Patterns of Directional and Fluctuating Asymmetry in Southern Ocean Sea Urchins

Bilateral symmetry is a fundamental organizational trait in many metazoans. However, deviations from this symmetry, manifested as directional or fluctuating asymmetries, offer valuable insights into developmental, functional, and environmental processes. This study quantified and characterized bilateral asymmetry in three related species of the genus Abatus using a dual approach that integrates linear and geometric morphometrics. Our analyses reveal consistent patterns of directional asymmetry across different species, with specific trends showing that the left body sides tend to be larger. In contrast, fluctuating asymmetry exhibited an inverse relation with directional asymmetry. Also, linear morphometric analysis showed no significant correlation between directional asymmetry and either sex or body size, while geometric analyses only identified subtle shape deviations related to size but not to sex. These findings allow us to discuss the possible origins of this trait, mainly related to developmental constraints due to reproduction or growth, or carried over on a genetic basis. Our results highlight the importance of combining different morphometric approaches to clarify complex patterns of morphological variation and emphasize the significance of asymmetry analyses in understanding evolutionary and ecological processes in irregular echinoids.