Sexual dimorphism, asymmetry, and allometry in the shell shape of Modiolus metcalfei (Hanley, 1843) collected from Dumangas, Iloilo, Philippines: A geometric morphometric approach

The shell is the most conspicuous and variable part of a bivalve and is largely affected by environmental conditions. Thus, this study was conducted to describe the changes in the shell shape of the horse mussel Modiolus metcalfei from Dumangas, Iloilo. Allometric changes in shape, sex-induced shape differences, and asymmetry were investigated using geometric morphometric methods. A total of 12 landmarks where used to obtain biological shape information from 60 horse mussel individuals. Multivariate analysis of variance revealed significant differences of shell shapes between sexes (Wilk’s λ=0.01, p=1.26E-219). Generally, males have an expanded shell but compressed mid-shell region while females have compressed shell but expanded midand postero-ventral region. Moreover, the multivariate regression of shape on centroid size was statistically significant (p<0.001, Goodall’s F-test). Smaller individuals were slender and elongate while larger individuals were slightly rounded and curved.Furthermore, directional and fluctuating asymmetry were highly significant (p<0.0001, Procrustes ANOVA), indicating developmental instability probably caused by the ecological health of the coastal waters of Dumangas, Iloilo. The differences in shape between sexes were attributed to the differences in reproductive roles while allometric shell morphology changes reflected its adaptation to the environment.