Modes of Life
- 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 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
- Opening the valves and pushing
the muscle foot downwards achieves burial.
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
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
- 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.
DEPTH: As a general rule, deep-burrowing
bivalves occur more in shallower water settings,
whereas shallow infaunal species tend to occur in deeper water
Bivalves tend to occur in well-oxygenated
environments, while only opportunists occur in low oxygenated areas.
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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