Bivalves are marine mollusks in the class
Bivalvia.
This class consists of over 9000 species that
can be found from the tropics to the Arctic
in shallow and deep waters.
This includes organisms like clams, mussels,
oysters, and scallops.
The largest living bivalve is the giant clam,
reaching over 3 ft (1 m) wide.
The majority of bivalve species are marine,
but some do inhabit freshwater.
Most bivalves are sedentary filter feeders
with a two-part hinged shell that covers their
soft-bodies.
Two adductor muscles are used to close their
shell.
They breathe through their gills and gather
food through their gills.
To do this, bivalves draw in water and filter
out the tiny suspended food particles like
algae and detritus as the water passes through
their gills.
Alternatively, some species will ingest mud
from the seafloor and extract the edible material
from that.
Many species have a retractable foot that
allows them to move and burrow into the sediment.
Species like razor clams will bury themselves
in the sediment and extend their siphon up
to the surface to suck in water.
Some species can swim through the water by
snapping their shell open and closed.
Bivalves like mussels, have byssal threads
that permanently anchor the adults to hard surfaces.
Bivalves like oysters permanently cement themselves
to a substrate.
Bivalves grow with their shells.
Their mantle secretes calcium carbonate that
leaves a pattern of growth lines on their
exterior.
Their shell is reduced or absent in some bivalves
like shipworms.
Bivalves can be hermaphroditic or have separate
sexes.
Fertilization is usually external, in which
eggs and sperm are released into the water
column.
Bivalves are important in keeping water clean.
For example, the average adult oyster can
filter 25 gallons (95 liters) of water each
day.
Contaminants like metals and naturally occurring
algal toxins will concentrate in their bodies
as they filter food.
This is called bioaccumulation.
This doesn’t hurt the bivalve, but eating
a bivalve that has filtered too many toxic
substances can be harmful.
Bivalves are valued as a food source throughout
the globe.
Global production for human consumption is
estimated at over 15 million metric tons per
year.
Most are harvested by aquaculture.
In the United States alone, the value of the
commercial harvest is about 1 billion dollars
annually for over 150 million pounds of bivalves.
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