ORGANIC MATRIX SIMILARITIES IN MOLLUSKS AND FORAMINIFERA

Molluscan shells, particularly among the bivalves, are well known for their microstructural diversity and complexity. Studies on their calcification suggest that the process is indirectly controlled by various attributes, physical and chemical, of an organic matrix. Unfortunately our understanding of the role of the matrix is hampered by the fact that only part of it, termed the "insoluble matrix," can resist decalcification and remain for examination. Thus results from studies on the insoluble matrix, like this one, must be interpreted with care. Scanning electron microscopy of insoluble matrix preserved by critical-point drying reveals that molluscan matrix has a basic form: strands of soft, apparently sticky material about 0.03 micrometers in diameter, often with random nodules about twice that diameter along their length. Such strands are found within the crystals of calcium carbonate that make up the shell, but being poorly connected are often lost during decalcification. More obvious are strands that form thin sheets with a felted or lacy appearance, or thicker sheets with no apparent openings, that form the boundaries between individual crystals. Also obvious are the irregular mats of strands that accumulate during pauses in calcification, or during short intervals of dissolution along surfaces of growth. Foraminifera can also calcify their tests, and like mollusks, have developed diverse microstructures. Preliminary investigations, based on two species from each of two suborders, have found the basic form of insoluble matrix to be physically the same as in mollusks. The 0.03 micrometer thickness, the sometimes nodular appearance, the tangled masses and felted sheets, are all present in this material. This raises several interesting questions about the biology and evolution of calcification, the first of which requires investigating other calcifying phyla.