Morphometric Analysis of Surface Utricles in Halimeda tuna (Bryopsidales, Ulvophyceae) Reveals Variation in Their Size and Symmetry within Individual Segments

Abstract

Calcifying marine green algae of genus Halimeda have siphonous thalli composed of repeated segments. Their outer surface is formed by laterally appressed peripheral utricles which often form a honeycomb structure, typically with varying degrees of asymmetry in the individual polygons. This study is focused on a morphometric analysis of the size and symmetry of these polygons in Mediterranean H. tuna. Asymmetry of surface utricles is studied using a continuous symmetry measure quantifying the deviation of polygons from perfect symmetry. In addition, the segment shapes are also captured by geometric morphometrics and compared to the utricle parameters. The area of surface utricles is proved to be strongly related to their position on segments, where utricles near the segment bases are considerably smaller than those located near the apical and lateral margins. Interestingly, this gradient is most pronounced in relatively large reniform segments. The polygons are most symmetric in the central parts of segments, with asymmetry uniformly increasing towards the segment margins. Mean utricle asymmetry is found to be unrelated to segment shapes. Systematic differences in utricle size across different positions might be related to morphogenetic patterns of segment development, and may also indicate possible small-scale variations in CaCO3 content within segments.

Morphological variation and haplotype diversity of \(\textit{Halimeda macroloba}\) and \(\textit{H. opuntia}\) (Chlorophyta: Halimedaceae) from Southern Vietnam

“…their morphologies that may be adaptive i the new changes [27]. Plasticity in the morphology of surface utricles is also found in H. tuna [6]. H. macroloba belongs to section Rhipsalis, exhibiting high morphological plasticity [28].…”

“…For example, the thallus morphology of H. macroloba Decaisne from Thai waters showed variation in different habitats such as subtidal, intertidal, or high light intensity areas [5]. The surface utricles within individual segments also showed variation in the case of H. tuna [6]. Therefore, genetic markers and morphological observation are used to identify the genus into genetically and morphologically distinct species [7].…”

Halimeda tuna (Bryopsidales, Ulvophyceae) calcification on the depth transect in the northern Adriatic Sea; carbonate production on the microscale of individual segments

“…In addition to among-species differences, variation in size and symmetry of peripheral utricles within a single segment of H. tuna has also been demonstrated. The utricles near the segment bases were considerably smaller than those located near the apical and lateral margins, and symmetry of the utricles decreased from the centre to the margins ( Neustupa & Nemcova, 2020 ).…”