Polychaetes perform important roles in the coral ecosystem. Before knowing the interaction between polychaetes and other members in coral ecosystem, the first thing we need to know is which species of polychaetes are present in the ecosystem. The aim of my research is to better understand the taxonomy and biodiversity of coral-associated polychaetes, especially tube worms. I use both morphological characters and molecular markers to try to clarify the phylogenetic relationships between genus Hydroides, the most specious genus in the families of the tube worms.