Noun | 1. | Costia necatrix - a flagellate that is the cause of the frequently fatal fish disease costiasis flagellate, flagellate protozoan, flagellated protozoan, mastigophoran, mastigophore - a usually nonphotosynthetic free-living protozoan with whiplike appendages; some are pathogens of humans and other animals genus Costia - a genus of flagellate protoctist |