Drop a lobster in boiling water and the lobster will thrash around wildly. Pierce an earthworm with a fishhook and the worm will twist and writhe in excruciating pain.
Or will it? Do these animals really feel pain? Or are their movements just muscles automatically contracting due to an outside stimulus?
"You're dealing with the fundamentals of pain and what pain is," said Tony Yaksh, professor of anesthesiology at the University of California at San Diego. "It's complicated -- how do you define pain?"
A recent scientific report from Norway has added fuel to this long-simmering debate. The study, funded by the Norwegian government, finds that animals like lobsters have nervous systems that are too simple to process what we call "pain."
According to Yaksh, primitive animals like lobsters have the ability to perceive and respond to a "noxious stimulus," that is, any agent that can cause physical harm like tissue damage.
"When you deal with a non-verbal animal, and when you see a lobster in boiling water, you know that's a noxious stimulus," said Yaksh.
But scientists like Yaksh stop short of calling what the lobster feels "pain" -- or pain as humans know it. The difference, Yaksh explained, is in our feelings. "There's a strong emotional component to what we call pain," he said.
It is this emotional component that helps us remember what causes pain, said Yaksh. "It's one of those things that drives you to avoid those [painful] things in the future," he said.
But animals with simple nervous systems, like lobsters, snails and worms, do not have the ability to process emotional information and therefore do not experience suffering, say most researchers.
"There are two types of animals, invertebrates and vertebrates," said Craig W. Stevens, professor of pharmacology at the Center for Health Sciences at Oklahoma State University in Tulsa.
Animals without spines, or invertebrates, "have chain ganglia -- groups of neurons connected by nerve fibers," said Stevens.
When stimulated, these chain ganglia cause muscles to contract. "It's a very quick neuron response," Stevens said.
According to Stevens, the chain ganglia network is so simple it doesn't even require a brain. "If you remove the head region of a lobster, the body of the lobster would still react the same way, because of the local reflexes ... involving those chain ganglia," he said.
"When you drop a lobster in boiling water, or put a fishhook through a worm, those stimuli cause those muscles to contract," Stevens said. "We describe that as pain because of the motor response, which is nothing more than neurons that have been stimulated."
But vertebrates with spines have much more advanced nervous systems and can feel real pain and suffering, Stevens explained.
"In humans, there are ... neurons talking through all parts of the brain. That's a big difference," Stevens said.
Reports like the recent one claiming lobsters feel no pain, however, arouse the ire of some animal-rights activists.
"I don't care what this report came out with -- I don't know how anyone could say that the lobster does not feel pain," said Mary Beth Sweetland, director of research and investigations for People for the Ethical Treatment of Animals.