Tridacna maxima in Sulawesi, Indonesia
Photo: Bernard Dupont

To understand how our warming oceans impact tropical cyclones, scientists need more information about storm patterns during the past. Unfortunately, a reliable historical record of these storms doesn’t extend all that far back—not even two centuries. Now a team of researchers thinks giant clam shells could help fill in some of the gaps.

Tridacna maxima is a species of giant clam found widely across the warm, coral-rimmed seas of the Indo-Pacific. These clams have a number of characteristics that make them well-suited as a living log of typhoons, tropical cyclones in the northwest Pacific. They can live for well over a century, they don’t move, and they accrete their thick shell relatively quickly, producing daily growth “rings” trees do. This means that brief events like a passing storm have a better chance of affecting the clam in a way that leaves a physical or chemical memento.

One place where powerful typhoons lumber through on the reg is Okinotori, a coral atoll south of Japan, whose rich reefs happen to be studded with giant clams. So, a team led by researchers at Hokkaido University in Japan took some of these clams to see if their shells could reliably retell the history of typhoons. They took size measurements of the shell growth increments, along with measurements of the ratio of certain oxygen and carbon isotopes, and the ratio of barium to calcium, on the youngest clam they collected. The idea here was that cold water upwelling from a typhoon makes the clam crank down its growth rate, and all the churning messes with the ocean’s normal chemical composition, both of which should get incorporated into the shell.

Since the rings accrue daily, and the researchers knew what day they pulled the live clam out of the water, they could count backwards and see if these chemical signatures lined up with recent records of typhoons over Okinotori.

And that’s exactly what they saw, according to the paper published recently in the Journal of Geophysical Research: Biogeosciences. Right around the time a typhoon had came by, shell growth basically stopped, and the barium and stable oxygen isotopes spiked. Though the particular clam the researchers focused on was young, the same day-scale resolution exists in much older, bigger individuals, potentially providing storm information over many decades.

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The researchers are hopeful exceptionally long-lived clams might increase the accuracy of spotty and incomplete records at the beginning of the 20th century, and that fossilized clam shells (which retain chemical markers of climate conditions in life) could reveal storm frequency much further back in time.

Enlarged image of the shell edge showing a stripe pattern of growth increments. Geochemical analysis of increments reveals the clam’s paleoenvironment.
Photo: Komagoe T. et al., Journal of Geophysical Research: Biogeosciences, April 19, 2018

It’s a result that doesn’t surprise Matthew Peros, an associate professor in Bishop’s University’s Department of Environment and Geography who was not involved in this study.

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“People have experimented with using [stable isotope ratios] from speleothems [stalagmites, stalagtites, etc.], microfossils, tree rings, and other archives as a tropical cyclone proxy before, so if anything I would say that I am surprised that people did not do this sooner,” Peros told Earther.

Amy Frappier, an associate professor of geosciences at Skidmore College who was also not involved in this study, told Earther that the study presents a creative addition to the “toolbox” of paleotempestology—the study of past tropical cyclones using historical records and geological proxies. “As this is a young and growing research area, I am delighted to see new studies like this one showing new techniques,” she said.

The whole Tridacna maxima valve. The shell was cut in two sections along the maximum growth axis.
Photo: Komagoe T. et al., Journal of Geophysical Research: Biogeosciences, April 19, 2018

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But Hali Kilbourne, a research associate professor at the University of Maryland Center for Environmental Science who wasn’t involved in the study, was less convinced of the predictive capacity of the team’s results.

“If I were to look at the data and try to pick out the storms, I’m not 100% sure I could do so if I didn’t already know when the storms existed,” Kilbourne told Earther. “I think that’s a problem for actually using something like this. For me that means that we’re just not there yet, there’s more work to be done.”

If giant clam shells can be used as a record of past storms, they would be the newest addition to a growing set of tools paleotempestologists use to peer into the past—from deposits of tiny crustacean shells that reveal past hurricanes to tree rings and old shipwrecks that register hurricane data. Being able to see the ghosts of these storms scarred into living things is awesome, but the data they provide is most valuable for helping us predict cyclone patterns today and tomorrow.

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“Paleotempestology can, relatively quickly, detect hundreds of ancient storms in the past, extending our understanding of storm patterns and risk before the historical records were available,” Frappier said. She added that because these records often dip back into periods when the climate was different, the data can help scientists test theories and model predictions about typhoon-climate interactions, and how things will change as the planet warms up.

As for the use of giant clams as a typhoon tally repository, the results need to be replicated, and more work is certainly needed to refine the method. But these silent, shelled sentinels of the reef seem like they have quite the story to tell.

Jake Buehler is a science writer living on Washington’s Olympic Peninsula with an adoration for the Tree of Life’s weird, wild, and unsung—follow him on Twitter or at his blog.