Geometric morphometrics shows a close relationship between the shape features, position on thalli, and CaCO3 content of segments in Halimeda tuna (Bryopsidales, Ulvophyceae).

Calcifying green algae of the genus Halimeda J.V. Lamouroux are typical for the modular thalli composed of serial segments. Their CaCO3 content gradually increases with age due to calcification, the intensity of which is largely linked to photosynthesis. The dynamics of segment phenotypic plasticity at different scales and its relation to CaCO3 content is not well known. We investigated the populations of Halimeda tuna in the upper sublittoral of four regions on the Adriatic Sea coast. Using geometric morphometrics, we explored the patterns of segment shape plasticity, their relationships with the spatial factors and CaCO3 content. The results showed that segment position on thalli was the main determinant of their shape features. This effect was considerably more prominent than the differences among plants, populations, or regions. Likewise, the segment shape proved to be a significant predictor of their CaCO3 content. Segments with inversely conical shapes, typical for the lower parts of branches, contained significantly less CaCO3 than the reniform and oval segments that probably contribute most to the overall carbonate budget of the Mediterranean Halimeda draperies. [ABSTRACT FROM AUTHOR]