Bivalvia

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Also known as pelecypods, the approximately 15,000 species of this taxon are found in marine and freshwater habitats throughout the world.

A bivalve is characterized by possessing two shells secreted by a mantle that extends in a sheet on either side of the body. The oldest part of the shell, the umbo, can be recognized as a large hump on the anterior end of the dorsal side of each shell. The two shells are joined at the dorsal end by a region called the ligament. The ligament is comprised of the tensilium and resilium. Together they open the shells at rest. A bivalve closes its shells by contracting its powerful adductor muscles. Commonly there are two, an anterior and a posterior one, but in some taxa (such as scallops) there is only a single, central one.

The body is laterally compressed. The only external structures are the labial palps; in some groups, there are sensory tentacles and photoreceptors at the edge of the mantle. Bivalves also possess two ctenida (in most cases) and a muscular foot. The edges of the mantle are fused in some taxa and prolonged to form tube-like siphons. One siphon carries water to the mantle cavity (the inhalent siphon) and one from it (the exhalent siphon); in many taxa they are fused but the water streams remain separate. A bivalve uses its muscular foot either to attach itself to a substrate or to burrow. Scallops propel themselves through the water by jet propulsion: rapid closing of the valves squirts water out of the mantle cavity, and the animal "swims" in the opposite direction.

Most bivalves are filter feeders, but some are scavengers or even predators. The four main feeding types of bivalves are defined by gill structure. In protobranchs, the ctenida are used only for respiration and food is caught by the labial palps. In filibranchs and lamellibranchs, the ctenida trap the food particles in their mucous coating and transfer the food to the labial palps via ciliary action. These two groups differ in that the branches of the ctenida are connected only by ciliary junctions in filibranchs whereas lamellibranchs have tissue connecting the branches of the ctenida. A septibranch bivalve has a septum across its mantle cavity, which functions to pump in food. Thus, Bivalvia is the only molluscan class characterized by the absence of a radula.

Most marine bivalves go through a trochophore stage before turning into a free-swimming veliger larva. This type of larva looks like a miniature bivalve with a row of cilia along the edge of the mantle. Freshwater species lack these stages. Instead, some go through a larval stage known as the glochidium. Rather than being free-swimming, a glochidium attaches to fish or other objects that will not be swept downstream. Glochidia can be serious pests of freshwater fish.

The evolutionary relationships of bivalve lineages are not currently well understood. There appear to be many parallel lines of evolution that obscure relationships between taxa. However, there is some consensus that Protobranchia (Palaeotaxodonta), Pteriomorpha, Heterodonata, and Anomalodesmata are legitimate taxonomic groups. Pteriomorphans, characterized by reduced or absent siphons, usually have filibranch ctenida. Heterodontans are typified by possessing large siphons and have lamellibranch gill structure. Lastly, anomalodesmatans are the septibranch bivalves.

Many bivalves (such as clams or oysters) are used as food in places all over the world. Pearl oysters are used for commercial production of pearls. Bivalves can also cause economic damage. The glochidia larvae of some freshwater mussels can be serious parasites of fish, and some marine bivalves bore through wood, causing damage to wooden ships, pilings, and other wood structures.

Bivalves dislodged from beneath a single rock in a coral lagoon in the South Pacific. Included are pen shells, pearl oysters and a young Tridacna. Note the bivalve with its inhalant (ventral) and exhalant (dorsal) siphons extended. Photograph: Jack Burch

Included is an anterior end view of a bivalve with its valves widely gaping, showing the mantle lining each valve, the foot, the two demibranchs on each side of the foot, and the labial palps. Photograph: Jack Burch


References:

Kolzoff, E. N. (1990) Invertebrates. Sauders College Publishing.

Purchon, R. D. (1968) The Biology of the Mollusca. Pergamon Press.

Tasch, P. (1973) Paleobiology of the Invertebrates: Data Retrieval from the Fossil Record. John Wiley & Sons, Inc.


Written by: Derek Kellogg and Daphne G. Fautin. Last updated: 26 January 2001

Glossary

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature