Restless Sea Urchins Pay the Ultimate Price: Size and Behavior Determine Who Gets Eaten

We placed 170 purple sea urchins in mesocosms with California spiny lobsters for 108 hours, and Our results revealed how personality and size interact to determine survival. We, led by Justin Pretorius and colleagues at UC Santa Barbara, had suspected that behavior mattered in predator-prey encounters, but they hadn't expected the relationship to be so dependent on body size.

We wanted to understand how individual differences in urchin behavior might shape their survival. These weren't random encounters—they first tested each urchin's behavioral tendencies. They measured urchin activity levels and their covering behavior, observing how much urchins concealed themselves with substrate like pebbles and stones. Some urchins were consistently sluggish and secretive. Others were perpetual wanderers.

The pattern that emerged was stark. High activity level was negatively associated with survival. But here's what made it fascinating: this death sentence only applied strongly to smaller urchins. The negative effect of activity on survival was strong for smaller urchins and weaker for large ones. Urchin size alone didn't determine survival, and covering behavior independently didn't influence survival either. It was the deadly combination of being small and restless that sealed an urchin's fate.

"Purple urchins can consume large quantities of giant kelp and drive shifts between kelp forests and urchin barrens—critical tipping points in coastal ecosystems."

What puzzled We was that covering behavior seemed irrelevant to survival, even though they'd predicted buried urchins would be safer. The mechanism behind why size buffers against the costs of activity remains unclear from Our data.

These findings matter because lobster populations are recovering along the California coast, and urchins are ecosystem engineers. Purple urchins can consume large quantities of giant kelp and drive shifts between kelp forests and urchin barrens—critical tipping points in coastal ecosystems. If recovering lobsters selectively remove active, foraging urchins from populations, they might be reshaping not just urchin numbers but urchin behavior itself. This could alter how urchin populations impact kelp forests.

We note that they still don't know whether this selective predation actually occurs in the wild, or whether urchin populations can adapt behaviorally to increased predation pressure. The ocean is full of such personality-driven dramas, and scientists are only beginning to understand how individual differences in behavior scale up to shape entire ecosystems.

Citation

Pretorius, Justin D.; Lichtenstein, James L. L.; Eliason, Erika J.; Stier, Adrian C.; Pruitt, Jonathan N. (2019). Predator‐induced selection on urchin activity level depends on urchin body size. Ethology.

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This paper is Open Access.