Cockles and mussels belong to a group of molluscs called bivalves, which includes oysters, clams, scallops and razor shells.
Bivalve means having two valves and all these creatures have two halves to their shells, joined by a hinge that opens rather like that of a locket.
Most bivalves feed by filtering food from the water through a tube called a siphon.
Cockles burrow in sand and mud on the seashore. Mussels cling to rocks and breakwaters between the high and low tide marks. 288 Sea creatures
Oysters and some other molluscs line their shells with a hard, shiny, silvery white substance called nacre.
When a lump of grit gets into an oyster shell, it is gradually covered in a ball of nacre, making a pearl.
The best pearls come from the Pinctada pearl oysters that live in the Pacific Ocean. The world’s biggest pearl was 12 cm across and weighed 6.4 kg. It came from a giant clam.
Scallops can swim away from danger by opening and shutting their shells rapidly to pump out water. But most bivalves escape danger by shutting themselves up inside their shells.
A giant clam found on the Great Barrier Reef was over 1 m across and weighed more than 0.25 tons.
There are colonies of giant clams living many thousands of meters down under the oceans, near hot volcanic vents.
The swan mussel is a bivalve — a mollusc type of shellfish similar to clams and oysters on the seashore. It lives in lakes and slow, deep rivers. It draws a current of water into its shell, both for breathing and to filter out tiny particles of food.