Description: A massively thick, elongate-oval, somewhat swollen bivalve known for its ability to produce pearls in the mantle. Surface black or dark brown, front end rounded, hind end more produced with slight incurving half way up. Rare and threatened in highland streams in the south-west, west and north-west. 90 - 140 mm.

World Distribution: Found throughout the North Temperate zone. Distribution type: Circumpolar Boreo-temperate (56).

Irish Distribution: Found virtually throughout highland parts of Ireland, at any rate where clear highland streams with cobble bottoms and native salmonids exist. Still widespread but declining in the south-west, west and north-west. Threatened with extinction because recruitment of juveniles has declined drastically in all areas due to pollution and siltation.

Ecology:

• Restricted to running water and commonest in streams with large cobbles and sandy substrata
• Predominantly a soft water species
• There are populations in one or two limestone streams in the Nore System where peculiar shell forms have developed

Key Identification Features:

• With a massively thick, elongate-oval, bivalve shell
• Surface black or dark brown but the matt periostracum is often worn away at the umbos
• Front end rounded, hind end more produced with slight incurving half way up
• Shell interior with a layer of nacre or mother of pearl
• Pearls are produced by the deposition of nacre on particles of grit lodged in the mantle

Taxonomy:

• The hardwater form durrovensis was accorded subspecific status when red-listed by IUCN, but its exact taxonomic status is unclear. It occurs in the RIver Durrow, a contributory to the River Nore.

Red list status:

• Critically endangered (CR).

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

iNaturalist: Margaritifera margaritifera at iNaturalist World Species Observations database.

GBIF data for Margaritifera margaritifera | Classification: Bivalvia, Unionida, Margaritiferidae, Margaritifera

Thumbnails for genus Margaritifera