Shell shape variation in populations of common cockle Anadara oceanica (Lesson, 1831) (Bivalvia Arcidae) from the intertidal areas of Margosatubig, Zamboanga del Sur (Philippines)

Abstract

The advent of geometric morphometrics opened an area to study morphological variations in organisms. Thus, the aim of this study is to use outline-based geometric morphometrics to describe variations in the shell shapes of the left and right valves of Anadara oceanica (Lesson, 1831) (Bivalvia Arcidae) populations from the two neighbouring intertidal zones of Margosatubig, Zamboanga del Sur, Philippines. Herein, there were two levels of analyses that were employed: first, the shell shapes of the outer left and right valves between populations were compared; second, the shell shapes within population were quantitatively determined in terms of its symmetry. Results revealed significant variations both in the left and right valves of A. oceanica between populations. The variations observed are characterized by the deformations in the umbonal and anteroventral angles and in the dorsal, anterior and ventral margins of the outer shell both in the left and right valves. Although further studies are necessary in order to elucidate these variations, the second analysis revealed that the detected asymmetry in the shell shapes within A. oceanica populations was the cause of variation within populations that contributed to the significant variations between populations. Considering that the two sites are not geographically isolated, the results herein clearly proved that shell shape variation could also occur in neighbouring populations. The variations in the shell shapes of A. oceanica populations may have implications to habitat adaptation which aid in understanding the nature of this species especially those dwelling in the intertidal areas of Margosatubig,