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. Structure and development of genicula are significantly different across evolutionary lineages, and yet biomechanical performance is broadly similar. Because chemical composition plays a central role in both calcification and biomechanics, we explored evolutionary trends in cell wall chemistry across crustose and articulated taxa. We compared the carbohydrate content of genicula across convergently-evolved articulated species, as well as the carbohydrate content of calcified tissues from articulated and crustose species, to search for phylogenetic trends in cell wall chemistry during the repeated evolution of articulated taxa. We also analysed the carbohydrate content of one crustose coralline species that evolved from articulated ancestors, allowing us to examine trends in chemistry during this evolutionary reversal and loss of genicula. We found several key differences in carbohydrate content between calcified and uncalcified coralline tissues, though the significance of these differences in relation to the calcification process requires more investigation. Comparisons across a range of articulated and crustose species indicated that carbohydrate chemistry of calcified tissues was generally similar, regardless of morphology or phylogeny; conversely, chemical composition of genicular tissues was different across articulated lineages, suggesting that significantly different biochemical trajectories have led to remarkably similar biomechanical innovations.

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. Genicula in the three articulated lineages differ in both cellular construction and development, which may result in different constraints on morphology. By studying the interaction between flexibility and morphological variation in multiple species, we investigate whether representatives of convergently evolving clades follow similar strategies to generate mechanically successful articulated fronds. By using computational models to explore different bending strategies, we show that there are multiple ways to generate flexibility in upright corallines but not all morphological strategies are mechanically equivalent. Corallinoids have many joints, lithophylloids have pliant joints, and metagoniolithoids have longer joints-while these strategies can lead to comparable thallus flexibility, they also lead to different levels of stress amplification in bending. Moreover, genicula at greatest risk of stress amplification are typically the strongest, universally mitigating the trade-off between flexibility and stress reduction.