Modes of Life

SHALLOW INFAUNAL

Shallow infaunal bivalves live just in the sediment on the sea or river floor.
They tend to have a height:length ratio of 1.
To make burrowing into the sediment easier, these bivalves tend to be smooth and streamlined.
Spines may be present so that predators are unable to remove them from the sediment.

DEEP INFAUNAL

Deep infaunal bivalves live deep within the sediment on the sea or river floor.
They are at least twice as long as they are high.
They first appear in the fossil record roughly 2 million years ago. This style of life took much longer to develop because fused siphons had to evolve, as deep sediment living is a difficult mode of life.
Opening the valves and pushing the muscle foot downwards achieves burial.

EPIFAUNAL

All bivalves have the ability to secrete sticky threads known as byssal threads. However, most bivalves tend to use them only during infancy when they are small and require stabilization.

Species such as Mytilus use their byssal threads into adulthood. Mytilus is an epibyssate bivalve, which means it lives on the sediment surface with its threads reaching down into the substrate for stability. Epibyssate forms tend to be squat in shape.

Bivalves can also be endobyssate whereby they are situated half in and half out of the sediment with their byssal threads securing them to the sediment. This type of bivalve characteristically displays a pointed anterior.

Byssally attached bivalves tend to be found in high-energy environments.

OTHER INTERESTING MODES OF LIFE

OYSTERS: Oysters tend to have irregularly shaped shells and are able to physically cement themselves to hard surfaces.
PECTINIDS: These species simply lie on the sea floor and they are at great risk from predators. Therefore they have a unique escape strategy; by quickly clapping their valves together, pecctinids are able to perform a swimming function over short distances.
GRYPHAEA (or Devil's Toenail): This is an extinct genus of bivalve that is thought to have existed roughly 150 million years ago. It is believed to have been an unattached recumbent recliner on the sea floor, with its very own self-righting mechanism if a strong current knocked it out of the sediment.
BORING BIVALVES: These tend to have a very thin soft shell with a hard tip able to bore into surfaces such as wood or rock. They may encase themselves within the surface they are boring, and in the past were sometimes responsible for the sinking of galleons.

CONSTRAINTS ON A BIVALVE'S MODES OF LIFE
There are several environmental factors that can determine a bivalve's mode of life:

TEMPERATURE: Bivalves have broad temperature ranges. However, there is a relationship between shell thickness and temperature. In colder water it has been noted that shell size and thickness decreases as calcium carbonate (the material bivalve shells are composed of) becomes unsaturated.
SALINITY: Bivalves are known to be able to tolerate broad salinity ranges, but most diversity occurs in marine settings, with fewer species occurring in fresh waters.
WATER DEPTH: As a general rule, deep-burrowing bivalves occur more in shallower water settings, whereas shallow infaunal species tend to occur in deeper water settings.
OXYGENATION: Bivalves tend to occur in well-oxygenated environments, while only opportunists occur in low oxygenated areas.
TURBULENCE: Generally bivalves tend to be found in mildly turbulent areas, although they are able to inhabit a wide range of turbulence levels.

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