Many molluscs have two shells that clamp together. In rockpools mussels can be very common, covering the rocks and thousands of pipis can be hidden in the sand. Often the larger mussels have been removed by people for food or fish bait. Around our city rockpools the smaller mussels are crushed when people walk on them. Then in some places there are so many sharp-shelled mussels on the rocks it’s difficult to swim anywhere near them. Other molluscs with two shells include scallops and clams.
Inside the two shells are the body and all the organs. Bivalves suck seawater into their shells so they can filter out microscopic animals and plants. This is called filter feeding.
The tiny microscopic young of molluscs float around in the sea until they find a suitable home. A mussel will settle and glue itself to a solid surface such as a rock. A pipi will live in the sand of a surf beach. A scallop will prefer the sandy bottom of a shallow sea. A clam will want to anchor itself on anything solid like a coral reef in the warm shallow seas.