Parasitic and commensal copepods occurring as planktonic organisms with special reference to Saphirella-like copepods

Many species of parasitic and commensal copepods have a planktonic phase in their life history. On the basis of the ecological features in the copepodid stages, the following four types of planktonic phase can be recognized: Type 1: non-feeding type copepodids (especially, adults) occur as plankton. All species belonging to the orders Monstrilloida and Thaumatopsylloida. Type 2: all copepodid stages occur as plankton. So-called "planktonic" members (Macrosetella gracilis, Sapphirina salpae, Ergasilus genuinus, Pontoeciella abyssicola etc.) of the orders Harpacticoida, Poecilostomatoida and Siphonostomatoida. Type 3: copepodid I and adult (female) stages occur as plankton. Pennellid siphonostomatoids having intermediate hosts. Type 4: only copepodid I stage occurs as plankton. Many species belonging to the orders Poecilostomatoida and Siphonostomatoida and parasitic members of the order Cyclopoida. Saphirella-like copepods are important members in this group. The term "Saphirella-like copepod" indicates the first copepodid stage of the order Poecilostomatoida, which has developed maxillipeds and a cephalothorax formed by fusion of head and first pedigerous somite. All or some species belonging to seven genera of four families have copepodid I stages of this type. Saphirella-1ike copepods occur occasionally in inlet and estuary waters with high abundance (maximum density, 4,771 inds.m(-3) in Tokyo Bay) and are numerically dominated by species of the genus Hemicyclops. The members of Hemicyclops have complex interspecific relationships with nektonic and benthic fauna through food-chain and "inhabitation-chain". Occurrence of parasitic and commensal copepods such as Saphirella-like copepods in planktonic fauna gives much important information for studies of species-diversity in coastal waters.