Fish Tales: What Squid Reveal About Chronic Pain

New research focused on squid has revealed why pain may be essential to survival. A study published in Current Biology revealed that an injury caused the aquatic animals to be on high alert, which in turn helped them survive an attack by a predator.

If living beings must exhibit special instincts to come out on top in a survival-of-the-fittest world, pain encourages animals and humans to act on those instincts. Although the premise seems obvious, researchers say the theory hasn’t been tested until now.

Pain helps organisms survive by putting them on high alert, a study shows.

The study focused on the longfin inshore squid, which live predominantly in the Atlantic waters off the East Coast of the United States, and examined how it interacted with the black sea bass, a natural predator. To put the squid on high alert, researchers at the University of Texas Medical School at Houston (UT) injured each one by cutting off the tip of an arm. This type of injury is common in the wild.

A group of control squid were injured in the same way, but they received local anesthetic to numb any sensations of pain. Researchers wanted to make sure the squid were reacting from the pain, and not from any reduced mobility resulting from the injury. A second control group had no injuries and were perfectly healthy.

Then, the groups of squid were set up for the ultimate test—a face-off with the black sea bass predator. The sea bass could tell which squid were injured and went after those two groups right away, opting for the easier kill instead of targeting the non-injured group.

However, the pain that injured squid felt ultimately helped them survive, with 45% of the experimental group surviving attacks from the black sea bass while less than 25% of the group that received local anesthetic survived.

When in defense mode, squid tend to move in a certain sequence designed to protect them against predators. The squid that felt pain were more likely to sense the impending danger posed by the sea bass and responded by escalating their defensive postures. In contrast, the uninjured squid and those who received local anesthetic did not identify the same level of danger, and in turn, their defense mechanisms were lowered.

Watching the defensive sequence gave researchers a way to easily compare the way injured and non-injured squid assessed dangerous situations, scientists say.

Researchers concluded that feeling pain—or detecting negative stimuli—made the squid extra alert, giving them the push needed to fend off an attack and survive. This is in contrast to other theories of pain that posit the changes resulting from discomfort are negative and not beneficial to continued survival, researchers say.

Do fish feel pain? Some researchers say no.

Although the research on squid was consistent with the idea that squid feel pain, that fact in itself is very much in debate. Scientists aren’t sure whether fish and other sea creatures feel pain the way humans do. “The [study] authors are careful not to claim that squid feel pain,” says Robert Elwood, an expert in animal behavior from Queen’s University Belfast. “However, their data are consistent with that idea.”

Despite the debate, researchers still concluded that pain—or negative stimuli without sensation—is an important trait for survival. For fish, which might not actually feel pain, scientists use the term “nociception.” Nociception refers to the ability for an aquatic creature to recognize potentially dangerous stimuli and react to escape it, even if they don’t feel pain. Research on the topic is ongoing.

In 2013, for example, British researchers conducted a study suggesting that crabs felt pain from electroshocks, but other research has not arrived at similar conclusions, according to National Geographic. Another possibility is that some species feel pain while others do not.

Chronic pain’s evolutionary roots may involve making humans more alert in order to increase survival odds, the Current Biology study says.

While people do not, in their daily lives, fend off attacks from sea bass, or even black bears, chronic pain likely triggered that same increased sensitivity and resulting defense mechanisms earlier in evolutionary history, says Edgar T. Walters, a UT neurobiologist. Chronic pain likely would not have endured through evolution unless it served a higher purpose, he adds.

However, researchers aren’t sure why humans experience chronic pain, a common condition affecting 100 million U.S. adults. Although much research has focused on modern causes of chronic pain, with risk factors including age, extra weight, smoking, and depression, not much discussion has surrounded its potential evolutionary basis.

Science has outlined pain’s biological underpinnings. Chronic pain is generally caused by overly communicative nerves that keep warning the brain about tissue damage, even if none exists. Still, little is known about why people suffer in the 1st place. That may in part be due to the wide range of possible causes for chronic pain.

Arthritis has its own set of causes and risk factors, while fibromyalgia and injuries that never healed correctly have an entirely different set. Severity ranges from mild to severe, and can accompany or be exacerbated by emotional distress, including depression.

Meanwhile, a person’s prognosis for chronic pain is heavily dependent on emotional factors, according to WebMD. Previously active people may no longer be able to do their favorite activities due to chronic pain. Resulting depression often worsens their suffering.

In the animal kingdom, many do not adjust their behaviors to show they’re suffering, according to rheumatology expert Dr. Mark Borigini. Such a display would be considered a weakness, putting the animal at an increased risk for an attack. Humans, however, split into different camps. Some people are very expressive with their pain while others manage it stoically and without expression, Borigini says.

The complex mix of physical and psychological factors determines how well people fare and if they’re able to maintain some semblance of a normal life, WebMD says.

Whatever the case, researchers say the squid research could help unravel the purpose of pain and other forms of nociception in order to better treat chronic pain in humans.

Do you think chronic pain has an evolutionary purpose?

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