Mytilus planulatus forms clumps where it attaches to reefs, rocks, pillions, or moorings. The shell is a two sided symmetrical long tear drop shape which is black in colour, lightening toward the point.
The shell is often covered with other sea vegetation. Inside is a orange to cream coloured mussel with a dark lip which attaches to to the shell by a strong white muscle.
Host/prey of Cercaria praecox Walker, 1971 accepted as Oceroma praecox (Walker, 1971) Cribb, Miller, Bray & Cutmore, 2014 Feedingtype: endoparasitic, Stage: larva
Host/prey of Oceroma praecox (Walker, 1971) Cribb, Miller, Bray & Cutmore, 2014 Feedingtype: endoparasitic, Stage: larva
Host/prey of Tergestia haswelli Dollfus, 1927 Feedingtype: endoparasitic, Stage: larva
Mytilus angustanus Lamarck, 1819
Mytilus aoteanus Powell, 1958
Mytilus corneus Lamarck, 1819
Mytilus dunkeri Reeve, 1857 sensu Angas, 1867
Mytilus lacunatus Lamarck, 1819
Mytilus obscurus Dunker, 1857
Mytilus paetelianus Clessin, 1887
Mytilus ungularis Lamarck, 1819