Hydrozoanthus gracilis is a slender zoantharian (family Hydrozoanthidae) growing epizoically on hydroid colonies in tropical and subtropical marine environments, with smaller and more gracile polyps than typical zoantharians adapted to life on the delicate branches of hydroid hosts. The family Hydrozoanthidae is unique within Zoantharia for its host specificity toward Hydrozoa, offering comparative material for studying the evolution of cnidarian-cnidarian symbiosis and the ecological dynamics of epizoic associations. Genomic data have been generated for this species and are available in public databases, enabling comparative studies of cnidarian evolution and ecology.