Abstract

Posidonia oceanica (L.) Delile produces buoyant fruits that disperse at the sea surface, yet the morphological basis of their stable floating orientation remains poorly quantified. We combined morphometric measurements, geometric and Archimedes-based estimates, and macroscopic sectioning to test whether a recurrent dorsal protrusion, reflecting dorsoventral asymmetry, is associated with a consistent floating orientation and to describe the internal organisation associated with this dorsoventral asymmetry.

Fruits showed a strong tendency to self-orient with the dorsal protrusion facing upward and maintained an emerged fraction of ~18-20% of total fruit volume under static conditions. Macroscopic sections showed that this external asymmetry corresponded to a consistent internal dorsoventral organisation, including thicker dorsal mesocarp above the seed. Simple proxies based on fruit mass and axial measurements closely matched direct measurements of mass and volume, supporting their use as practical non-destructive estimators of dispersal-relevant traits.

Together, these results identify morphological traits associated with stable floating orientation in P. oceanica fruits and provide quantitative descriptors of the floating phase that may inform future trait-based approaches to near-surface dispersal and connectivity.