Sea urchins are underwater lawnmowers and can modify whole shore ecosystems with unabated vegetarian appetites. But new research suggests that these spiny invertebrates also sink their teeth into something a bit more challenging — and dangerous. In a first, scientists found out recently that urchins attack and eat butcher sea stars. Researchers Dr. Jeff Clements and Dr. Fredrik Jutfelt at the University of Science and Technology collaborated with Dr. Sam Dupont at the University of Gothenburg reported that the observations are based on a classic predator-prey script in the journal Ethology.

In 2018, marine ecologist Dr. Clements and his associates studied familiar sun stars (Crossaster papposus). Dr. Clements wanted to separate one of the sun stars for some time and needed a space for the aquarium. In a tank of 80 green sea urchins, he placed the starfish (Strongylocentrotus droebachiensis).

“These people are urchins’ predators; nothing will happen,” recalls Dr. Clements. “I thought, ‘Okay, there are a few marine sluts there. In two weeks, he said, the urchins hadn’t eaten anything.
The following day Dr. Clements was not able to find the Sunstar when he came into the laboratory. On the side of the tank was a stack of urchins with some red underneath. Clements revealed the victims, pried the urchins off.

“Decimated was the marine star,” he says. “It had just been ribbed off by the urchins.”

This behaviour was soon noticeable to Dr. Clements and his colleagues. Therefore, the team conducted two tests, each with the sun star at an urchin tank, recording how this “predator-bearing reversal” occurs.

An urchin would come close to the sun star, feel around and eventually attach himself to one of the many arms of the sun star. Other urchins, covering the arms of the sun star, would go along. After approximately an hour, when the team removed the urchins, the arms, eyes, and other sensory organs of the sea star were chewed out.

This incapacity has been dubbed “urchin pinning” by the team.

If these urchin attacks occur in the wild, Dr. Clements believes there could be some exciting ramifications for kelp forest ecosystems. When overabundant, urchins can graze kelp forests to extinction, leaving urchin “barrens” in their wake. It would be easier for urchins to maintain their population if they fed on whatever animals were left behind.

“If [the urchins] use animals to survive in these urchin barrens when kelp is scarce or nonexistent, it may delay the recovery of these kelp forests to their original state,” Dr. Clements says.

The synchronized attacks could be based on the chemicals of the continuous feeding of smells into the water, says Dr. Clements. Once the first sweat begins to chew on the sun star, the other sweaters can recognize the sun star as food. To see what factors affect their appetite for the sun stars, Dr. Clements wants, in future, to carry on experiments manipulating hunger and the density of Urchins.

Dr. Clements says the findings show that invertebrates like sea urchins can perform surprisingly complex behaviours even with simple nervous systems. “These animals aren’t just kicking around on the [sea] bottom doing nothing.”

Journal Reference:

Clements, JC, Dupont, S, Jutfelt, F. “Urchin pinning”: Behavioural observations reveal how hungry urchins actively prey upon their sea star predators. Ethology. 2021; 127: 484– 489.

Main image credit: Brian Dewey

About the Author

Dr. Jeff C. Clements, PhD

Dr. Jeff C. Clements, PhD is a marine ecologist with a particular interest in benthic invertebrates, currently working as a Research Scientist at Fisheries and Oceans Canada (Gulf Region) in Moncton, NB. His primary research goals are to understand how environmental conditions influence marine animal behaviour and ecophysiology, determining the proximate mechanisms mediating such influences, and elucidating functional ramifications at organismal, population, and community scales. The environmental conditions he is interested in can be biotic (e.g. predation, conspecific associations, parasitism) or abiotic (i.e., environmental conditions), and can be natural (i.e., natural variation) or anthropogenic (e.g. acidification, warming, deoxygenation, invasive species). His research questions span the fundamental-applied research continuum and are applicable to basic biological understanding as well as fisheries and aquaculture.