Macrotritopus – synonym of Octopus defilippi
Megalocranchia pardus – synonym of Teuthowenia pellucida
Megalocranchia richardsoni – synonym of Teuthowenia pellucida
Mesodinium phialinum – synonym of Mesodinium acarus
Mesodinium rubrum – synonym of Myrionecta rubra
Metacylis annulifera – synonym of Helicostomella subulata
Metacylis jörgensenii – synonym of Metacylis jorgenseni
Metacylis mediterranea var. pontica – synonym of Metacylis sp.
Metacylis mereschkowskyi – synonym of Metacylis sp.
Metacylis sp. aff. mereschkowskyi – synonym of Metacylis sp.
Metamysidopsis elongata atlantica
Mithrax forceps – synonym of Mithraculus forceps
Moneuplotes vannus – synonym of Euplotes vannus