This is a Saddle Oyster (Pododesmus patelliformis), a species of bivalve in the family Anomiidae found in the northeastern Atlantic. Unlike the fragile, translucent shells of many anomiids, this species has a thin but robust and opaque shell, typically up to 4 cm in diameter, often white with brown markings. This example has only the upper half of the valve (shell) intact.

Location:
Northeastern Atlantic Ocean—from Norway down to the Mediterranean—attached to coarse sand, gravel, rocks, or shells, from the intertidal zone to depths of around 60 metres.

Average Size:
Up to approximately 4 cm in diameter.

Family:
Anomiidae (Saddle Oysters / Jingle Shells)

Species:
Pododesmus patelliformis (formerly Anomia patelliformis).

Shell Characteristics:
The shell is inequivalve: the upper valve is cupped with multiple weak radial riblets (up to approximately 60), while the lower valve is flattened to conform to its substrate. The interior of the upper valve is glossy, typically white or yellowish-green, with two furrowed muscle scars; the lower valve is transparent and thinner.

Ecological Role:
A sessile filter feeder, the Saddle Oyster helps maintain water clarity by filtering plankton and fine particles. It contributes to fouling communities on hard substrates and can provide microhabitat for epifauna such as barnacles, tube worms, and algae.

Reproduction:
Like other anomiids, this species has separate sexes. Fertilisation is external, with planktonic larvae that later settle and attach using a byssus (a bundle of strong, silky fibres that some bivalves and other molluscs produce to attach themselves firmly to a surface) through the lower valve.

Conservation Status:
Not currently listed as threatened. It is commonly found in areas where marine life grows on surfaces like rocks and shells. It is a hardy species, but its numbers could be affected if there aren’t enough suitable surfaces to attach to, or if the water becomes polluted.