Calcification in marine molluscs: how costly is it?

Although crucial to our understanding of skeletal evolution in marine invertebrates, the cost of calcification has remained elusive for a simple reason: CaCO3 is an inorganic material. Its cost thus derives solely from the metabolic expenses of accumulating, transporting, and precipitating CaCO3 and cannot normally be separated from other metabolic costs. Traditionally, calcification cost has been ignored and total shell cost has been assumed to derive solely from skeletal organic matrix. The cost estimated here was permitted by the substantial natural variation in shell thickness in two rocky-shore gastropods (Nucella lamellosa and Nucella lapillus). In both the field and laboratory, data from three separate experiments revealed that groups of snails producing extra shell material under a particular set of experimental conditions also consumed extra food. The cost of calcification was estimated by computing the extra energy assimilated per unit extra shell produced at a common rate of tissue growth and then subtracting the cost of the organic matrix. At 1-2 J/mg of CaCO3, the calcification cost reported here is roughly 5% of that for the predominantly proteinaceous organic fraction of molluscan shells on a per-gram basis. This may explain why calcareous microstructures high in organic content have become less common evolutionarily.