Allometric patterns were important in determining the shell shape in both C. galea and C. caribaea. In both species, shell length explained minor variation in shell shape (R2 = 0.072; p = 0.001 and R2 = 0.041; p = 0.049 for C. galea and C. caribaea, respectively), and these relationships depended on the host on which snails were found (shell length and host species interaction, R2 = 0.046; p = 0.001 and R2 = 0.133; p = 0.004 for C. galea and C. caribaea, respectively). The presence of such host-associated differences was also implied by the regressions between the CAC and shell length, where the slope of these linear regressions varied among snails associated with different host species (regressions were only done for hosts with ≥ 5 specimens; Fig. 11). Post-hoc tests further confirmed the presence of host-associated differences in allometric patterns in both C. galea and C. caribaea. Predicting the hypothetical shell shape of shells associated with a certain host species for a specific shell length, based on linear regressions of PC scores against shell length, clearly showed these differences (Fig. 12). Allometric patterns were subtler in C. caribaea compared to C. galea. In case of C. galea, both the distance (i.e., amount of change per unit of growth) and correlation (i.e., the direction of change) among vectors of shell length were, after Bonferroni corrections (n = 45), different for six pairs of host species (Fig. 12a; Table S6 in Online Supplementary Material 4). In C. caribaea, the correlation of vectors differed for a single pair of host species, while no differences were observed in the distances among vectors (Fig. 12b; Table S7 in Online Supplementary Material 4).
Fig. 11. Allometric patterns in Coralliophila galea (a) and C. caribaea (b). Common allometric component (CAC) is plotted against shell length (a). Separate regressions for host species with five or more specimens are plotted, symbols on the lines represent the R2-value (first symbol) and p-value (second symbol) of the regression: ns: p > 0.05; *: R2 < 0.1, p < 0.05; **: R2 ≥ 0.1, p < 0.01; ***: R2 ≥ 0.5, p < 0.001; ****: R2 ≥ 0.75, p < 0.0001. Points and regression lines per host are colour-coded as in Figs. 9-10. Shell length is plotted on a logarithmic scale.