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Urchin mobs band together to slaughter sea stars that prey on them

In an overpowering predator-prey role reversal, the spiny invertebrates pin the sea stars’ bodies.

Sea urchins are underwater lawnmowers, capable of uprooting entire nearshore habitats with their insatiable vegetarian appetites. New research indicates that the spiny invertebrates would also sink their teeth into something a little more difficult — and dangerous.
Researchers recently found urchins attacking and consuming predatory sea stars, which is a first. Researchers announce their findings in the June issue of Ethology, which flips the traditional predator-prey script.

Jeff Clements, a marine behavioral ecologist, and his colleagues were researching common sun stars at the Kristineberg Marine Research Station in Fiskebäckskil, Sweden, in 2018. (Crossaster papposus). Clements needed aquarium space at one point because he decided to separate one of the sun stars for a short time. The starfish was put in a tank with around 80 green sea urchins (Strongylocentrotus droebachiensis).

“I thought, ‘Okay, there’s a bunch of sea urchins in there, these guys are predators of urchins, nothing’s gonna happen,’” recalls Clements, of Fisheries and Oceans Canada in Moncton. The urchins, he says, hadn’t eaten anything in two weeks.

When Clements returned to the lab the next day, he couldn’t locate the sun star. On the side of the tank, there was a pile of urchins with something red barely visible underneath. Clements yanked the urchins away from the victim.

“The sea star was absolutely decimated,” he says. “The urchins had just ripped it apart.”

Clements and his coworkers quickly noticed that this conduct had never been documented before. So the researchers conducted two experiments, each with a single sun star in the urchin tank, to see how this “predator-prey role reversal” works.
One urchin will approach the sun star and feel around before grabbing one of the sun star’s many sides. The sun star’s arms will be covered by other urchins. When the team removed the urchins after about an hour, they discovered that the arm ends, as well as the eyes and other sensory organs positioned there, had been chewed off.

This feature of the sun star’s anatomy will work against it.

“[The tips] are the first part of the sun star that the urchin is going to encounter as it approaches,” says Clements. “So if the urchin consumes those first, the sun star is going to be less effective at escaping the attacks.”

The team has named this incapacitation “urchin pinning.”

It’s likely that the urchins are behaving in self-defense, eliminating a predator before it attacks them. According to Julie Schram, an animal physiologist at the University of Alaska Southeast in Juneau who was not associated with the study, the urchins’ relative hunger may be the cause of the attacks. Urchins can switch up their diet in surprising ways in crowded lab conditions with limited food, as in this study. Some animals, for example, have been observed eating each other.

“This would suggest to me that when starved, adult urchins will seek out alternate food sources,” she says. 

According to Jason Hodin, a marine biologist at the University of Washington in Friday Harbor, urchins’ ability to prey on parasitic sea stars had been speculated at previously, with sea stars showing up in urchin stomach contents. However, this was often misinterpreted as scavenging. “Active predation was the more interesting possibility, and it’s satisfying to see that possibility confirmed, at least in the lab,” says Hodin, who was not involved with the research.

If these urchin attacks occur in the wild, Clements believes there may be some interesting implications for kelp forests habitats. Urchins will graze kelp forests to extinction if they become overabundant, leaving behind urchin “barrens.” It would be easier for urchins to maintain a high population if they fed on whatever animals were left behind.

“If [the urchins] are using animals to persist in these urchin barrens when kelp is low or nonexistent, it could actually delay the recovery of these kelp forests back to their original state,” says Clements.

According to marine ecologist Megan Dethier, such discussions of ecosystem influences are premature and make much too much of a “rare lab situation.” Even in urchin barrens, where food is scarce, such attacks haven’t been reported, according to Dethier of the University of Washington Friday Harbor Laboratories.
The urchin attacks can’t be deliberate, she says, since the animals lack a brain and central nervous system.

“Urchins doing a coordinated predatory attack is not biologically feasible.”

According to Clements, the coordinated attacks may be the result of chemical reactions caused by the ongoing feeding, which releases odors into the water. When the first urchin begins to chew on the sun star, the other urchins will begin to recognize it as food. Clements plans to conduct studies in the future to manipulate the hunger and density of urchins to see what factors affect their desire for sun stars.
The findings serve as a reminder that invertebrates like urchins can perform surprisingly complex behaviors despite having basic nervous systems, according to Clements. “These animals aren’t just kicking around doing nothing on the [sea] bottom.”

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