Bivalves

Bivalve Chlamys antiaustralis, from the Miocene of Victoria. Source: Museum Victoria.

Bivalves are very common shallow-water molluscs, and include scallops, oysters, and clams.

Bivalves are mostly benthic (bottom dwelling) and marine, although some live in fresh water. Some bivalves are burrowers, some attach themselves to rocks or to the sea floor, while others live in cavities.

All bivalves have a shell consisting of two hinged components ('valves') which enclose the animal's soft body parts. The valves are partly open when relaxed, but are held shut by muscles when danger from predators (birds, starfish, gastropods) threatens. Some bivalves can swim short distances by clapping their two valves together.

The body of bivalves includes a mouth, gills that are also used in filtering food from the water, and a muscular foot used for digging or movement. Bivalves that burrow into the sediment also have tube-like structures called siphons that extend to the sediment surface in order to draw water into the shell cavity.

Bivalve Panenka gippslandica, from the Lower Devonian of Jordan River, Victoria. Source: Museum Victoria.

Bivalves first became common in the Ordovician Period (434 to 490 million years ago), and became widespread in the Mesozoic Era (65 to 251 million years ago) as burrowing species evolved. Since then they have become the dominant group of hard-shelled marine animals.

Some bivalve families have changed very little since the Palaeozoic Era (prior to 251 million years ago), while others show greater evolutionary change. Fossil bivalves are sometimes used to help date rocks of the Tertiary Sub-era (about 2 to 65 million years ago).

Fossil bivalves are found in various parts of Victoria, including Torquay, Lakes Entrance and Mornington.