A relatively solid shell up to 50mm long and 25mm high. Yellow or red-brown often mottled with dark streaks. The internal 'shelf' is white while the rest of the interior is typically tan. This species typically lives in chains of animals stacked on top of one another which are often thrown up onto the strand line.
- Oval shaped shell which is half blocked by a shelf near the spire (only visible when empty)
Feeds by filtering plankton from the water. The species lives in chains of up to fifteen animals (usually fewer). The smallest animals at one end are male with larger females at the other end (the intermediate animals are changing sex). Males fertilise the females in the same chain. The chains persist through the addition of juvenile males at the end while the females die at the other.
This is a non-native species, originally introduced to the UK through oysters imported to Essex in 1887-90. The species is native to the Atlantic coast of North America.
Has been recorded from the south of Norway and Sweden, through Denmark and to the Atlantic coast of France.