Morphological allometry constrains symmetric shape variation, but not asymmetry, of Halimeda tuna (Bryopsidales, Ulvophyceae) segments

Green algae of the genus Halimeda have modular siphonous thalli composed of multiple repeated segments. Morphological variation among the segments has been related to various environmental factors, which often jointly affect their size and shape. The segments are bilaterally symmetric, which means that their shape variation can be decomposed into the symmetric and asymmetric components. Asymmetric variation might reflect both environmental heterogeneity and developmental instability of morphogenetic processes during the development of segments. In the present study, we examined if segment shape in H. tuna is related to their size and if an allometric relationship can also be found with respect to their asymmetry. Relative contributions of directional and fluctuating asymmetry to the segment shape variation within individual plants were investigated at two close localities in the northern Adriatic Sea. A series of equidistant semilandmarks were set along the outline of the segments, and analyzed by geometric morphometrics using two parallel methods to optimize their final position. Symmetric variation was strongly constrained by allometry, which also explained differences between populations. Smaller segments were significantly more asymmetric, but the difference in asymmetry between populations could not be explained solely by this allometric relationship. These differences between populations might have been caused by variation in local environmental factors. We conclude that members of the genus Halimeda represent an intriguing model system for studies of morphometric symmetry and asymmetry of sessile marine organisms, including effects of allometric relationships and infraspecific variation in relation to environmental factors of the benthic coastal habitats.