A large wedge-shaped bivalve standing point-down in the sand with its gape at the surface.
A large thin-shelled bivalve, wedge-shaped and up to about 40 cm along its length, buried point-down in soft sediment with only the broad gaping end showing above the surface. The exposed edge is often the only visible part, and it is usually encrusted with algae, sponge, hydroids and small ascidians, so a live pen shell reads as a short line of growth standing out of the sand. The shell is thin, brittle and translucent at the margin, and broken pieces are common on muck ground.
The animal is anchored by byssal threads spun from the pointed end and attached to buried shell fragments and stones. The mantle edge is visible in the gape as a dark line, and the shell closes with a snap when a shadow crosses it. Feeding is by filtration.
The cavity inside is one of the reliable places to find commensals. Small shrimps of the genus Conchodytes live between the mantle and the shell in many individuals, along with small crabs, and they are visible only in an open animal and only from directly above. Recorded from about 3 to 35 m on sand, silt and seagrass, and dead shells are far more often seen than live ones on heavily worked ground.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.