By Jonny Lupsha, Wondrium Staff Writer
Clams and oysters are bivalves that filter water through their bodies. Clams even have a small foot-like muscle that helps them burrow into soft sand. One clam may pay off big for a New Jersey couple.
Usually, the only way someone gets awarded thousands of dollars after eating clams is if they win a food-poisoning lawsuit, but one New Jersey man stumbled upon a better way. While digging into his favorite appetizer, he found a pearl that would be worth several thousand dollars if he sold it. The fact that clams may contain pearls was almost as surprising to other restaurant patrons as it was for the man who discovered the pearl in his appetizer.
Both clams and oysters belong to a group of animals found in the ocean called bivalves. In his video series Zoology: Understanding the Animal World, Dr. Donald E. Moore III, director of the Oregon Zoo and senior science advisor at the Smithsonian’s National Zoo and Conservation Biology Institute, discusses the fascinating world of bivalves.
The World Is Your Clam
Bivalves include 20,000 species of clams, oysters, mussels, and scallops.
“The bivalves have so much morphological diversity that it is most useful to define them by the features that unite them,” Dr. Moore said. “All bivalves have a hinged outer shell, also called a ‘valve’; and a mantle, and most have a foot or threads used for burrowing or anchoring the creature into the substrate—oysters do not have a foot in adulthood.”
According to Dr. Moore, the mantle is essentially an envelope formed by the dorsal body wall. It wraps itself around the soft parts of the body and secretes the creature’s shell, which is calciferous, from its outer surface. The shell protects the mantle, which houses respiratory organs that actually develop from the mantle tissue. Finally, digestive and reproductive products are emptied into the mantle cavity.
Bivalves breathe through gills in their mantles. In fact, their gills are so diverse that their morphology is very helpful in classifying bivalves taxonomically.
What Would They Have on Their Bibs?
Many bivalves may be delicious—from clam chowder to oysters Rockefeller—but how do they eat?
“Most bivalves are sedentary filter feeders that use microscopic, hair-like structures called cilia within the inner part of their mantle to develop currents,” Dr. Moore said. “The currents move oxygen along gill filaments; blood flow is counter to this current, creating a counter-current exchange system for oxygen diffusing into the blood. And these gills serve a dual purpose; not only do they absorb oxygen from the water through gill membranes, but they [also] strain food particles out of the water current created by the respiratory action of these ciliary movements.”
As bivalves strain said food particles out of the water current, they separate them from indigestible food particles at the same time. Gland cells on their gills secrete large amounts of mucous that envelopes food particles. These small bits of food slide down “food grooves” into its simple mouth and then into the stomach. Indigestible particles, on the other hand, are released and swept away down in the water.
“During feeding, one oyster can filter over one gallon of water per hour, which it does by drawing water over its gills through the ciliary action,” Dr. Moore said. “Scientists estimate that Chesapeake Bay oysters historically filtered the entire Bay’s 15 trillion gallons of water every three to four days.”
Pearls may be the most valuable commodity to come from bivalves.