Phylogenetic Position of the Order Monstrilloida (Crustacea, Copepoda) inferred from 18S rDNA Sequences

Abstract

Monstrilloid copepods are unique and puzzling marine crustaceans, known as endoparasites of polychaetes and mollusks during larval stages, though they become free-living and non-feeding plankton in their adult stage. Since their mouthparts are highly reduced or nearly disappeared in the adult stage, the phylogeny solely based on morphological characters may not well resolve genealogical relationships. Monstrilloida is often treated as one of ten copepod orders or as a subgroup within the order Siphonostomatoida, a parasitic/commensal copepod. To clarify the phylogenetic relationship of the order Monstrilloida, we attempted to determine 18S rDNA sequences for copepod species including three monstrilloid genera (Cymbasoma, Monstrilla and Monstrillopsis). Based on the present data with those previously reported, phylogenetic analyses focusing on Monstrilloida were done applying four different general tree algorithms (i.e., NJ, MP, ML and BI). All four trees consistently and strongly supported the monophyly of the order Monstrilloida, which was validated as a sister group to Siphonostomatoida. Our results provide more robust and reliable perspective on the evolutionary history of Monstrilloid copepods.are highly reduced or nearly disappeared in the adult stage, the phylogeny solely based on morphological characters may not well resolve genealogical relationships. Monstrilloida is often treated as one of ten copepod orders or as a subgroup within the order Siphonostomatoida, a parasitic/commensal copepod. To clarify the phylogenetic relationship of the order Monstrilloida, we attempted to determine 18S rDNA sequences for copepod species including three monstrilloid genera (Cymbasoma, Monstrilla and Monstrillopsis). Based on the present data with those previously reported, phylogenetic analyses focusing on Monstrilloida were done applying four different general tree algorithms (i.e., NJ, MP, ML and BI). All four trees consistently and strongly supported the monophyly of the order Monstrilloida, which was validated as a sister group to Siphonostomatoida. Our results provide more robust and reliable perspective on the evolutionary history of Monstrilloid copepods.