Oct. 14, 2004 — Researchers have found fossil clams from Antarctica that lived for more than 120 years and defy conventional wisdom about longevity in nature.

Cold climates, coupled with short growing seasons, are usually called on to explain how some critters live for decades or centuries. But the Antarctic waters where the clam Cucullaea raea lived some 45 million years ago weren't any colder than those off of North Carolina, said Syracuse University geologist Linda Ivany.

"When I first saw these things, I didn't think (their growth bands) could be annual," said Ivany about the fossil clam shells.

But one of her then-undergraduate students, Devin Buick (now at the University of Cincinnati), expressed an interest in checking it out to be sure. Buick and Ivany published what they discovered in the October issue of the journal Geology.

Using a specially mounted dental drill and camera to extract samples precisely from light and dark-colored individual growth bands, Buick and Ivany were able to measure oxygen and carbon isotopes changes as the clam grew. What the isotopes revealed were regular cycles of warming and cooling — seasons.

That means not only are the bands indeed annual, like tree rings, said Ivany, but they also reveal that the clams stopped growing every year at an unlikely time: in summer.

"It's really, really strange that they stop growing in the summer," said Ivany.

What she expected was for the clams to slow down and stop growing in winter, when the water was colder, sunlight dimmer and food scarcer.

Ivany speculated that the clams might have diverted their energy from growing shells and into reproduction in the summer.

"We really don't know the answer to this," she said.

As for why the clams attained such an old age, it turns out that cold is also not a factor for some long-lived modern mollusks, said biologist Teresa Newton of the U.S. Geological Survey. Newton has worked on long-lived mussels, clams and other mollusks in such temperate places as the lower Mississippi River and Ireland.

"I'm not convinced it's a cold thing," said Newton. "One of the evolutionary advantages (of a long life) is just reproductive length."

In other words, nature might have favored long-lived clams because they had a better chance of reproducing and continuing the species if they lived to see many mating seasons. Call it evolution's "Try, try again" strategy.

Another longevity possibility for the Antarctic clams is that they starved a lot of the time, said Ivany. Caloric restriction has been found to extend the lives of many animals, and Ivany isn't ruling it out.

"It's possible," she said.