How a squishy clam conquers a rock

Burrowing giant clams have perfected the ship-in-a-bottle trick, and the one big thing that scientists convinced themselves couldn’t explain it, actually can.

Tridacna crocea, the smallest of the 10 or so giant clam species, grows a shell that eventually reaches the size of a large fist. Starting as youngsters, the burrowers bore into the stony mass of an Indo-Pacific coral reef, trapping themselves behind a too-skinny exit for their entire decades-long lives.

Only the extravagantly colored upper edges of the clam’s body can push out the thin slit in the reef. These protruding frills teem with algae related to those in corals. Basking in sunlight, the algae pay rent in the form of a substantial portion of a giant clam’s nourishment.

The clams “actually have eyes in this tissue,” says environmental physiologist Richard Hill of Michigan State University in East Lansing. At the slightest shadow — a predator, perhaps — the clam yanks in such vulnerable parts through the very narrow crack. “It’s as if the clam vanished,” he says.

The colors the clam creates on this extendable tissue approach the psychedelic and can bewitch aquarium hobbyists into paying three-figure sums. “The ones that get the big money are turquoise,” Hill says, but he’s seen indigo blue as well as yellow, crimson and even a boring brown.

The opposite rim of the clam body, hidden deep inside the coral crevice, looks anything but colorful.  Through a hole near the shell hinge, a soft cream-colored mass called (quite unfairly) the “boring organ,” emerges. Like pulling on a gym sock, this tissue spreads upward over the shell. A sock embrace puts the tissue in touch with the cave wall that the clam must erode as it grows.

A leading mollusk biologist of the 20th century, Sir Charles Maurice Yonge, proposed that boring organs gradually dissolve the rock by secreting acid. Yet he shot down his own theory (and fretted about it through the rest of his life) by adding a color-changing pH indicator to the clam’s seawater. There was no sign of a surge in acidity.

Hill decided to reexamine the mystery. After some epic bouts of Googling the options for testing pH in a liquid, he suddenly wondered if direct clam contact mattered. He found a device measuring pH at lots of points across a flat piece of foil that a clam, he hoped, could be persuaded to press its boring organ against.

The project required a trip to Japan plus world-class mollusk management to coax clams to touch organs to foil in the desired way. When they did, however, the detector revealed red blotches of acidity, like lipstick smudges on a napkin.

The clams may not be able to bring down the pH of a lot of alkaline seawater, but they can acidify a surface, Hill and his colleagues report June 13 in Biology Letters. Turns out that this organ is boring in the very best sense of the word.