Bivalves are molluscs belonging to the class Bivalvia. They have two-part shells, and typically both valves are symmetrical along the hinge line. The class has 30,000 species, including scallops, clams, oysters and mussels. Other names for the class include Bivalva, Pelecypoda, and Lamellibranchia.
Bivalves are exclusively aquatic; they include both marine and freshwater forms. However some, for instance the mussels, can survive out of water for short periods by closing their valves.
Bivalves are unique among the molluscs for lacking a radula; they feed by siphoning and filtering large particles from water. Some bivalves are epifaunal: that is, they attach themselves to surfaces in the water, by means of a byssus or organic cementation. Others are infaunal: they bury themselves in sand or other sediments; these forms typically have a strong digging foot. Some bivalves, such as scallops, can swim.
The systematic layout presented here is according to Newell's 1965 classification based on hinge teeth morphology. There exists no robust phylogeny, and due to the plethora of fossil lineages, DNA sequence data is only of limited use should the subclasses turn out to be paraphyletic. The monophyly of the Anomalodesmata is especially disputed, but this is of less consequence as that group does not include higher-level prehistoric taxa. It is, however, currently accepted that Anomalodesmata resides within the subclass Heterodonta.
Subclass Pteriomorphia (oysters, mussels, etc)
There also exists an alternative systematic scheme according to gill morphology (Franc 1960). This distinguishes between Protobranchia, Filibranchia, and Eulamellibranchia. The first corresponds to Newell's Palaeotaxodonta + Cryptodonta, the second to his Pteriomorphia, and the last contains all other groups. In addition, Franc separated the Septibranchia from his eulamellibranchs, but this would seem to make the latter paraphyletic.
The shapes of bivalve shells vary greatly - some are rounded and globular, others are flattened and plate-like, while still others, have become greatly elongated in order to aid burrowing. The shipworms of the family Teredinidae have greatly elongated bodies, but the shell valves are much reduced and restricted to the anterior end of the body, where they function as burrowing organs, allowing the animal to dig tunnels through wood.
Scallops have complex eyes with a lens and retina, but most other bivalves have much simpler eyes, if any. There are also light-sensitive cells in all bivalves, that can detect shadows falling on the animal.
In the septibranchs the inhalant siphon is surrounded by vibration-sensitive tentacles for detecting prey.
Statocysts within the organism help the bivalve to sense its orientation, which can then be corrected if need be.
The shell is composed of two calcareous valves, which are made of either calcite (as with, e.g. oysters) or both calcite and aragonite, usually with the aragonite forming an inner layer, as with the pterioida. The outermost layer is the periostracum, composed of a horny organic substance. This forms the familiar coloured layer on the shell. The shell is added to in two ways - at the open edge, and by a gradual thickening throughout the animal's life.
Typically the marine bivalve will start life as a trochophore, later becoming a veliger. Freshwater bivalves have a different life cycle: they become a glochidium, which attaches to any firm surface to avoid the danger of being swept downsteam. Glochidia can become serious pests of fish.
Bivalves appeared late in the Cambrian explosion and came to dominate over brachiopods during the Palaeozoic; indeed, by the end-Permian extinction, bivalves were undergoing a huge radiation in numbers while brachiopods (along with around 95% of all species) were devastated.
It had long been considered that bivalves are better adapted to aquatic life than the brachiopods were, causing brachiopods to be out-competed and relegated to minor niches in later fossil strata. In fact, these taxa even appeared in textbooks as an example of replacement by competition. Evidence adduced for this included use of an energetically-efficient ligament-muscle system for opening valves, requiring less food to subsist. Lately, however, this interpretation of the interaction between brachiopods and bivalves has been largely exploded. It seems instead that the reason for the prominence of bivalves over brachiopods has to do with chance disparities in their response to extinction events.
The impact of the "pull of the recent" on the history of marine diversity.(testing the statistical validity of biodiversity models)
May 16, 2003; The history of global marine biodiversity is controversial because the observed pattern is difficult to separate definitively...