Bivalvia
| Bivalves | |
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| "Acephala" from Ernst Haeckel's Artforms of Nature, 1904 | |
| Scientific classification | |
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| Class: | Bivalvia Linnaeus, 1758
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Subclass Protobranchia Subclass Pteriomorphia - oysters, mussels Subclass Paleoheterodonta
Subclass Heterodonta - clams, zebra mussels Subclass Anomalosdesmata | |
Bivalves are molluscs belonging to the class Bivalvia. They typically have two-part shells, with both parts being more or less symmetrical. 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.
Bivalves lack a radula and feed by siphoning and filtering small particles from water. Some bivalves are epifaunal: that is, they attach themselves to surfaces in the water, by means of a byssus. Others are infaunal: they bury themselves in sand or other sediments. These forms typically have a strong digging foot. Some bivalves can swim.
Anatomy
Bivalves are filter feeding mollusks that feed using their gills. They have an open circulatory system that covers the organs with blood. Neparidia remove the excreted waste.
Palaeontology
The oldest known fossil bivalves are 500 million years old. Some are now extinct, such as the Jurassic Gryphaea, also known as "devil's toenails".
Examples of fossil freshwater bivalves are the large mussel Archanodon, found in the Old Red Sandstone formation of the Devonian Period in the United Kingdom. The non-marine bands of Carboniferous Coal Measures contain 'mussel bands' which are crowded with specimens such as Carbonicola and Naiadites. These freshwater bivalves prove useful as zone or index fossils for subdividing the strata of the Coal Measures.
Most fossil bivalves are relatively small, typically being a few centimetres (about an inch) long. However, a few giant forms are known, such as Inoceramus from the Cretaceous-period Chalk Formation of Europe, which grew to well over a metre (3 feet) in length.
The shells of bivalves are composed of calcium carbonate and are quite strong in most species, and hence easily become fossilized. However, in some bivalves the shell is made up of a form of calcium carbonate known as aragonite, which easily dissolves away when the fossil is entombed in sediment, leaving only an impression of the shells' internal and external surfaces (known as internal and external moulds).
Remarkable bivalve fossils have been found in which the original colour banding of the shell has been preserved, such as in specimens of Aviculopecten planoradiatus from the Carboniferous Period of Derbyshire in England.
Bivalves seem to have evolved at a relatively slow pace, especially when compared to some of the other molluscs - such as the ammonites.
The bivalves which appeared in the early Ordovician Period were marine, and like many other groups of molluscs they radiated out to fill many of their niches (which they still occupy today) by the end of the Ordovician. Bivalves begin to become reasonably common fossils in the rocks of the Silurian and Devonian Periods. The shells of bivalves in rocks older than those of the Carboniferous are not generally preserved; these fossils largely consist of mere impressions in the rock. Freshwater species first appear at the boundary between the Silurian and Devonian Periods. All of the major groups of bivalves were in existence by the end of the Palaeozoic Era.
Bivalves are much more numerous and varied in rocks from the Mesozoic Era, and they may occur in great numbers at some localities. Many of the ancient bivalve forms became extinct during the Triassic Period and were replaced by many new forms, some of which have persisted to the present day.
Like the gastropods, bivalves are very abundant fossils in rocks of the Cenozoic Era. Many of these bivalves are related to living forms so it is possible to state with some confidence the conditions in which the rocks they are found were deposited, such as shallow marine water, brackish or freshwater, under sub-tropical or a cold climate, etc.
Some living forms of bivalve have a very long ancestry, such as the modern Nucula, which has relatives in rocks from the Ordovician that are not very different from those found living today.
History
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 ~95% of all species) were devastated.
This raises two questions: how did the bivalves come to challenge the brachiopoda niche before the extinction event, and how did the bivalves escape the fate of extinction? Although inevitable biases exist in the fossil record and our documentation thereof, bivalves essentially appear to be better adapted to aquatic life. Far more sophisticated than the brachiopods, bivalves use an energetically-efficient ligament-muscle system for opening valves, and thus require less food to subsist. Furthermore, their ability to burrow allows for evasion of predators: buried bivalves feed by extending a siphon to the surface (indicated by the presence of a palial sinus, the size of which is proportional to the burrowing depth, and represented by their dentition). Additionally, bivalves became mobile: some developed spines for buoyancy, while others suck in and eject water to enable propulsion. This allowed bivalves to themselves become predators.
With such a wide range of adaptations it is unsurprising that the shapes of bivalve shells vary greatly - some are rounded and globular, others are flattened and plate-like, while still others, such as the razor shell Enis, have become greatly elongated in order to aid burrowing.