Pain's Benefit to Squid May Hold Clues to Chronic Human Pain, Scientists Say

For the longfin inshore squid, pain can mean the difference between life and death, according to a new study. That's because pain prompts injured squid to behave in ways that help it survive encounters with a fish predator, researchers said.

That finding may also provide hints as to why other animals, including humans, experience long-lasting or chronic pain, behavior experts say.

It's long been thought that pain causes an animal to act self-protectively, says Robert Elwood, an animal behavior researcher at Queen's University Belfast who was not involved in the study. Pain teaches an organism to avoid situations that will bring it on. It seems obvious, but it hasn't really been tested until now, Elwood said in an email interview.

In a study published today in Current Biology, researchers report that the sensitivity with which injured squid reacted to aggressive moves from a predator, in this case a black sea bass, gave the squid better odds of surviving an attack.

This study is one of the few experiments that have looked at pain in a context closer to real-world conditions, says Zen Faulkes, an animal behavior researcher at the University of Texas-Pan American in Edinburg, Texas, who was not involved in the project. Specifically, the study looked at pain in the context of a predator-prey interaction rather than as an isolated sensation in an individual.

Going Numb

Study co-author Robyn Crook, a neuroscientist at the University of Texas Medical School at Houston, and colleagues wanted to see how injured squid reacted when confronted with a predator.

The researchers induced a minor injury common in wild squid in their test animals by snipping off the tip of one arm in each of 20 longfin inshore squid. This resulted in a hyperaware or sensitive state in the squid long after the initial damage.

The scientists wanted to make sure that it was this sensitive state affecting an injured squid's behavior, rather than any mechanical effects of losing an arm tip, so they injured a second group of 16 squid in the same way as the first group. But before cutting off the tip of an arm, Crook and colleagues applied a local anesthetic to the area. This prevented those 16 squid from registering their injury for about six hours.

Crook and colleagues compared their injured groups to two control groups: 20 uninjured squid and 16 uninjured but locally anesthetized squid.

The scientists found that black sea bass preferentially targeted injured squid—and that when attacked, both injured groups had lower survival rates than the uninjured groups did. But of the two injured groups, about 45 percent of the squid that knew they were hurt survived their encounters with the black sea bass. Less than 25 percent of the locally anesthetized injured squid, who could not feel the pain of the injury, survived.

Once animals are injured, "they're at high risk of death and they need something to cope with it," Crook says. Their sensitive or hyperaware state post-injury seems to be that coping response.

What Is Pain?

Simpler invertebrates like the sea hare, a sea slug relative, also go into a sensitized state after an injury, says Crook. The phenomenon occurs in an unknown number of other animals as well, which suggests that it could be a widespread or even universal response, she says.

In people, long-lasting pain is generally thought to be a bad thing, says Edgar T. Walters, a neurobiologist at the University of Texas Medical School at Houston. Walters says the squid study reminds us that anything that's as prevalent as chronic pain probably served an important function in our ancestors, which is why it has persisted through millennia of evolution.

It's unclear whether invertebrates like squid, crabs, or lobsters feel pain in the same way that people do. Rather, researchers label an animal's reflexive reaction to harmful stimuli "nociception"—the ability to detect a harmful stimulus and react in a knee-jerk fashion while not necessarily feeling an accompanying sensation. (See "Debate Continues: Did Your Seafood Feel Pain?")

"Nociception is sensory information," says Faulkes, "just like sound is sensory information." Pain is our interpretation of that information, much like music is an interpretation of a series of sounds, he says.

"The [study] authors are careful not to claim that squid feel pain," says Elwood. "However, their data are consistent with that idea."

At the very least, this study shows that pain can be an important motivator for survival, Elwood says.

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