Reproductive and recruitment potential of Halophila hawaiiana

Abstract

Understanding the details of seagrass species’ vegetative and sexual reproductive capacity is important to inform restoration efforts for these habitat engineers. Previous work has shown that vegetative fragments of Halophila species can settle and establish in new areas. However, it is unclear whether fragment size impacts vegetative reproduction success, and whether the isolated species Halophila hawaiiana has retained this ability. To determine whether H. hawaiiana can settle and establish from vegetative fragments, we took fragments of various sizes and fixed them to a string held just above the sediment in situ. We found that fragments could settle and establish into the sediment; 39 % of fragments grew roots into the sediment by day 14. Fragment settlement was strongly associated with month and location. Fragment size non-linearly influenced fragment settlement, with medium-sized fragments (four to six nodes) being the most successful. Regarding sexual reproduction for H. hawaiiana, observed seeds have previously been considered a result of apomixis; however, the presence of staminate flowers indicates that sexual reproduction may be possible. We collected flowers and artificially pollinated them in the lab, with unpollinated flowers isolated as a control. Endosperm growth and seed production was observed in the treatment group but not the control - suggesting sexual reproduction and not apomixis is the method of seed production for this species. To guide seagrass conservation efforts, future research is needed to understand the links between vegetative fragment settlement and patch expansion, and to determine environmental factors that induce flowering and seed germination in H. hawaiiana.