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.
Highlights
The green macroalgae of the genus Halimeda J.V.Lamouroux are important calcifying organisms in tropical and sub-tropical marine ecosystems
The measures of polygon size were only weakly positively correlated with their shape asymmetry (Table 1)
The primary subject of this study was the quantitative evaluation of the size and symmetry of
Summary
The green macroalgae of the genus Halimeda J.V.Lamouroux are important calcifying organisms in tropical and sub-tropical marine ecosystems. Their macroscopic thallus is composed of branched filaments (siphons) which contain multiple nuclei but are not divided by any cell walls. The above-ground part of the thallus consists of a serial sequence of segments that are separated by short and narrow nodes (see Figure 1a). The segments are composed of medullar filaments (located in the central part) and a cortex formed by several layers of swollen utricles. The peripheral (primary) utricles are usually laterally appressed, forming an analogy of an epidermis which separates seawater from the internal interutricular spaces, the prime location of CaCO3 precipitation
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