THE PHYLOGENETIC POSITION OF RHOPALURA-OPHIOCOMAE (ORTHONECTIDA) BASED ON 18S RIBOSOMAL DNA-SEQUENCE ANALYSIS

The Orthonectida is a small, poorly known phylum of parasites of marin e invertebrates. Their phylogenetic placement is obscure; they have be en considered to be multicellular protozoans, primitive animals at a ' 'mesozoan'' grade of organization, or secondarily simplified flatworm- like organisms. The best known species in the phylum, Rhopalura ophioc omae, was collected on San Juan Island, Wash. and a complete 18S rDNA sequence was obtained. Using the models of minimum evolution and parsi mony, phylogenetic analyses were undertaken and the results lend suppo rt to the following hypotheses about orthonectids: (1) orthonectids ar e more closely aligned with triploblastic metazoan taxa than with the protist or diploblastic metazoan taxa considered in this analysis; (2) orthonectids are not derived members of the phylum Platyhelminthes; a nd (3) orthonectids and rhombozoans are not each other's closest relat ives, thus casting further doubt on the validity of the phylum Mesozoa previously used to encompass both groups.