7 - Shell Repair

Abstract

Molluscs are capable of repairing damage inflicted to the shell. The repair, often called shell regeneration , is accomplished by a deposition of new shell material in the damaged area. The process of repair may or may not be similar to that of normal shell formation, and the characteristics of the repaired shell vary with the organisms and the region of the shell damage. This chapter discusses (1) the processes of shell repair, (2) structure and mineralogy of repaired shell, (3) chemical characteristics of organic matrix and its role on crystal initiation, (4) histology and histochemical aspects of the mantle epithelium during shell repair, and (5) factors controlling shell repair. Shell repair is a highly regulated process, and attention is increasingly being paid to its on–off control mechanisms. Recent physiological and ultrastructural investigations have revealed that the mechanisms are intimately associated with neurosecretory or hormonal actions of the organisms. It will probably not be long before the control substances and their targets, that is, ion concentration and transport, synthesis and secretion of organic matrices for shell repair, and inhibition of normal shell formation, are identified and their mode of action and counteraction established. Characterization of organic matrix in repaired shell is an important topic, which deserves concentrated research efforts. In many cases, repaired shell consists of a number of layers, each composed of crystals, the type and/or morphology of which differ from the normal, and very often a bimineralic layer is present as well. The organic matrix has also been shown to differ from that of the normal shell in its composition and/or structure. It is hoped that chemical and structural characteristics of insoluble and soluble (if any) protein and carbohydrate fractions, and lipid components of the matrix will be investigated extensively and their roles in nucleation, orientation, morphology, polymorphisms of crystals, and inhibition of crystal growth clarified. Shell repair is an excellent experimental system in which to investigate various phases of molluscan shell formation, and the information obtained not only will contribute to the knowledge of shell repair but also will be very useful in understanding the mechanisms of biological calcification.