Frequent Flowering but Low Reproductive Success of the Dioecious Seagrass Enhalus acoroides (L.f.) Royle in Li’an lagoon, China

Enhalus acoroides (L.f.) Royle is a large and important foundation seagrass species in the tropical Indo-Pacific. The northern marginal populations in China have been declining over the last decades. The reproductive phenology and fruit set of this dioecious seagrass were investigated from June 2017 to February 2018 in two areas in Li’an lagoon, Hainan Island, China. We found that E. acoroides flowered year-round. However, the flowering intensity varied among seasons and was highest in the summer and lowest in the winter. The shoot sex ratio was male-biased, and the pollen-ovule ratio was approximately 104: 1, based on the sex ratio and the number of male florets per inflorescence. Female and male plants flowered at the same time in this population. Despite the high reproductive investment in terms of flower formation observed throughout the year, the abundance of the mature fruits was low, possibly due to physical disturbance and cover by epiphyte and algae. This study provides insight into the reproductive ecology of E. acoroides, which will be useful for the future conservation of this threatened seagrass.