Madoniactis lofotensis Danielssen, 1890 – synonym of Urticina eques
Madrepora pertusa Linnaeus, 1758 – synonym of Lophelia pertusa
Madrepora prolifera Pallas, 1766 – synonym of Lophelia pertusa
Medusa palliata Bohadsch, 1761 – synonym of Adamsia carciniopados
Milneedwardsia carnea Carlgren, 1921 – synonym of Edwardsiella carnea