Environmental and biological controls on bivalve shell mineralogy.

Abstract

SummaryBivalves lay down two forms of calcium carbonate in their shells, aragonite and calcite. Shells may be wholly aragonitic, or may contain both aragonite and calcite, in separate monomineralic layers. Shells are built up of several layers of distinct aggregations of calcium carbonate crystals. These aggregations are referred to as shell structures and their general features are described. Aragonite occurs as prismatic, nacreous, crossed‐lamellar, complex crossed‐lamellar and homogeneous structures. Calcite occurs as prismatic or foliated structures. Myostracal layers (calcium carbonate laid down below sites of muscle attachment) are always aragonitic. The ligament and byssus when calcined are also invariably aragonitic. A summary of the occurrence of calcite and aragonite and the associated shell structures is given. Calcite is found only in the outer layer of superfamilies belonging to the subclass Pterio‐morphia with the exception of two species only from the Heterodont superfamily Chamacea. Generally within a superfamily shell structure and mineralogy are very constant. In all superfamilies these combinations have existed for many millions of years.It is therefore demonstrated that the prime control on shell mineralogy is genetic. Possible controls on mineralogy by the mantle cells, nature of the extrapallial fluid, nature of the periostracum and the organic matrix of the shell layers are discussed.It is known that environmental factors may modify the basic mineralogy/shell structure pattern within a superfamily. Thus there is an inverse relationship between the percentage of calcite in the shell and the mean temperature of the environment inhabited by the bivalve.A critical examination of published data shows that the evidence is convincing only in the superfamily Mytilacea. The species Mytilus californianus, which shows the greatest temperature effects, is peculiar amongst the Mytilacea in having an inner calcite layer as well as an outer one.Conflicting evidence for an inverse relationship between salinity and aragonite content is reviewed. The differences of opinion cannot be resolved without experimental work.We are grateful to the following for much useful discussion, and encouragement in many ways: Dr J. R. Baker, Dr G. E. Beedham, Dr B. C. M. Butler, Dr A. Hallam, Dr J. D. Hudson, Dr R. P. S. Jefferies, Mr J. Macrae, Dr W. S. McKerrow, Mr N. J. Morris, Mr C. P. Palmer, Mr N. Tebble, Dr E. R. Trueman and Professor A. Williams. Our best thanks are to Mr R. Cleevley for critically reviewing the manuscript.The following have rendered us considerable technical assistance: the staff of the electromicroscopy unit of the British Museum (Natural History), under the direction of Mr B. Martin; the technical staff of the Department of Geology, King's College, London and of the Department of Geology and Mineralogy, Oxford; Mrs J. M. Hall, and Mr G. Burton.