Burrowing giant clams have perfected the ship-in-a-bottle trick. Scientists had convinced themselves they couldn’t explain how the clams carved out a big home within a tiny opening. New research finally shows how the mollusks do it. In the process, that research highlights a flaw in tests that had aimed to answer this question decades ago.
Tridacna crocea (Try-DAK-nuh KROH-see-uh) is the smallest of some 10 or so giant clam species. It grows a shell that eventually reaches roughly the size of a large fist. But you would likely never get a chance to see that shell. Starting as youngsters, the burrowers bore into the stony mass of an Indo-Pacific coral reef. As the clams grow, they trap themselves for life behind a too-skinny exit. There, these clams can thrive for decades.
Only the brightly colored upper edges of the clam’s body can push out of the thin slit in the reef. The frills sticking out teem with algae related to those in corals. Basking in sunlight, the algae pay “rent” by providing a great share of the giant clam’s energy.
The clams “actually have eyes in this tissue,” Richard Hill says of the frills. Hill is an environmental physiologist at Michigan State University in East Lansing. At the slightest shadow — a predator, perhaps — the clam yanks in its vulnerable parts through the very narrow crack. “It’s as if the clam vanished,” he says.
The colors the clam creates on its extendable frills approach the psychedelic. And they can bewitch aquarium hobbyists into paying hundreds of dollars to own one. “The ones that get the big money are turquoise,” Hill says. He’s also seen some that are a deep indigo blue, yellow, crimson — even a boring brown.
The opposite rim of the clam body is hidden deep within the coral crevice. It looks anything but colorful. Through a hole near the shell’s hinge, a soft cream-colored mass called (quite unfairly) the “boring organ” can slide out. 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 away, giving it room to grow.
Sir Charles Maurice Yonge was a leading 20th-century mollusk biologist. He had proposed that boring organs might gradually dissolve the rock by secreting acid. Later, he shot down his own hypothesis (and fretted about it throughout the rest of his life). He had tested the idea by adding a color-changing pH indicator to the clam’s seawater. This showed no sign of a surge in acidity.
Hill decided to revisit the mystery. After some epic bouts of Googling options for testing the pH in a liquid, he wondered if what the clam directly contacted was what mattered. To investigate, he found a flat device that could measure pH at lots of points on a piece of foil. He and his colleagues then placed clams beside the device — and hoped the boring organs would push out against this device.
The project required a trip to Japan. It also needed world-class mollusk management to coax clams to reach out their organs and touch the foil in the desired way. When they did, red blotches emerged. They looked like lipstick smudges on a napkin, and pointed to spots of acidity.
The clams may not be able to bring down the pH of a lot of alkaline seawater. They can, however, acidify a surface. That acid can dissolve rock to carve out a hidden cave. Hill and his colleagues shared their discovery June 13 in Biology Letters. Turns out that this organ is boring in the very best sense of the word.
algae Single-celled organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.
alkaline An adjective that describes a chemical that produces hydroxide ions (OH-) in a solution. These solutions are also referred to as basic — as in the opposite of acidic — and have a pH above 7.
biology The study of living things. The scientists who study them are known as biologists.
colleague Someone who works with another; a co-worker or team member.
coral Marine animals that often produce a hard and stony exoskeleton and tend to live on reefs (the exoskeletons of dead ancestor corals).
dissolve To turn a solid into a liquid and disperse it into that starting liquid. (For instance, sugar or salt crystals, which are solids, will dissolve into water. Now the crystals are gone and the solution is a fully dispersed mix of the liquid form of the sugar or salt in water.)
erode Gradual removal of soil or stone, caused by the flow of water or the movement of winds.
hypothesis (v. hypothesize) A proposed explanation for a phenomenon. In science, a hypothesis is an idea that must be rigorously tested before it is accepted or rejected.
indigo A deep blue dye made from Indigofera, a plant belonging to the pea family. One of this dye’s best known contemporary uses: tinting the denim used to make blue jeans.
Indo-Pacific A merging of Indian and Pacific that refers to the tropical Indian Ocean and adjoining western and central parts of the Pacific Ocean.
organ (in biology) Various parts of an organism that perform one or more particular functions. For instance, an ovary is an organ that makes eggs, the brain is an organ that makes sense of nerve signals and a plant’s roots are organs that take in nutrients and moisture.
Pacific The largest of the world’s five oceans. It separates Asia and Australia to the west from North and South America to the east.
pH A measure of a solution’s acidity or alkalinity. A pH of 7 is perfectly neutral. Acids have a pH lower than 7; the farther from 7, the stronger the acid. Alkaline solutions, called bases, have a pH higher than 7; again, the farther above 7, the stronger the base.
physiologist A scientist who studies the branch of biology that deals with how the bodies of healthy organisms function under normal circumstances.
predator (adjective: predatory) A creature that preys on other animals for most or all of its food.
psychedelic An adjective (especially popular in the 1960s) that refers to the abnormal mental experiences (such as hallucinations) brought on by use of certain drugs (such as LSD), and sometimes described as swirling, kaleidoscope-like, color patterns.
reef A ridge of rock, coral or sand. It rises up from the seafloor and may come to just above or just under the water’s surface.
seawater The salty water found in oceans.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
tissue Made of cells, it is any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.
Journal: R.W. Hill et al. Acid secretion by the boring organ of the burrowing giant clam, Tridacna crocea. Biology Letters. Vol. 14, June 13, 2018, 20180047. doi: 10.1098/rsbl.2018.0047.
Journal: C.M. Yonge. Mode of life, feeding, digestion and symbiosis with zooxanthellae in the Tridacnidae. Great Barrier Reef Expedition 1928–29. Scientific Reports. Vol. 1. Published February 2, 1922, p. 283. London, The British Museum (Natural History.)