When a burrowing giant clam is very young, only about half a millimeter long, it picks out the home it will have for the rest of its life. It attaches itself to the rock of a reef, and as time passes, it grows larger (although as the smallest of giant clams, it reaches only 10 to 15 centimeters in length). Simultaneously, it sinks into the rock at an imperceptibly slow rate.
When a scuba diver swims by an adult clam on one of the Pacific reefs where they live, all she will see is what looks like a protruding pair of beautiful turquoise lips. These are the clam’s feeding tissues, basking in the sun and filtering food from the water — the rest of its body is safely encased in a cave of its own making.
How exactly a clam could do that has long been a mystery. But in a new study in Biology Letters, researchers revealed at long last a probable tool: The clam’s foot releases acid.
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That’s illuminating because reefs are built by tiny coral organisms that create their own skeletons out of calcium carbonate. When they die, they leave behind their hard shells, and their descendants, as well as many other creatures like the clams, make their homes in and on the resulting hummocks. Mix calcium carbonate and acid, however, and the molecules of the rock dissolve, in the same reaction you would see if you dropped an Alka-Seltzer into Coca-Cola, which is weakly acidic.
Decades ago, researchers had suspected that the clams used acid, as well as perhaps sanding down the stone using their rough shells. But when they released pH-sensitive fluid into the seawater around clams, there was no change, even very close to the foot, the meaty tissue that emerges from the base of the shell and presses against the rock. Baffled, they turned their attention elsewhere.
Richard Hill, a biologist at Michigan State University who is an author on the new paper, and his co-authors wondered whether looking for acid in the seawater wasn’t the wrong way to go about it — the acid might only be found on the flat surface where the foot was pressing.
“So we looked for a 2-D way to detect acidity,” said Dr. Hill.
Soon, the team had nine burrowing giant clams resting comfortably on flat, color-changing pH-sensitive foil in lab tanks. When they used a camera to photograph the foil from the bottom, the evidence was clear: The shape of the clams’ feet was visible in light red, indicating that they had created an acidic environment at least two pH levels stronger than that of the seawater. Further testing showed that the feet contained a molecular pump that moves around hydrogen ions, or protons, to make acid.
The reason earlier tests did not pick up on acid around the clams might come down to the rate at which they can make it, suggests Dr. Hill. When the clams are pressing their feet against a surface, even a fairly small number of protons will be concentrated together, leading to a substantial drop in pH at that spot. A large volume of water would pose a much bigger challenge.
“My guess is the reason they don’t acidify seawater is they simply aren’t making protons fast enough to do that,” said Dr. Hill.
Earlier reporting on shellfish
The research not only answers an old question about how these clams burrow, but also implies ways in which they contribute to the reef ecosystem. By dissolving old coral, the clams release the calcium carbonate back into the water for living coral to use.
“They are big time players in these particular reefs,” said Dr. Hill, referencing a study in which researchers found more than 100 of these clams, ranging from very young to fully grown, peacefully coexisting in a single square meter of the Great Barrier Reef. “They are as important as the corals are, in terms of their biomass and their roles.”