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It’s possible the urchins are acting in self-defense, preemptively destroying a predator in their midst. Though, it could be the urchins’ relative hunger that’s behind the attacks instead, says Julie Schram, an animal physiologist at the University of Alaska Southeast in Juneau not involved with the research. In crowded lab conditions with limited food — similar to this study — urchins can switch up their diet in surprising ways, she notes. Some species have been documented cannibalizing each other, for instance.
“This would suggest to me that when starved, adult urchins will seek out alternate food sources,” she says.
Urchins’ capacity to feed on predatory sea stars had been hinted at before, with sea stars turning up in urchin stomach contents, says Jason Hodin, a marine biologist at the University of Washington in Friday Harbor. But this was often interpreted 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 are something that also happens in the wild, Clements thinks there could be some interesting ramifications for kelp forest ecosystems. When overabundant, urchins can graze kelp forests down to nothing (SN: 3/29/21), leaving behind urchin “barrens.” If urchins are feeding on whatever animals are left behind, it’d be easier for their numbers to remain high.
“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.
Such discussions of ecosystem influences are premature, says marine ecologist Megan Dethier, and are making way too much out of a “peculiar lab situation.” Such attacks haven’t been documented even in urchin barrens, where food is scarce, notes Dethier, of the University of Washington Friday Harbor Laboratories.
And the urchin attacks can’t be intentional since the animals don’t have a brain or central nervous system, she says. “Urchins doing a coordinated predatory attack is not biologically feasible.”
The synchronized attacks may be based on chemical consequences of the ongoing feeding releasing smells into the water, Clements says. Once the first urchin starts chewing on the sun star, the other urchins may start recognizing the sun star as food. In the future, Clements wants to run experiments manipulating the hunger and density of urchins to see what factors influence their appetite for sun stars.
The findings are a reminder that even with simple nervous systems, invertebrates like urchins can execute surprisingly complex behaviors, Clements says. “These animals aren’t just kicking around doing nothing on the [sea] bottom.”