Bivalves, the Twin-Shelled Mollusks

These spiny pink scallops belong to one of about 9,200 living species of bivalves.
These spiny pink scallops belong to one of about 9,200 living species of bivalves. Photo © Jeff Rotman / Getty Images.

Bivalves are a group of mollusks that includes clams, scallops, oysters, mussels, razor shells, cockles, venus shells, borers, trough shells and many others (some of which live in the deep sea and have yet to be identified). Bivalves are the second most diverse group of mollusks, ranking only behind gastropods in number of species.

Bivalves are so named for their paired shells. The shells of a bivalve consists of two halves, mirror images of one another, that are joined at one edge by a flexible hinge.

Each half is asymmetrical and rounded, so that when it's closed against its opposite number, this forms a domed space near the hinged edge of the shell that accommodates the bulk of the bivalve's body and narrows towards the edge of the shell that opens. (Bear in mind that although most bivalves have paired shells, a few species either have drastically reduced shells or no shells at all.)

Bivalves live in marine and freshwater habitats; the most diverse, consisting of 80 percent of all species, live in ocean habitats. These invertebrates have four different lifestyles: epifaunal, infaunal, boring and free-moving. Epifaunal bivalves attach themselves to hard surfaces and remain in the same spot for their entire lives. Epifaunal bivalves, such as oysters, adhere to surfaces using either cementation or byssal threads (sticky chitinous threads secreted by a gland in the foot). Infaunal bivalves bury themselves in sand or sediment on the seafloor or in riverbeds; they have thin, soft shells armed with hard tips, and they bore into solid surfaces such as wood or rock.

Free-moving bivalves, such as scallops, use their muscular single feet to dig into sand and soft sediments; they can also move through the water by opening and closing their valves.

Most bivalves have a pair of large gills located in their mantle cavity. These gills enable the bivalves both to extract oxygen from the water (in order to breathe) and to capture food; water rich in oxygen and microorganisms is drawn into the mantle cavity and washes through the gills.

In species that burrow, a long siphon extends to the surface to take in water; mucus on the gills helps capture food and cilia transfer the food particles to the mouth.  

Bivalves have mouths, hearts, intestine, gills, stomachs and siphons, but do not have heads, radulae or jaws. These mollusks possess abductor muscles that, when contracted, hold the two halves of their shells closed. Bivalves are also equipped with a muscular foot, which in many species, such as clams, is used to anchor their bodies to the substrate or to dig down into the sand.

The bivalve fossils date back to the Early Cambrian period. During the ensuing Ordovician, bivalves diversified in terms of both number of species and the variety of ecological niches occupied.

Species Diversity

Approximately 9,200 species

Classification

Bivalves are classified within the following taxonomic hierarchy:

Animals > Invertebrates > Mollusks > Bivalves

Bivalves are divided into the following taxonomic groups:

  • Protobranchia
  • Pteriomorpha - This group includes animals such as scallops, oysters, pearl oysters, mussels, arcs and various other families
  • Anomalodesmata
  • Rostroconchia
  • Heterodonta
  • Palaeoheterodonta

Edited on February 10, 2017 by Bob Strauss