Common but Least Noticed: Diversity of the Mangrove Fern <i>Acrostichum aureum</i> in Hainan Island, China

ABSTRACT Acrostichum aureum across nine coastal sites in Hainan Island was examined using morphological, cytometric, and genomic approaches. Cluster analysis identified five ecologically distinct clades linked to pinna morphology and habitat. Flow cytometry suggested sexual reproduction without ploidy‐spore number correlation. Genetic analyses revealed cohesive population structure with minor introgression from Acrostichum speciosum (12 samples), relatively low nucleotide diversity (π = 0.0647–0.0780), outcrossing dominance indicated by high heterozygosity ( H o = 0.0771–0.1042) and negative inbreeding coefficients ( F is = −0.0407–0.0017), and minimal differentiation ( F st &lt; 0.05) among populations. Despite widespread distribution, the species exhibits moderate genetic variation, highlighting vulnerability to environmental changes. Based on the heterozygosity and π, three priority conservation areas are proposed to preserve extant genetic diversity and maintain habitat connectivity. These findings underscore the need for targeted protection of this ecologically significant mangrove fern.