Bivalves are soft-bodied invertebrates that are contained within the protective area between two valves or halves of a shell. In the Pacific Northwest these animals include scallops, clams, mussels, and oysters.
Where I live in Victoria, BC there is an abundance of swimming scallops in an area with good water current near the end of the Ogden Point Breakwater. Since bivalves are filter feeders, this current provides plenty of drifting phytoplankton which is gleaned from the water by the animal’s gills. Swimming scallops are very interesting as they will open and close their shells quickly pulling themselves up into the water, where they will then relax and float down to another safer location as a means of avoiding predators like sea stars.
The general anatomy of a bivalve is unusual as this animal is lacking some basic parts that other molluscs have such as a head! The shell of a bivalve is made of calcium carbonate, and the mantle secretes new shell material at the free edge enabling increased growth to each of the two valves over time. A hinge ligament joins the two halves of the shell together, and a large adductor muscle holds them closed. When the muscle relaxes the ligament then springs the shell open. A foot can extend beyond the bivalve’s shell and is often used in some species for moving the animal away from predators, or for assisting in burying into the soft ocean floor.
The anatomy of bivalves varies slightly between the different species. For example a Pacific geoduck is buried in sand for protection, but has a very long siphon that extends above the sand with two openings. One opening allows fresh ocean water to be drawn down to the filtering gills, after which the filtered water is expelled up and out the other opening. This specialized feeding adaptation is much different than that of the swimming scallop mentioned earlier.
I think it is astonishing that some bivalves have such long lifespans, like the Pacific geoduck which has been recorded living up to 168 years.