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

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

“…Out of the factors examined within this study, only calcium carbonate content was found to be significantly influenced by depth. In general, the average CaCO 3 content of 60.3% agrees with the values obtained by Neustupa & Nemcova (2022) who investigated upper sublittoral H. tuna populations over four regions on the latitudinal gradient of the Adriatic Sea (61.6%). Within other investigated Mediterranean Sea populations of this species, lower calcium carbonate content (59.7%) was found by Prát & Hamáčková (1946) and Ballesteros (1991) (45.7%).…”

“…However, the depth factor did not contribute significantly to the observed H. tuna segments shape and size variation. Segment shape variation spanned by the first two principal components resembled very closely the results of similar analyses on a larger set of segments (982) from two nearby sublittoral populations ( Neustupa & Nemcova, 2018 ) and on a smaller set of segments (535) from distant regions on the Adriatic coast ( Neustupa & Nemcova, 2022 ). This is a striking example of the deeply conserved pattern of segment shape plasticity in H. tuna applied independently of environmental conditions or location ( e.g.…”

“…Thus, we were able to track slight changes in mineral content on the microscale of the segments. Compared to our previous studies ( Neustupa & Nemcova, 2018 ; Neustupa & Nemcova, 2022 ), the Halimeda plants studied, consisted of a lower number of segments (3–12; average 6.3), and therefore only four position categories were defined, compared to our previous study that distinguished five positions ( Neustupa & Nemcova, 2022 ).…”

“…Individual segments were decalcified in 5% HCl for 10 min until bubbling had ceased ( Vroom et al, 2003 ), rinsed in deionized water, dried (60 °C for 45 min) and weighed again. The proportional mineral content of each segment was determined as previously described in Neustupa & Nemcova (2022) . C = 1 −dcw/drw , where drw is the total dry weight and dcw is the dry weight after decalcification.…”

“…Although both populations were sampled from the same depth in the upper sublittoral, there was a significant difference in the mean shape of the segments, indicating that local environmental factors may have influenced segment morphogenesis ( Neustupa & Nemcova, 2018 ). Later, Neustupa & Nemcova (2022) examined size and shape plasticity, and calcium carbonate content of H. tuna segments in populations along the Adriatic latitudinal gradient. They showed, similarly to the previous study, that segment position on the thallus was the main determinant of its shape.…”