We begin our study of bivalves where most science begins — by marveling at their beauty and amazing variety! We wonder, how was such diversity achieved, and how is it maintained?
Bivalves do everything but fly. They are the second largest class of mollusks after gastropods, the group that includes snails and slugs. Nearly 20,000 living bivalves have been identified.
Bivalves are missing a feature that most other animals have — a head! Despite this, most bivalves have a mouth, some have lips, and a few even have eyes (like Sammy Scallop). Most have two shelly valves and a large pair of gills. But beyond these similarities, bivalves can be very different from each other — a result of each species’ unique evolutionary history.
Smallest & Largest
The smallest living bivalve is one of the Nut Clams, Condylonucula maya. An adult Condylonucula, measures only 500 microns — less than 2 one-hundredths of an inch — about half a millimeter. The largest living bivalve is the Giant Clam, Tridacna gigas. This gentle giant can grow almost 5 feet in length (up to 1½ meters).
Hottest and Coldest
Bivalves inhabit most of the aquatic environments on Earth, including many that experience temperature extremes. The hydrothermal vents along the margins of the Earth’s tectonic plates harbor several bivalve species in temperatures ranging from 100 to 400 degrees Celsius (200-700 degrees Fahrenheit). At the other extreme, Antarctica — the coldest place on Earth — is home to many species of bivalves, in waters of -2 to 1 degree Celsius (28-34 degrees Fahrenheit).
Shallowest and Deepest
Although bivalves have never evolved air breathing and therefore cannot live on land (like snails and insects have done), one species actually lives above the water line — the limpet-like Saddle Oyster, Enigmonia aenigmatica, on Australian seawalls or the underside of mangrove leaves, moistened only by the spray of the surf. One of the deepest living bivalves is the Hydrothermal Vent Clam, Calyptogena magnifica, which survives by metabolizing sulfur (instead of oxygen) in great beds on the East Pacific Rise, 1½ miles beneath the surface of the ocean!
Youngest and Oldest
We know very little about normal life spans of bivalves in the wild. Those we know best are raised in aquaculture — species like the Blue Mussel. It takes only a year for a Blue Mussel to reach marketable size — over 2 inches long. We can also estimate the ages of bivalves by counting growth bands in sectioned shells — not as accurate as annual tree rings, but close. A single specimen of the Ocean Quahog, Arctica islandica, dredged from off of Iceland in 2006, was calculated to be over 400 years old using this method. We can use this method on fossils too. Cucullaea raea is a 50-million-year-old ark from Antarctica — its annual shell bands often total more than 100 years!