Rare occurrence of only female flowers suggests a lack of sexual reproduction and potential clonality of the seagrass Halophila baillonii Asch. on the Pacific coast of Costa Rica

Halophila baillonii Asch. is a rare seagrass species found in the Caribbean, Atlantic, and Eastern Tropical Pacific (ETP). It is listed as Vulnerable in the IUCN Red List yet very little is known about its dispersal mechanisms. For this study, samples were collected at five sites on the Pacific coast of Costa Rica. Each site was visited once between 2019 and 2021 as part of a larger molecular study. At each site, H. baillonii foliar shoots were manually collected along three transects. Presence of flowers or fruits was assessed for 1292 foliar shoots using a dissecting microscope. Additionally, sediment samples were collected to assess seed density at three of the sites. Flowering frequency was very low overall (0.7 %), flowers were only found at two of the five sites, and only female flowers were present. There were no fruits or seeds at any of the sites. This is the first report of H. baillonii flowering on the Pacific coast of Costa Rica and within the ETP. Meanwhile, H. baillonii flowers of both sexes and fruits have been found from limited sampling efforts at multiple sites in the Caribbean and Brazil. The findings of this study suggest a lack of sexual reproduction and potential clonality of the seagrass H. baillonii on the Pacific coast of Costa Rica. This supports the recent notion that H. baillonii is the first non-native seagrass in the ETP, yet more detailed field and molecular studies are needed.