Fragum unedo can grow to 6.5 cm (2.5 in) in length but 4 cm (1.5 in) is a more usual size. The two domed valves are equal in size and asymmetric, with the beaks in front of the mid line. The outline is sub-quadrate with steeply sloping dorsal margins. The posterior margin is long and almost straight while the anterior margin is evenly rounded. They meet at a distinct, nearly square angle. The strong radial ribs often have small red, thorn-like sculpturing. The margin is deeply crenulated. The texture of the shell is robust, the colour being mainly white but sometimes mottled with reddish mosaic patches.
Fragum unedo has a symbiotic relationship with certain micro-algae, zooxanthellae, which live in the mantle and other soft tissues. It is heliophilous, meaning that it needs to expose its mantle above the substrate in order to maximise the amount of sunlight available for photosynthesis. The closely related Fragum fragum is sciaphilous, its symbionts having a much lower light requirement for photosynthesis. It has a large posterior gape and can remain more completely buried in the sediment with less likelihood of predation.
