Phenganax marumi is a zoantharian (family Hydrozoanthidae) found in tropical Indo-Pacific marine environments, growing epizoically on hydroid colonies or other substrates. Like other members of this family, its polyps are small and adapted to colonizing the branches of hydroid hosts in a commensal or parasitic relationship unique among cnidarians. Its inclusion in comparative genomic datasets helps clarify the molecular basis of host specificity in epizoic zoantharians.