June 3, 2013
In recent weeks, photos of lobsters have been floating around social media with captions calling the crustaceans “biologically immortal.” Anyone with an Internet connection can even create a meme juxtaposing this fact with a joke of his or her own. But is this fun fact actually, well, fact?
The viral scientific tidbit can be traced back to a brief 2007 news story that reports that lobsters don’t show typical signs of a phenomenon known as senescence. In plain terms, the report says that lobsters don’t age the way other living creatures do—they don’t lower their reproductive ability, slow their metabolism or decrease in strength. This led to extrapolations that lobsters, if left undisturbed, can’t die. Other websites write that lobsters’ incredible longevity is courtesy of their DNA, with credit specifically due to telomerase, an enzyme used in cell division that protects the ends of chromosomes.
The popular graphic and its different textual iterations caught the eye of Zen Faulkes, an invertebrate neuroethologist at the University of Texas-Pan American, who sussed out the situation on his blog and came to this conclusion: “If there is evidence supporting that claim,” he wrote in an email, “I have not found it yet.”
It’s true that lobsters continue eating, reproducing and growing until the end. And there is an end—they’re not immortal. But like most decapod crustaceans, which also include crayfish and shrimp, they have indeterminate growth. That means they don’t reach a set size limit in their lifetimes, continuing to grow until they die of natural causes or are killed.
Lobsters grow by molting their hard exoskeleton, and they do so a lot: the average lobster can molt 44 times before it’s a year old. By the time lobsters reach the age of seven, they molt once a year, and after that, once every two to three years, growing larger with each successive shedding of its exoskeleton. The largest lobster on record, caught in Nova Scotia in 1977, weighed 44 pounds, six ounces and measured 3.5 feet in length. Last year, fishermen caught a 27-pound lobster, roughly the size of a toddler–the largest in Maine’s history. For lobsters, bigger bodies translate into more reproductive success: females can carry more eggs as their body volume increases, and they keep producing them until they die.
Molting is a stressful process. Losing an exoskeleton leaves the critter, now without a hard shell and strong pincers, temporarily vulnerable to predators. But predation isn’t senescence. So what would be a natural death for lobsters?
According to Carl Wilson, lead lobster biologist with the Maine Department of Marine Resources, between 10 and 15 percent of lobsters die naturally each year as they shed their exoskeletons because the exertion proves to be too much. Each molting process requires more and more energy than the one before it as lobsters grow in size.
Finally, older crustaceans stop shedding their exoskeletons altogether—a clue that they’re near the end of their lifespans. They run out of metabolic energy to molt, and their worn-and-torn shells contract bacterial infections that weaken them. Shell disease, in which bacteria seeps into lobster shells and forms scar tissue, adheres the crustaceans’ bodies to their shells. The lobster, attempting to molt, gets stuck and dies. The disease also makes lobsters susceptible to other ailments, and in extreme cases, the entire shell can rot, killing the animal inside.
“Is that senescence? Maybe not in how we think about it,” says Jeffrey D. Shields, a marine science professor in the Virginia Institute of Marine Science of the College of William & Mary. “But it is senescence in the way that older people die of pneumonia.”
But one question about lobsters’ lifespans still remains. Scientists do not yet have a truly validated way of determining the age of lobsters. “The problem with lobsters is when they molt, they molt their entire exoskeleton, including their digestive tract and gastric mill and the like, so there are no hard parts that are left,” Wilson says. These hard parts, if a trace of them were left after every molt, would help determine a creature’s age—without them, approximating lobsters’ birth years is difficult.
Previous research has suggested that the biggest European lobster males in the wild live an average of 31 years, and the females an average of 54 years. The work is based off assumed accumulation rates of fat residues found in the creature’s eyestalk. Other scientists are approximating the age of lobsters by measuring a pigment called neurolipofuscin that builds up in the crustaceans’ brains over time. Still more are studying discarded exoskeletons and counting growth bands deposited in the calcified body structures (PDF) to determine an average rate of growth for a given lobster, allowing scientists to estimate its age.
Scientists, however, are not looking for the secret of lobster immortality—it doesn’t exist.
January 16, 2013
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.
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.
January 21, 2011
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.
Check out the entire collection of Surprising Science’s Pictures of the Week on our Facebook page.