The Shell ofCardium Edule, Cardium GlaucumandRuditapes Philippinarum:Organic Content, Composition and Energy Value, As Determined by Different Methods

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Abstract
The organic content of the shells of molluscs can represent a significant fraction of the total organic content (Bernard, 1974) but it is often neglected in calculations of energy budgets in these animals. This may be in part due to uncertainty about the true values, since published estimates of the organic content of shells show quite wide variation. The species examined and also the provenance of the selected samples contribute to this variation as does also the method of measurement. The methods principally used have been by ignition at various temperatures from 400 to 550°C for various durations between 2 and 36 h (see Shumway & Newell, 1984; Jørgensen, 1976; Mohlenberg & Kiorboe, 1981; Vahl, 1981; Shafee, 1979; Priceet al, 1976) or by acid extraction using different extraction proceedures (see Ivell, 1979; Dame, 1972; Horn, 1986; Griffiths & King, 1979). To calculate the energy content of the organic component some investigators have used the Hughes (1970) coefficient of 5.037 cal mg-1, while others have used Paine's (1971) protein coefficient of 2.39 J g-1. Wilbur & Saleuddin (1983) have called attention to the need for more study of these analyses. We present here the results of a study of the shell organic content of three species of molluscs, using two methods for the measurement and giving data on biochemical composition and energy value.