The respiratory system
In the primitive bivalves the paired gills are small and located posteriorly. The gills in all other bivalves (save septibranchs, which have lost their gills) are greatly enlarged and possess a huge surface area. While the gills are thought to serve a respiratory function, respiratory demands are low in these mostly inactive animals, and, since the body and mantle are both bathed in water, respiration probably takes place across these surfaces as well. Such a mechanism has been demonstrated for a few bivalves, most notably freshwater species that are exposed to occasional drought. In such species, drying induces slight shell gaping posteriorly, the mantle margins exposing themselves to air. For most intertidal bivalves (which are alternately exposed to wetting and drying), respiration all but ceases during the drying phase.
The vascular system
The heart, enclosed in a pericardium, comprises a medial ventricle with left and right auricles arising from it. Blood oxygenated within the ctenidia flows to the auricles and from there to the ventricle, where it is pumped into anterior and posterior aortas. The blood then enters hemocoelic spaces in the mantle and visceral mass and returns to the heart via the ctenidia or the kidneys. The blood serves both to transport oxygen and metabolic products to tissues deep within the body and as a hydrostatic skeleton (for example, in the extension of the foot during locomotion and siphons during feeding). There are amoeboid corpuscles, but, except in a few bivalves, no hemoglobin or other respiratory pigment occurs.
The reproductive system
The reproductive system is simple and comprises paired gonads. These gonads discharge into the renal duct in primitive bivalves but open by separate gonopores into the suprabranchial chamber in more modern bivalves. Typically, the sexes are separate, but various grades of hermaphroditism are not uncommon. Eggs and sperm are shed into the sea for external fertilization in most bivalves, but inhalation of sperm by a female permits a type of internal fertilization and brooding of young, usually within the ctenidia.
Features of defense and aggression
The most significant adaptation is the earliest division of the shell into two valves within which the animal was wholly contained. Slow components of the adductor muscle permit sustained adduction, while the interlocking hinge teeth prevent shear. In addition, the shell may be strongly ridged, forming an interlocking shell margin, and it may be concentrically ringed with spines or sharp ridges projecting outward. Posterior sense organs, including photophores and eyes, are developed around the siphons and mantle margins. Detection leads to withdrawal deep into the sediment by burrowing species. In such animals the shell is smooth and compressed. Scallops respond to predation by swimming; shallow-burrowing cockles can leap using the foot. In the razor clams the siphons can break off (autotomize) when bitten, to be regenerated later. Similarly, noxious secretions are produced by the similarly autotomizing long tentacles of the Limidae (file shells). The unique pallial organ of fan shells (family Pinnidae) produces a secretion of sulfuric acid when bitten.
Only the deepwater subclass Anomalodesmata (families Verticordiidae, Poromyidae, and Cuspidariidae) and the scallops are predators. Prey is captured either in the sudden rush of water into the mantle cavity or by the rapid eversion of the inhalant siphon.
Evolution and paleontology
The oldest known bivalves are generally believed to be Fordilla troyensis, which is best preserved in the lower Cambrian rocks of New York (about 510 million years old), and Pojetaia runnegari from the Cambrian rocks of Australia. Fordilla is perhaps ancestral to the pteriomorph order Mytiloida, Pojetaia to the Palaeotazodonta order Nuculoida.
By the Ordovician Period (488.3 million to 443.7 million years ago) most modern subclasses were represented by definable ancestors. The oldest Ordovician bivalves are, however, the subclass Palaeotaxodonta, which are thought to have given rise to the Cryptodonta by elongation. Modern assessment of their shell structure and body form, notably with the possession of posterior protobranch gills and with palp proboscides for deposit feeding in the Palaeotaxodonta, generally supports this view. An extinct subclass Actinodontia also arose in the Ordovician Period and may be represented today by the superfamily Trigonioidea (placed in the subclass Palaeoheterodonta), which are an aberrant group of the subclass Pteriomorphia. The remaining, more typical, members of the Pteriomorphia also arose at this time and persist today, still characteristically occupying a range of substrate types but with byssal attachment and a trend toward loss of the anterior adductor muscle. The common mussels (family Mytilidae) are thought to be derived from an extinct group, the family Modiomorphidae. The subclass Orthonotia also arose in the Ordovician Period and are the probable ancestors of the deep-burrowing razor shells (Solenoidea). The origins of the subclass Anomalodesmata are less clear, but they too arose in the Ordovician Period and may have links to the order Myoida, which presently includes deep-burrowing forms and borers. Representatives of the superfamily Lucinoidea are very different from all other bivalves, with an exhalant siphon only and an anterior inhalant stream. Some of these deposit feeders also possess, like the subclass Cryptodonta, sulfur-oxidizing bacteria in the ctenidia and are thought to have ancient origins, represented by the fossil Babinka. Babinka is itself interesting and is closely related either to Fordilla, one of the oldest bivalves, or to the ancestors of the molluscan class Tryblidia. Today the superfamily Lucinoidea is generally placed within the subclass Heterodonta, which is a younger group that traces back to the Paleozoic Era, when the first radiation of all bivalves took place.
The stamp of modernity was placed upon the Bivalvia in the Mesozoic Era (251 million to 65.5 million years ago), when virtually all families currently recognized were present. Throughout time, the fortunes of the subclasses have waxed and waned, with repeated modification of form allowing repeated diversification into different habitats. Similarity of habitat is matched by similarity in structure and form, allowing for various interpretations of the fossil record. It is clear, however, that most modern bivalves can trace their ancestry back a long way and that the inherent plasticity of the bivalve form is responsible for the success of a molluscan experiment in lateral compression of the shell.