Ask any biologist and they'll tell you — whether they agree or not — that there's no conclusive evidence that crabs and lobsters feel pain. In fact, unless a study involves an octopus or a squid (both animals that possess complex nervous systems), researchers don't have to obtain clearance from their institutions before performing experiments on animals devoid of spines, commonly called invertebrates. But, as The Washington Post reports, one researcher is questioning the "invertebrates don't feel pain" rule and finding interesting, albeit disquieting, results.

The first problem researchers have to tackle when studying pain in animals is how to define it. What we humans might perceive as a pain response (such as a lobster yanking a pincer away from a hot surface), might actually just be a reflex that bypasses the pain centers of the brain. So any researcher wishing to find evidence of pain in animals like crabs and lobsters would have to find a way to avoid mistaking pain for a simple reflex. And Robert Elwood, an animal behaviorist at Queen's University in Belfast, thinks he's done just that.

One of Elwood's first pain experiments, published in 2007, involved brushing acetic acid on a prawn's antennae. To his surprise, the prawns began to groom their antennae in a prolonged and complex way — a behavior that could be compared with a human nursing a wound. Just to make sure, however, Elwood repeated the experiment, but he did so after applying a local anesthetic to the antennae. This, he found, diminished the prawn's grooming. He's obtained similar results in experiments involving hermit crabs and electric shocks. "This is prolonged and complicated behavior," Elwood told The Washington Post, "which clearly involves the central nervous system." These behaviors, he said, couldn't possibly be defined as simple reflexes.

The results of Elwood's experiments are extremely controversial. Despite the numerous studies that have identified pain receptors and pain-related behaviors in invertebrates with complex nervous systems, such as octopi and squids, Elwood has gained only a small amount of support from the scientific community. Some researchers worry that if we start to think of less complex invertebrates in the same way we think of an octopus or a giraffe, it become will be hard to decide where to draw the line.