For all individuals observed, the f

For all individuals observed, the following measurements were made with digital calipers: maximum dimension (dorsoventral diameter) of the whole bivalve and, for each valve separately, the distance of the ventral margin of the nacre from the hinge along the direction of maximum growth (see Fig. 2). From these data, we calculated the ratios LV/RV to capture the degree of symmetry in microstructure and determined the linear extent of the prismatic flange for each valve as a proportion of the entire valve. Where possible we made measurements for multiple individuals (over as wide a range of size as possible) of a particular species to obtain mean values but this was not possible in all instances. Although less than ideal, the use of smaller numbers appears justified because for those instances where ten individuals were available for study (e.g. Pinctada margaritifera and Pteria colymbus) there were no significant differences (p > 0.05) for the proportion of asymmetry over the range of sizes available。An exception, however, appears to be Electroma alacorvi, where of five individuals there was a greater asymmetry in the larger individuals although our sample size was not sufficient to demonstrate the significance of this.