Description: This small oval to rhomboid bivalve reaches up to 13 mm in length and is light yellow to pale brown in colour, and is usually mostly uniform with a few darker patches. It may be partially covered by the light brown periostracum. This sometimes has a pale green tinge which darkens at the margins. There are 8-12 wide ribs towards the front of the shell and 30-50 finer ribs towards the back. Inside of the shell has the iridescent sheen of mother-of-pearl, sometimes with hints of purple.

Habitat: Under rocks on rocky shores from the intertidal to shallow sublittoral. Also found amongst small algaes.

Distribution: Found all around Britain and Ireland.

Similar Species: Could be confused with Modiolula phaseolina, Musculus niger or Musculus subpictus. Could potentially be confused with juvenile Modiolus adriaticus, Modiolus modiolus, Modiolus barbatus or Mytilus edulis.

Key Identification Features:

• Mostly uniform colour with some darker patches.
• Oval to rhomboidal shape.
• 8-12 wide anterior ribs.
• 30-50 fine posterior ribs.

Distribution Map from NBN: Musculus discors at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Musculus discors at iNaturalist World Species Observations database.

GBIF data for Musculus discors

WoRMS: Musculus discors at World Register of Marine Species. Accepted name: Musculus discors (Linnaeus, 1767). AphiaID: 140472.

Classification: Biota; Animalia; Mollusca; Bivalvia; Autobranchia; Pteriomorphia; Mytilida; Mytiloidea; Mytilidae; Crenellinae; Musculus