Description: This bivalve has a distinctive distorted elongate oval outline, and reaches up to 2.5 cm. Level of distortion and the overall shape can vary depending on the shape of the occupied crevice/hole. The shell is brittle, and dirty white to light brown colour. There can be a darker red-brown to purple staining at the umbones but this is not always present. There are 10-15 (rarely more) prominent thin, concentric ridges on the outer surface of the shell, which protrude so far that they have a frill-like appearance. These are especially prominent towards the edge of the shell. Between the ridges there are grooves and lines. 3 cardinal teeth in each valve, 2 (centre, posterior) bifid in right valve, 1 (centre) bifid in left. Inside of shell is white to pale yellow and may have some darker staining in red-brown to purple. Pallial sinus is short and triangular to u-shaped.

Habitat: Holes or crevices in rock, namely limestone, or Laminaria holdfasts. Found from the lower shore to the shallow subtidal. Seems to often inhabit holes previously occupied by Hiatella.

Distribution: Recorded from south and SW coasts of Britain and Ireland, and western to NW coast of Ireland.

Similar Species: Although there are many other species in the family Veneridae present in this area the frilly ridges should help distinguish this species from others.

Key Identification Features:

• Distorted elongate oval with distinctive frilly ridges.
• Grooves and lines visible between ridges.
• May have dark staining at umbones.
• 3 cardinal teeth in each valve, 2 bifid on right. 1 on left.

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

iNaturalist: Irus irus at iNaturalist World Species Observations database.

GBIF data for Irus irus

WoRMS: Irus irus at World Register of Marine Species. Accepted name: Irus irus (Linnaeus, 1758). AphiaID: 141917.

Classification: Biota; Animalia; Mollusca; Bivalvia; Autobranchia; Heteroconchia; Euheterodonta; Imparidentia; Venerida; Veneroidea; Veneridae; Tapetinae; Irus