Smithsonian.com

As part of a new study, shore crabs that were given a mild electrical shock responded in a way indicating they felt pain. Credit: Queen’s University Belfast

Can crabs feel pain? New research on the clawed crustaceans suggests the answer is yes.

A group of UK researchers came to this conclusion by examining the reactions of common shore crabs to mild electric shocks in a study released today in the Journal of Experimental Biology. The key to their finding is the distinction between the nervous system activity known as nociception and pain, which is defined as an unpleasant sensory and emotional experience. For years, many researchers assumed crustaceans such as crabs experienced the former, but not the latter.

Nociception—which differs from pain in that it isn’t subjective—is produced by the peripheral and central nervous systems in reaction to potentially tissue-damaging stimuli. All animals experience this reflex, including humans—for example, the nerve endings (called nociceptors) under our skin transmit a signal along our spinal cord to the brain when we touch a too-hot plate, and we automatically jerk our hands back.

For crabs, nociception provides immediate protection following a small electric shock, but it shouldn’t trigger any changes in its later behavior. That’s a job for pain—it helps organisms learn to avoid the harmful source in the future.

In this study, the crabs appeared to do just that. Ninety crabs were placed in a tank with two areas without a light source, one crab at a time. After the crabs scuttled toward the dark area they liked best, they were removed from the tank and exposed to a mild electric shock.

Following a rest period, each of the crabs was returned to the tank. Most of the crustaceans returned to the shelter they’d picked the first time. Those who had received a shock in the first round were zapped again, and when they were introduced into the tank for the third time, the majority moved to the other, presumably shock-free safe area. Crabs who hadn’t been shocked returned once again to their first-choice area.

Shore crabs chose on which side of the tank to seek shelter. Credit: Queen’s University Belfast

Dark hideaways, like under rocks along waterbeds, are important to these creatures because they offer protection from predators. After receiving the electric shocks, the decapods chose to trade in safety to avoid the unpleasant experience in the future.

“Having experienced two rounds of shocks, the crabs learned to avoid the shelter where they received the shock,” said study co-author Bob Elwood, an animal behavior professor at the School of Biological Sciences at Queen’s University Belfast, in a statement. “They were willing to give up their hideaway in order to avoid the source of their probable pain.”

So did the crabs remember the pain? The researchers say it’s possible, and previous work by Elwood and others supports the idea.

In a 2009 study with hermit crabs, wires attached to the creatures’ shells delivered small shocks to their abdomens, which they typically protect by crawling into empty mollusk shells. The only crabs to abandon their shells in search of others had previously incurred electric shocks, which researchers say means the crabs found the experience unpleasant—and perhaps ouch-worthy.

A new shell was then offered, and those crabs that had been shocked but remained in their original homes moved quickly toward the new option, investigated it for a shorter time and were more likely to make the switch than those who hadn’t been shocked. Experiencing shocks changed the hermit crabs’ motivation, much like the way we choose not to touch that hot plate again.

Such behavioral changes were also the subject by a 2007 paper by Elwood, with a different crustacean, the prawn. Various noxious stimuli introduced to prawns’ antennae elicited a reflexive tail flick. But after that, the prawns groomed their antennae and rubbed them against the side of their tanks, prolonged activities that, researchers say, signal the experience of pain.

While it’s impossible to explicitly demonstrate that crustaceans like crabs, prawns and lobsters feel pain, researchers hope these findings spur investigation of how the marine animals are handled in aquaculture and in the kitchen, where chefs often declaw or boil crabs alive.

The red and aquamarine highlights, and its giant size, set this crayfish apart (Photo courtesy of Carl Williams)

Crayfish, crawfish, crawdads. Call them what you will (tasty?), there are some 600 species found all over the world, and half of those in the United States and Canada. But if you’re looking for the real hotspot of crayfish diversity, head to Tennessee or Alabama. That said, scientists weren’t expecting to find a new species in Shoal Creek in Tennessee; aquatic biologists had been studying life in that little waterway for decades.

The tale starts in 2009, when Eastern Kentucky University biologist Guenter Schuster received some photos of a large crayfish found in Shoal Creek and shared them with Chris Taylor, an aquatic biologist at the University of Illinois. The crayfish had bearded antennae covered in bristly setae that enhance their sensory capabilities, and it looked a lot like Barbicambarus cornutus, a species that lives about 130 miles away from the creek in Kentucky and can grow as big as a lobster. Schuster and Taylor thought that perhaps a fisherman had brought the crayfish to Tennessee in a bait bucket. But when a colleague in Tennessee told them he’d found another giant crayfish in the creek, they had to check it out for themselves.

After a couple hours of wading through the water and upturning boulders, they struck the crayfish jackpot. Beneath a big, flat boulder under a bridge they found a male twice the size of any other crayfish they’d seen that day. And under an ever bigger rock they spotted a female. DNA analysis showed that these large Shoal Creek crayfish were their own distinct species, now named Barbicambarus simmonsi; a description of the new species appears in the Proceedings of the Biological Society of Washington.

The scientists aren’t sure why no one noticed the large crustacean before. “If you were an aquatic biologist and you had seen this thing, because of the size and the setae on the antennae, you would have recognized it as something really, really different and you would have saved it,” says Schuster. However, it appears that these crayfish are not common (only 5 have ever been caught) and their preference for living under large rocks in deep water may have made them easy to overlook, especially in times of high water.

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