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Musculus cf. impactus (Hermann, 1782)

Description: Shape equivalve, inequilateral, light in weight, inflated; umbo near anterior end, incurved, projecting above dorsal margin. Valves wider posteriorly; anterior end broadly rounded to subtruncate; posterior end subtruncate, elongate in some specimens. Margin weakly crenulated by external sculpture. Hinge without teeth. Ligament moderately long, curved. Interior smooth, muscle scars obscure, surface nacreous. Exterior with 3 sculptural panels: anterior panel with at least 6 radial ribs; posterior panel with more numerous radial ribs; central panel with only concentric growth ridges. Colour of exterior purple-brown; interior iridescent with external sculpture showing Periostracum smooth, green or brown.

Size: Up to 35 mm in length.

Distribution: New Zealand and Australia. In Australia, from southern Queensland southwards and around southern Australia to south-western Western Australia, including Tasmania.

Habitat: Lives inside a felt-like mass of byssal hairs attached to solid substrates such as jetty piles (Fig. 2). Macpherson & Gabriel (1962) reported on the habitat in Victoria: “These mussels are gregarious, living in crevices of rocks at low tide level to 25 fathoms, sometimes embedded in sponge-like byssal masses”.

Comparison: This is the largest species of the genus that occurs in NSW.

Synonymy: There has been considerable confusion over the taxonomic identity and name(s) of this NSW species. Iredale (1936, p. 270) considered the names of the (larger) NSW species of Musculus to be cumingianus Reeve and varicosus Gould, implicitly rejecting the New Zealand name impactus Hermann. Consequently, the name Musculus cumingianus was used for the southern Australian species. But Macpherson & Gabriel (1962) said “having examined a series we are of the opinion that the Australian and New Zealand shells are conspecific”, hence reinstating the name of the New Zealand species for the southern Australian shell. But comparison of New Zealand specimens with the specimen illustrated here allows doubt that they are conspecific, so the name Musculus cf. impactus is used here.

Fig. 1: Off pylons, old Spit Bridge, The Spit, Sydney, NSW (C.97899). Top specimen of group in Fig. 2

Fig. 2: Off pylons, old Spit Bridge, The Spit, Sydney, NSW (C.97899)

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