Reproductive biology of a natural mangrove hybrid Rhizophora annamalayana and its parent species (R. apiculata and R. mucronata) (Rhizophoraceae)

Rhizophora annamalayana is a natural mangrove hybrid with a rare seed set; the species is fast disappearing. We investigated possible causes of rare fruit set and explored ways of overcoming reproductive difficulties in this mangrove. Flowers open in the early morning in the hybrid and its parent species. Anthers dehisced after flower opening only in Rhizophora mucronata and Rhizophora apiculata. Anthesis and anther dehiscence took up to 5 days in all three species. Stigma receptivity peaked during anthesis; receptivity was lost within 8 h in R. annamalayana and R. mucronata. In R. apiculata, anthesis occurred inside the flowers, confirming that cleiostogamy occurred. Pollen of R. mucronata was 100% viable. Viability was 53% in R. apiculata. In R. annamalayana, only 3% of pollen was viable. R. mucronata flowers offered pollen and nectar to visitors, which were mainly Lucilia caesar (flies, bees, ants, etc.). Pollinators of R. annamalayana were restricted to Monobia quadridens, Vespa tropica and Ocybadistes walkeri. R. annamalayana reproduced by cross-pollination.