Hydrozoanthus antumbrosus is a zoantharian (family Hydrozoanthidae) that grows epizoically on hydroid colonies, making Hydrozoanthus one of the few cnidarians parasitizing or commensal with other cnidarians—a remarkable lifestyle offering unique insights into cnidarian-cnidarian interactions. The Hydrozoanthidae is a small, phylogenetically distinctive family within Zoantharia whose host specificity for hydrozoans offers unique insights into the evolution of epizoic lifestyles and cnidarian-cnidarian coevolution. Genomic data have been generated for this species and are available in public databases, enabling comparative studies of cnidarian evolution and ecology.