STEWART, Hannah L.; Univ. of California, Berkeley: Ontogenetic changes in tissue strength, buoyancy and reproductive output enhance dispersal by the rafting macroalga Turbinaria ornata

Distributions of benthic macroalgae rely on dispersal distances of spores, gametes or detached pieces of fertile thalli. Many species of algae are buoyant and individuals detached from the substratum by hydrodynamic forces can drift long distances and colonize new areas. The tropical macroalga Turbinaria ornata was used to study how strength of stipe tissue and buoyancy change with ontogeny and sexual maturity. Thalli of T. ornata can be positively or negatively buoyant. Thalli from wave-swept forereef locations lack gas-filled floats (pneumatocysts) and experience negative buoyant forces (-0.011 N), while thalli from calm lagoon sites experience positive buoyant forces (+0.023 N) with up to 85% of their blades containing pneumatocysts. Thalli transplanted from forereef to lagoon sites developed pneumatocysts in their blades (70%) and became positively buoyant (+0.005 N) after 8 weeks, indicating that pneumatocyst production and positive buoyancy are phenotypically plastic traits. For buoyant lagoon thalli, tissue strength is negatively correlated with percent of thallus mass dedicated to reproductive tissue (percent reproductive mass). Buoyant force is positively correlated with percent reproductive mass. Percent reproductive mass increases with thallus age, indicating an ontogenetic change from tissue strength in younger thalli to buoyancy and reproductive capacity in mature thalli. Thus, as thalli become reproductively mature, they also become more susceptible to breakage and dislodgment by hydrodynamic forces, and the result is large floating rafts of reproductively mature thalli that help to disperse T. ornata throughout the South Pacific.