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Bivalves are easily recognized by their two-halved shell. They can burrow into the sediment or live on the ocean floor. Some can even move around through the water by snapping their shell open and shut to swim. Not all bivalves still have a shell though; some have evolved a reduced shell or have completely lost the shell.
Throughout history, bivalves have been one of the most important marine animals to humans. They have been used for food, jewelry, decoration, even money.
Two extinct Cambrian bivalved groups include the Stenothecoida and the Siphonoconcha (Parkhaev 1998) these may be nested within the Mollusca, but the absence of bilateral symmetry, enigmatic hinge structures, and shell composition places them outside of the Bivalvia as currently diagnosed.
One of the most interesting groups of bivalves, the rudists, went extinct during the Cretaceous-Tertiary mass extinction. These bivalves could be very large and often acted as the major substrate in Mesozoic reefs. Read more about rudists.
Life history & ecology
Many bivalves use their large foot to bury into the sediment on the ocean floor. They then extend a long siphon up to the surface to suck water in for filtering and breathing. You can often see these holes and the excurrent holes in mud flats at low tide. In many bivalve larvae or juveniles, a special gland, the byssal gland, can produce organic threads used for temporary attachment. In some groups, such as mussels, byssal threads permanently anchor the adults. A few groups of bivalves, such as oysters, are cemented permanently to the substrate.
Bivalves are hermaphroditic or have separate sexes. The eggs are typically small and not very yolk-rich. Fertilization is usually external, but in brooding species occurs in the mantle pallial cavity. For external fertilizers where the eggs and sperm are released into the water, the embryos often develop in the water column and go through both trochophore and veliger larval stages. Although morphologically similar to the gastropod veliger stage, phylogenetic analyses suggest that these larval stages are convergent and not homologous. Many other bivalves retain their eggs in the pallial cavity and suck in sperm with the inhalant water current. In some brooding bivalves the larvae develop in special pouches in the gills, or simply lie in the pallial cavity in others. Many brooding bivalves release their young as swimming veliger larvae while others retain them longer and release them as juveniles. Freshwater mussels (Unionoidea) have glochidial larvae that attach to fish as ectoparasites.
More on morphology
The bivalve shell consists of two valves that are hinged dorsally, usually with shelly interlocking teeth (the hinge), and always with a horny ligament that connects the two valves along their dorsal surfaces and acts to force the valves apart. The interior of the valves contain scars of the various muscles attached to it, in particular the (usually two, sometimes one) adductor muscles that, on contraction, close the valves. The shell can also be internal, reduced or even absent as in shipworms (Teredo, Bankia).
Bivalves typically display bilateral symmetry both in shell and anatomy, but there are significant departures from this theme in taxa such as scallops and oysters.
Bivalves lack a head, radula, and jaws. Although the plesiomorphic feeding state for bivalves is probably deposit feeding utilizing long labial palps, the ctenidia provide an effective filter feeding mechanism in most taxa with numerous levels or grades of organization. In most bivalves, the mantle pallial cavity contains a pair of very large gills that are used to capture food particles suspended in the inhalant water current.
The visceral mass is primarily situated above the pallial cavity and continues ventrally into the foot. The intestine is irregularly looped and opens dorsally into the exhalant area. Also opening into this region are the paired kidneys and, when separate from the kidneys, the gonopores of the paired gonads. The heart typically lies below the center of the valves and consists of two auricles and a single ventricle that supplies both anterior and posterior aorta. The nervous system is made up of three pairs of ganglia. These innervate the musculature, mantle, viscera, ctenidia, and siphons.
The bivalve foot is modified as a powerful digging tool in many groups, while in those that live a permanently attached life (e.g., oysters), it is very reduced.
In most bivalves the two halves of the mantle are fused around the edges, with openings anteriorly for the foot and posteriorly for the exhalant opening through which the water is expelled from the pallial cavity. This exhalant also carries waste products and gametes. The inhalant opening, through which water is carried into the pallial cavity, is also posteriorly located in most bivalves, lying just below the exhalant opening. The mantle edge is also where contact is made with the external world and is, consequently where any sense organs are located. These are usually simple sensory cells but in some there are pallial eyes and/or sensory tentacles.
All protobranchs are shallow burrowers. One group, the Solemyidae, farm symbiotic bacteria in their gills for food and have a very reduced gut. There are about ten, mostly small-sized families in all. Many are found only in deep water.
Many pteriomorphs live as epifaunal animals being free-living, byssally attached, or cemented and have a reduced foot. The mussels seen on rocky shores in great numbers are an example of this type of lifestyle. Other pteriomorphs are shallow burrowers.
Scallops are interesting, as some species have primitive "eyes" which allow them to sense light variation. They often combine this with the ability to quickly flee from predators using jets of water squirted from between their shells.
The shells of heterodonts have a complex hinge comprised of relatively small numbers of different types of teeth and the shell is never nacreous. A few of their members live in fresh water (notably Corbiculidae and Sphaeriidae). The lucinids farm symbiotic bacteria in their gills which provide most of their food requirements.
Other important members of Heterodonta are the extinct rudist bivalves. These bivalves, once very important components of reef ecosystems, went extinct at the end of the Cretaceous period. Also included in the Heterodonta are the invasive zebra mussels of the superfamily Dreissenoidea.
Original text by Paul Bunje, UCMP. Bankia setacea and Chlamys sp. by E. Eugenia Patten, © 2001 California Academy of Sciences; all other photos courtesy of www.jaxshells.org, with Nucula proxima by Bill Frank and Xylophaga atlantica by Joel Wooster.
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