Marine and freshwater mollusks that possess hinged shells, the Bivalvia class includes scallops, clams, mussels, oysters, and many other creatures that can be found in abundance all over the world. There are more than 9,150 known bivalve species. Let’s take a closer look at some of the more commonly seen varieties.
Bivalve Basics
Identifying bivalves is simple, since all have shells that consist of a pair of asymmetrically rounded halves which are identical to one another. The shell halves themselves are called valves, while the flexible ligament that joins them is called the hinge. The smallest known bivalves are nut clams, properly known as nuculidae; these lovely little clams inhabit a variety of intertidal zones throughout the world. The largest bivalves known to exist today are the giant clams, Tridacna Gigas, which are also one of the planet’s most endangered bivalve species.
Bivalves are for the most part extremely sedentary creatures; while some shipworms do burrow, and while others are capable of movement and can swim, they have extremely simple nervous systems; in fact, they have no brains. Instead, they have ganglia and a few nerve fibers that control the foot, the esophagus, and the adductor muscles that act to open and close the hinge. Some have tentacle mechanoreceptors or chemical receptors, and scallops have complex eyes with retinas and lenses. All bivalves have light-sensitive cells that are capable of detecting shadows falling over their bodies.
Bivalves possess open circulatory systems that include a triple-chambered heart with two auricles that receive blood from the gills and a single ventricle that pumps unpigmented hemolymph into the aorta and through the rest of the body. Their gills allow them to absorb oxygen, and they also aid in the filter feeding process.
Almost all bivalves are filter feeders, opening their shells to a certain extent in order to allow current to enter and exit. Unwanted sediment is removed via cilia on the gills, and desirable food particles are transported via a steady mucosal stream that leads to their mouths. If you spend time studying bivalve taxonomy then you’ll find that gill structure is one of the primary ways in which these animals are classified into groups.
Bivalves’ bodies are surrounded by a thin membrane, or mantle, which is attached to their shells by tiny muscles. As they grow, their shells thicken and additional layers form at the open edge. Look closely at a bivalve shell, and you will be able to see the lines that show growth; much in the same way a tree’s rings show growth. They are capable of tucking their entire internal structure into their shells and closing themselves tightly, preventing all but the most determined predators, such as oystercatchers, sea stars, octopus, otters, and humans from opening them.
In addition to their shells, bivalves have some fantastic defenses. Razor clams can dig themselves into the sand, cockles are capable of leaping, and file clams and scallops are able to swim, creating a jet of water by clapping their shells together. Others produce noxious secretions, with fan shells being equipped with an acid producing organ that is unique among bivalves.
No matter where you dive, you’re quite certain to find bivalves, even in areas that are relatively devoid of other life. Use caution, particularly on shore dives; these animals have sharp shells that act as a first defense, which can cause nasty scrapes and irritation. Last, but not least, if you enjoy eating them, be sure to check local regulations and understand the risks that come with such phenomenon as red tides. Many species are threatened, including some types of scallops; choosing responsibly raised shellfish can help to bring wild populations back to stability and prevent consumer illness.