Characterization of thallus mechanical and physiological traits of tropical fucoids: A preliminary study

SUMMARY Thallus mechanical strength, as well as production of secondary metabolites as defensive compounds, of tropical macroalgae is often essential for protection from herbivory. In tropical macroalgae, thallus mechanical strength is negatively correlated with productivity — a trade‐off between productivity and thallus toughness. The tropical fucoids, Turbinaria ornata which has defensive morphological traits against herbivory and Sargassum ilicifolium which is expanding its distribution in Japanese waters, were examined to determine thallus traits related to mechanical strength and productivity and their pair‐wise relationships were also examined. These traits that are directly or indirectly related to the trade‐off between productivity and thallus toughness were compared to data for various other temperate macroalgae by regression analyses. We found two strong positive correlations between thallus mechanical strength and thallus mass or thallus thickness, confirming that higher levels of mechanical strength for tropical fucoids is associated with higher biomass or thallus thickness. Also, negative correlations between thallus toughness and productivity were found indicating structural and physiological trade‐offs. However, the tropical fucoids exhibited relatively high productivity regardless of their higher level of thallus toughness. These traits of the tropical fucoids slightly deviate from the typical conservative strategy with higher thallus toughness and lower productivity as a trade‐off between productivity and thallus toughness.