Asked by: Chris Harris, Leeds
Seashells are the exoskeletons of animals called molluscs, including snails, nautiluses, mussels, scallops and oysters.
The biggest are giant clams, Tridacna gigas. Their twinned shells can grow to well over a metre across and tip the scales at 200kg, the same as two newborn elephants.
Giant clams, like all shell-making molluscs, sculpt their protective homes from calcium carbonate and gradually expand them throughout their lives. They inhabit coral reefs and can live for at least a century. Demand for their meat as a delicacy in many countries is making them vulnerable to extinction.