Phenganax subtilis is a slender zoantharian (family Hydrozoanthidae) found in tropical Indo-Pacific waters, forming epizoic colonies on hydroid branches. Like its congeners, it possesses the distinctive two-cycle tentacle arrangement of zoantharians adapted to the compact morphology required for life on hydroid hosts. The three described Phenganax species collectively provide comparative material for understanding the genomic basis of epizoic lifestyle evolution within Hydrozoanthidae, a unique family of cnidarian-cnidarian epizoics.