bivalves The phylum Mollusca is extremely diverse and contains a number of classes that at first sight can appear to be so different as to be unrelated. One of these is the bivalves (sometimes called pelecypods or lamellibranchs), an important group of bivalved molluscs familiar to all from the numerous shells that litter beaches. Bivalves are aquatic suspension-feeders, inhabiting a variety of infaunal and epifaunal habitats (i.e. living in and on the bottom) and are particularly characterized by their ability to burrow, some of them even into rock and wood. About 50 000 species have been described, of which about 10 000 are modern. It is their adaptability that has made them so successful today, although their geological history goes back into the Early Cambrian.
All bivalves have two shells or valves (Fig. 1a) that are usually virtual mirror images of each other about the commissure or edge along which the valves are in contact. The valves are connected to each other by a ligament which forms a hinge and is instrumental in opening the valves. Inside the hinge area the valves feature a set of teeth and sockets, the dentition, which is extremely variable in morphology and ensures a tight fit when the valves close. In addition the valves may show large internal scars from the attachments of the adductor muscles that operate to close the valves, and a linear scar parallel to the margin of the valves termed the pallial line. This is the line along which the muscles of the inner muscular part of the mantle are attached, the mantle being a sheet of tissue that forms the inner lining of the shell and secretes it. The mantle encloses the mantle cavity, and within this lie the gills, elongated ciliated feeding structures termed the palps, and the stomach. In burrowing forms a muscular foot projects outside the valves, and a siphon formed from the mantle projects to the surface (Fig. 1b). The presence of the siphon can be recognized by a deflection in the pallial line termed the pallial sinus.
Bivalve classification has always been a difficult problem because shell shape is often closely related to life habits, and important taxonomic characters are often present in the soft parts which do not get preserved. Virtually every organ system and hard-part structure has been used as the basis for grouping, and for many years a classification based on the gills was used. This has now been superseded by a multi-character classification in which hard-part structures, particularly the dentition, are important.
The shape and general morphology of bivalve shells directly reflects their mode of life; hence our understanding of the ways in which modern bivalves live enables us to make inferences about the way in which fossil forms lived. Many bivalves are shallow or deep burrowers, using the muscular foot to pull themselves through the sediment while rocking motions of the valves also help the digging process. Deep burrowers generally have elongate streamlined shells that enable them to move rapidly, while shallow burrowers tend not to have elongate shells. These infaunal bivalves live within the sediment for protection and remain connected to the surface by their siphons. Attached epifaunal bivalves are also common; they may be attached by a mass of threads, the byssus, as in the mussel, or be cemented down as in oysters. An extinct group of Mesozoic bivalves, the rudistids, became closely adapted to a cemented mode of life: one valve became conical while the other formed a lid. In some areas these animals were numerous enough to form reefs. In this way they parallel some Late Palaeozoic brachiopods and, remotely, even the corals. Swimming is a way of life adopted by some modern bivalves, such as the scallops, which are free-lying on the sea floor. Rapid opening and closing of the valves by the large adductor muscle allows the animal to eject water forcefully from the mantle cavity and can move it erratically just above the sea floor. This is an exhausting activity for the animal and it is used only to escape predators such as starfish. Some bivalves are adapted for life in hard substrates, boring into rock and wood by rocking and scraping actions of the valves and, in some instances, by using corrosive secretions.
Bivalves first occur in the Early Cambrian of Australia, North America, Denmark, and Siberia. These are extremely small (1 mm) and show similarities to an extinct molluscan class, the rostroconchs, which appear to have been ancestral to them. The rostroconchs were bivalve-like but possessed no hinge; the bivalves seem to have developed from them by reducing mineral deposition and increasing the deposition of ligamentous tissue in the hinge area. During the Ordovician the major groups of bivalves appeared and burrowing and bysally attached forms developed during a rapid burst of adaptive radiation. Although bivalves were less important than brachiopods (which are also bivalved benthonic suspension-feeders) during the Palaeozoic they were able to displace them in the early Mesozoic after the Permo–Triassic extinction event. This was mainly due to their ability to exploit infaunal habitats as a result of their development of a muscular foot and siphons formed by fusion of the mantle edges. At this time they appear to have first extensively colonized intertidal habitats—environments that they inhabit to the present day.
The bivalves have a long time-range and their biostratigraphic utility is limited. They are, however, used locally in Pennsylvanian (Late Carboniferous) coal measures, and in the Upper Cretaceous.
David K. Elliott
Bibliography
Boardman, R. S., Cheetham, A. H., and Rowell A. J. (eds) (1987) Fossil invertebrates. Blackwell Scientific Publications, Oxford.
Morton, J. E. (1967) Molluscs. Hutchinson, London.
