Humans Made Conchs Shrink (And One Kid Saw It Coming)

By Elizabeth Preston
Mar 21, 2014 5:39 PMNov 20, 2019 4:08 AM


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The classic, swirling shell of a conch helps protect it from hungry birds and sea creatures, but when a human decides to pluck one from shallow water and boil it for supper, there's not much the animal can do. Its only defense is to evolve, as a species, to be smaller and less appealing to people. That's what conchs in the Caribbean have done—today's humans get 40 percent less food out of a conch than our ancestors did. But that's not so surprising to a 12-year-old girl who described almost the same thing in a piece of fiction. To explain the fictional version, I have to insert myself into this story—hello! In my day job I edit a kids' science magazine called Muse. We recently ran a contest that asked readers to write a brief, made-up story in the style of the magazine's science news page. (The results were sometimes hilarious. "Too Much Coffee Makes Adults Feel Young" was an office favorite.) One winner of this contest was a 12-year-old named Madeline. Her submission was about a new species, the "broken-shelled hermit snail" (Mendacious latibulum), discovered on the beaches of North America. It had gone undetected until now, the story went, by evolving a mangled-looking shell that kept human collectors away. "She is almost certainly right in theory," says Aaron O'Dea, a marine historical ecologist and paleobiologist at the Smithsonian Tropical Research Institute in Panama. As long as the mollusk were still alive while on the beach and vulnerable to collection, evolving an ugly shell could help it survive. O'Dea knows all about mollusks evolving to avoid humans; he recently discovered that the West Indian fighting conch (Strombus pugilis) has done exactly that. O'Dea and his coauthors studied conchs living on the Caribbean side of Panama. To see how the animals had changed over time, they gathered shells from three general time periods. The first was modern shells, which they collected by snorkeling, visiting tourist shops where the shells are sold as souvenirs, and sampling trash piles left by locals who eat the animals. (People sometimes denied gathering conchs for food, even when heaps of shells under their homes told a different story. The authors explain that there's a regional stigma around eating the conch, nicknamed el raton del mar or "the sea mouse." In Brazil, though, it's sold as an aphrodisiac. Earmuffs, Madeline.) Prehistoric conch shells, from humans' earlier days living in the region, came from an archaeological site with material dating from around 690 to 1410 AD. In these days humans probably hunted the conch as they do today, wading into the shallows whenever they needed some for dinner. Finally the authors looked at local fossils, about 7,000 years old, to see conchs that had never met a human. The researchers measured the length and width of the shells, as well as the thickness of each shell's "lip," the part overhanging the opening. O'Dea explains that this reveals whether a conch is a juvenile or an adult. When they're young, conchs hide under the sand, slowly growing the spiky whorls of their shells. When they've stopped growing and are ready to emerge and find a mate, conchs develop the final portion of their shell, a thickened lip that makes it harder for predators to get inside. By measuring the lip thickness of each shell, the scientists could tell whether a conch was mature. They found that over time, conchs have grown smaller and begun reaching maturity at a smaller size. The shells shrank between the 7,000-year-old fossils and the 1,500-year-old archaeological samples, and again between the archaeological samples and today. For as long as humans have been gathering conch, we've been driving their evolution. "I had seen the conch in the field and noticed size differences," O'Dea says, "but when I put it together empirically it was astounding!" Using each shell's dimensions to estimate the size of the animal inside, he found that ancient conchs contained a "truly impressive" two-thirds more meat than modern ones. Today's humans need more animals to make a meal than their ancestors did. Conch sizes varied somewhat among the five modern areas the researchers sampled. The smallest mature shells came from a reef surrounded by homes with shell piles. Another group of small conchs came from a site that sells many shells to tourists. The largest conchs lived in a lagoon where no one is known to eat them. Other larger shells were found at a site surrounded by humans who prefer farmed meat to shellfish, and another site that's been protected in recent years by scientists. This shows that the conchs may be able to bounce back in size when people hunt them less. Still, none of these populations was as large as the ancient conchs. O'Dea says it's not clear whether shrinking has hurt the species as a whole. Conchs may have initially evolved to be large, he speculates, because it helps them win fights for mates. He hopes to find the answer by comparing the fitness of larger and smaller conchs. O'Dea says that Madeline's broken-shelled snail story may be coming true in other ways. For example, "There is evidence that the desire for big, impressive antlers for trophies by deer hunters has led to the evolution of less impressive, or less 'beautiful,' antlers," he says. "This is exactly what this girl is talking about, and I wouldn’t be surprised if we start to see fish or anything else that is selected against by humans for a certain trait follow suit." I tracked down Madeline herself to tell her the story of the fighting conch. She wrote back to say, "It’s really cool that something similar to what I thought might be plausible as an adaptation really works." Madeline says it's funny that humans can alter the evolution of a whole species without knowing it. But given how easily we reshaped the conch, she's more concerned about how many other, still-unseen effects we're having on the world. Focusing on the single example of the conch while ignoring the rest, she says, "would be making a mountain out of a mole-usk hill."

Image: by Loren Sztajer (via Flickr)

O'Dea, A., Shaffer, M., Doughty, D., Wake, T., & Rodriguez, F. (2014). Evidence of size-selective evolution in the fighting conch from prehistoric subsistence harvesting Proceedings of the Royal Society B: Biological Sciences, 281 (1782), 20140159-20140159 DOI: 10.1098/rspb.2014.0159

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