by normally herbivorous urchins (Jeff Clements)

Sun stars, a form of starfish, normally prey on sea urchins. But in an unintentional experiment, fisheries scientists observed the prey turning the tables on their predator, as hungry sea urchins swarmed and decimated a sun star. 

Jeff Clements, a research scientist with Fisheries and Oceans Canada in Moncton, was studying sun stars. He placed one in a tank for what he thought would be safe keeping. He knew the tank was full of sea urchins, but assumed that they would keep their distance. That's not what happened. 

Sea urchins in the process of attacking and eating their sun star predator (Jeff Clements)

The little guy wins sometimes

Within a few minutes, 70 or 80 small sea urchins swarmed the sun star, a large animal about the size of a dinner plate. . Despite the sun star's best efforts to 'run' away, the animal, was overwhelmed and suffered considerable damage.

Clements told Bob McDonald that he could barely see that the sun star was even there, it was so covered in urchins. When he pried the urchins off, the sun stars arms had been reduced in length by a half. Clements and his colleagues call this 'urchin pinning'.  The team described the phenomenon in a new study in the journal Ethology.

The tip of a sun star arm is the easiest place for the sea urchin to start snacking (Jeff Clements)

Sea urchins are known as voracious herbivores; they can completely destroy entire kelp forests. But what Clements did not understand at the time was the urchins in the tank had not been fed in a while. The usually herbivorous urchins were driven to attack the sun star by hunger.

As soon as one sea urchin attacks, others start to move in (Jeff Clements)

Re-thinking urchin behaviour

Although this behaviour had never been seen before, there were some hints that it might have taken place in the wild. Sun star tissue had turned up in the stomach contents of urchins. Researchers had attributed this to scavenging, but that now may need to be re-thought. 

Urchins rely heavily on smell to tell them about their surroundings. They detect chemicals in the water to tell them when predators - like sun stars - are near. This sense of smell is key to understanding what happened in this case.

Clements believes that all it took was for one urchin to nibble the tip of one of the sun star's arms for chemical cues to be released into the water, telling the other urchins that this is food, and not a predation risk.

The sun star attempts to defend itself by raising one arm over the attacking sea urchin (Jeff Clements)