Umimayanthus parasiticus is a zoantharian (family Hydrozoanthidae) with a potentially parasitic or closely commensal relationship with its hydroid hosts, reflected in its specific epithet. Found in tropical Indo-Pacific waters, it forms small colonies on hydroid branches with morphology adapted to this intimate association. Comparative genomics of Umimayanthus species with their hydroid hosts may reveal molecular signatures of host recognition and specificity.