Actual pain, though, requires learning, Elwood says, because its purpose it to help a creature avoid future painful and damaging experiences.

And so scientists have come up with a list of criteria for what pain might look like in an invertebrate: whether the animal rubs itself, if it gives up essential resources to avoid the potentially painful experience, and if can learn to avoid them in the future. "If it does that, it's clearly making decisions," Elwood says. The animal, probably, feels pain.

A few years back, Elwood found that prawns would begin grooming and rubbing themselves when they had acetic acid dabbed onto their tentacles. And hermit crabs that are shocked out of their shells likewise show stress-related behavior after the incident, such as grooming their abdomens.

And in a study published this week in the Journal of Experimental Biology, Elwood reports that crustaceans exhibit the learning component of pain.

For the experiment, Elwood and student Barry Magee got a bunch of common British shore crabs. They took a crab and placed it in a tank that had two dark shelters (crabs love the dark), one of which was wired to give its occupant a mild electric shock. The crab picked a shelter, got its shock or not, and then was removed from the tank.

A short while later, the researchers put the crab back into the tank and let it choose again. By the time that crab had been shocked twice, it had learned to avoid the wired hideaway and pick the safer one.

"To our surprise, the data are consistent with pain," Elwood says.

If Elwood were to respond to Stein today, he'd say that crabs do feel pain. Probably. Elwood hasn't yet looked at stress hormones in crabs to see if they change in response to the electrical shock or other potentially painful experience — another key sign that actual pain is occurring. And if there's no difference in the hormones, "it would speak against pain," he says.

As for what this might mean for those of us who occasionally dispatch a crustacean or two, the best way to minimize potential pain is likely electrocution or driving a knife through the creature's brain, Elwood says. But as most of us lack specialized machinery and knowledge of crustacean anatomy, the easiest way is still dropping the crab in a pot of boiling water.

Copyright 2013 National Public Radio.

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Yes, Virginia, Crabs Likely Feel Pain, But They're Still Delicious