Some people can pick up dropped socks with their toes, but they can't stretch their feet all the way to the basement to get them.
Scientists at the Scripps Institution of Oceanography in San Diego are studying a clam that can. At least, it can extend its foot up to 30 times the length of its shell into the mud.
Researchers believe it's the most extreme case of animal structure elongation ever discovered.
The clams stretch their feet into the muck to gather food - not for themselves but to nourish bacteria that grow in their gills. Then they snack on the bacteria. "The benefit to the clam is it can get some food from the bacteria. I like to think of it as growing a little garden in the gills," said Suzanne Defour, a doctoral candidate in marine biology at the University of California, San Diego.
Defour and marine biologist Horst Felbeck are studying symbiosis - two species living together to their mutual benefit. Their study was reported in the Nov. 6 issue of the journal Nature.
They focused on a family of tiny clams called Thyasiridae, smaller than watermelon seeds, that live in marine sediments around the world. Some thyasirids live in symbiotic relationships with bacteria camped in their gills.
The clams don't slurp the bacteria into their digestive tract, as they do with most of their diet, Defour said. Instead, "the cells of the gills just engulf the bacteria and digest them within those cells."
The clams, in turn, provide the bacteria with a place to live and with sulfides that the bacteria oxidize, yielding the energy they need to live.
What wasn't clear was how the clams - which need oxygen to live - acquired the sulfides, which are produced by decomposition of organic matter in deeper sediment where there is little or no oxygen.
To learn more, Defour set up an experiment to compare the behavior of three thyasirid species that hold bacteria in their gills, and two that don't.
Scientists put the clams in thin aquariums that could be X-rayed to reveal the clams' activity. Some of the tanks contained sediments rich in sulfides; the others were sulfide-poor.
After three weeks, the X-rays revealed long, sinuous trails beneath some of the clams living in the sulfide-free environment. Each trail was a burrow dug into the mud by a clam's foot. One trail stretched almost 5 1/2 inches from the clamshell - 30 times the length of the shell.
The symbiotic clams in sulfide-poor mud extended and withdrew their feet as many as 11 times in three weeks. Those in sulfide-rich sediments did less burrowing. The species without symbiotic bacteria did none. Defour concluded that all the burrowing was probably "sulfide-mining" by the symbiotic clams. "If they're more desperate for sulfides, they had to be more active with their foot," she said.
The transport mechanism is unclear, but Defour believes that the sulfides are absorbed through the clam's thinly stretched foot tissues and carried in the blood to the gills.
The symbiotic relationship evolved long ago, Defour said. Fossil thyasirids - complete with preserved burrows 15 times their shell length - have been found in Miocene sediments more than 10 million years old.
Defour's clams appear to have set a new record for animal elongation. Some corals can stretch appendages 20 to 25 times their original length, she said, "but nothing like this."