Half shell. This cross section of a Cucullaea raea clam reveals dark growth bands in the shell linked to age.

Centenarian Clams Cut Calories

A long-lived clam that inhabited the waters of Antarctica 45 million years ago may provide further evidence for the virtues of a calorie-restricted diet. In a study published in the October issue of Geology, researchers suggest that a shortage of food helped the fist-sized clam Cucullaea raea survive for up to 120 years, making it one of the longest-lived animals known. The findings challenge the prevailing explanation for bivalve longevity, and they may provide clues about which factors extend life spans for all animals.

Studies over the past 20 years have shown that clams in the frigid waters of high latitudes live up to 10 times longer than their warm-water counterparts, leading researchers to believe that chilly conditions contribute to longevity by slowing metabolic rate. But scientists have been stymied at figuring out whether something other than temperature might be playing a role in the long lives of these high-latitude clams.

Paleontologist Linda Ivany of Syracuse University in New York and her student, Devin Buick, realized they had a unique opportunity to test these other factors when they examined clam fossils from marine sediment on an island off the coast of Antarctica. The sediment was laid down during the Eocene period, a time when the waters off Antarctica were about 10 degrees warmer than they are today. The team sliced the fossils in half to reveal a series of dark bands in the shell that, like the rings of a tree, record the organism's age. From this, Ivany and Buick calculated that many of these clams lived as long as 120 years, despite the warmer water. Ivany and Buck next measured the ratios of carbon and oxygen isotopes in the shells, which indicated that the clams grew in the winter, when food was scarce, and stopped growing in the summer, when food was plentiful.

"This is the opposite of what we'd expect," says Ivany, who believes that the clams stopped feeding in the summer so they could focus on reproduction. The limited winter diet may be the key to their longevity, she says, because studies have shown that calorie restriction slows aging in other animals, including humans (ScienceNOW, 20 April). Ivany also believes that natural selection may have played a role, as clams who lived longer would have had more opportunities to spawn.

"This is a very impressive study," says Karl Flessa, a paleobiologist at the University of Arizona in Tucson. He believes the work will not only help explain the factors that influence life span but also provide insight into how animals cope with extreme environments. "It's a cool way to see how animals adapt to their world," he says.

Related Sites
Linda Ivany's site
Information on calorie restriction