Evolutionary patterns in chemical composition and biomechanics of articulated coralline algae.

Seaweeds inhabiting wave-battered coastlines are generally flexible, bending with the waves to adopt more streamlined shapes and reduce drag. Coralline algae, however, are firmly calcified, existing largely as crusts that avoid drag altogether or as upright branched forms with uncalcified joints (genicula) that confer flexibility to otherwise rigid thalli. Upright corallines have evolved from crustose ancestors independently multiple times, and the repeated evolution of genicula has contributed to the ecological success of articulated corallines worldwide.

Cellulose-rich secondary walls in wave-swept red macroalgae fortify flexible tissues.

Cellulosic secondary walls evolved convergently in coralline red macroalgae, reinforcing tissues against wave-induced breakage, despite differences in cellulose abundance, microfibril orientation, and wall structure. Cellulose-enriched secondary cell walls are the hallmark of woody vascular plants, which develop thickened walls to support upright growth and resist toppling in terrestrial environments. Here we investigate the striking presence and convergent evolution of cellulosic secondary walls in coralline red algae, which reinforce thalli against forces applied by crashing waves.

Bending strategies of convergently evolved, articulated coralline algae.

The evolution of uncalcified genicula in upright calcified corallines has occurred at least three times independently, resulting in articulated corallines within Corallinoideae, Lithophylloideae, and Metagoniolithoideae. Genicula confer flexibility to otherwise rigid thalli, and the localization of bending at discrete intervals amplifies bending stress in genicular tissue. Genicular morphology must, therefore, be balanced between maintaining flexibility while mitigating or resisting stress.

Convergence of joint mechanics in independently evolving, articulated coralline algae.

Flexible joints are a key innovation in the evolution of upright coralline algae. These structures have evolved in parallel at least three separate times, allowing the otherwise rigid, calcified thalli of upright corallines to achieve flexibility when subjected to hydrodynamic stress. As all bending occurs at the joints, stress is amplified, which necessitates that joints be made of material that is both extensible and strong.