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Mytilus planulatus Lamarck, 1819 x Mytilus galloprovincialis Lamarck, 1819

Description: Shape equivalve, inequilateral, elongate, with umbo at anterior end; variable in proportions. Dorsal margin straight or convex; postero-dorsal end extended, broadly rounded. Hinge with small teeth at anterior end, 2-5 in small shells, but becoming obsolete in larger specimens. Ligament long, narrow, more than half length of dorsal margin. Interior smooth, scars of posterior pedal retractor muscle and adductor muscle confluent so as to give the appearance of a single elongate scar posteriorly. Exterior with irregular, concentric growth ridges. Colour of exterior brown to bluish black; interior silvery white anteriorly, bluish at margin. Periostracum smooth, brown, usually irregularly worn away.

Size: Up to 105 mm in length in NSW, but larger in southern Australia.

Distribution: In Australia, there are records from Port Stevens, NSW, southward and around southern Australia, to southern WA, including Tasmania. The Australian taxon of Mytilus is part of two circum-global bands which occur in temperate waters in the northern and southern hemispheres. The extent of the Australian taxon within this distribution is not clear.

Habitat: Lives attached to hard substrates on rocky shores, wharves and piles from mid-tide level to about 10 m deep. It occurs in outer harbours and on the exposed coast and is sometimes abundant in dense aggregations.

Synonymy: This species has been referred to in Australian literature, either correctly or incorrectly, as Mytilus edulis Linnaeus, 1758, M. planulatus Lamarck, 1819, M. galloprovincialis Lamarck, 1819, M. edulis planulatus Lamarck, 1819, and M. (edulis) galloprovincialis (Lamarck, 1819). However, as explained in the Remarks section below, the complex genetics of this taxon defies conventional nomenclature.

Remarks: The shell varies in proportions depending on the extent of crowding during growth.

The Blue Mussel that occurs in NSW is a hybrid of a native species Mytilus planulatus Lamarck 1819 with the invading species Mytilus galloprovincialis Lamarck, 1819, which is endemic to the Mediterranean Sea and the coast of western Europe. There was an initial invasion during the Pleistocene period. Additional introductions of M. galloprovincialis occurred from shipping during European settlement in Australia, resulting in further hybridisation (Popovic, Matias, Bierne & Riginos, 2019). 

Fig. 1: Narooma, NSW (C.097316)


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