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The Pacific coast of North America is littered with black turban snails, and many of these thimble-sized snails bear triangle-shaped shards on their shells. These shards are scars of predation, proof that at some point in its life, this turban snail was attacked by a hungry crab.
Scientists know that by studying these scars, they can glean important information about predatory crabs, such as population numbers. In a new studythe researchers have shown that by looking at the number of scars on modern snails and those on snail shells taken from the fossil record, they can extend this comparison up to 120,000 years old.
“It’s really so fascinating to think about the fact that you could walk along the beach and pick up a snail and see this recording of a predator-prey interaction,” says Carrie Tyler, a paleontologist at the University of Miami Florida. who did not participate in the study. “I think it’s amazing to go all the way back to the Pleistocene and make this direct comparison.”
For the study, Kristina Barclay, a paleoecologist at the University of Victoria in British Columbia, compared the scars of black turban snails collected from study sites on the California coast near Los Angeles and San Diego with fossilized shells collected from the same region that date back to the Upper Pleistocene, between 120,000 and 80,000 years ago.
She found that modern snails have a 10-15% lower rate of scarring than their Pleistocene counterparts, likely reflecting a decline in crab numbers.
To ensure that the lower scarring rate represents a decline in the crab population, rather than being a reflection of the crabs becoming weaker predators over the 120,000-year period, Barclay also looked at a measurement called the attack size, or snail size. was when the crab tried to eat it. The scars that mark the snail’s shell near its center mean that the crab tried, and failed, to eat the snail when it was still small, and was therefore a weaker predator. Conversely, if the size at attack was larger at a sampling site, it means that the crabs were better predators. Barclay found that despite the long period, attack size remained relatively unchanged, suggesting that the decline in modern scars reflects a decline in crab numbers rather than a change in crab hunting efficiency.
The finding is consistent with previous estimates and anecdotal accounts that indicate crab populations are in decline. “I was really pleased to see that this is additional information that can back up the story we’re already hearing about crabs,” Barclay says. “This study is a useful piece of evidence to say that maybe we should have a little more management of these fisheries.”
Given the relative ease of studying snail scars, Barclay hopes the method could be used by local or citizen science groups interested in monitoring crab populations.
“I think that kind of approach is really important, and it’s not widely used enough,” Tyler says. “We need to draw attention to this type of method which is really profitable [and] which can provide us with really important data.