Bivalves
Classification
 PHYLUM: Mollusca
 CLASS: Bivalvia
Bivalve Morphology 1
 Look at page 32 of Black. Copy
diagram 18a.
 The soft body is held between two
shells (valves) which are usually
made of calcite or aragonite.
 The valves act as protection from
carnivores and wave energy.
 Bivalves are bilaterally
symmetrical, the plane of
symmetry occurring along the
line where the two valves meet.
 Therefore the valves are more or
less mirror images.
Bivalve morphology 2
VALVES:
 The shell is secreted by the
animal like in brachiopods
from the mantle.
 The valves are called left
and right.
 What are the two valves
called in brachiopods?
UMBONES:
 The point on the valve from
which the bivalve grew.
Bivalve morphology 3
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CONCENTRIC GROWTH
RINGS:
There are concentric growth
rings increasing in size away
from the umbones.
These are added as the
bivalve grows much in the
same way as the growth
rings on a tree.
If the growth season is good
then the rings will be more
widely spaced.
RADIATING RIBS:
The shell ornament and it
may also have spines.
These give the shell extra
strength and rigidity.
Bivalve morphology 4
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EQUIVALVE/INEQUIVALVE:
In most bivalve species the
valves look very similar
(equivalve) unlike Brachiopods
although sometimes the valves
do look different (inequivalve).
DORSAL/VENTRAL:
The dorsal side is the hinge
area the opposite is the ventral.
Difficult to remember.
ANTERIOR/POSTERIOR:
The anterior end is the mouth
end.
The posterior is the anus end.
Difficult to work out on a fossil.
Bivalve morphology 5
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SOFT PARTS:
Most soft parts are in the dorsal
half, which contains the main
organs and muscles.
MANTLE:
This is the fleshy part that
extended out towards the edge of
the valves.
It enclosed most of the soft parts
and was also responsible for the
growth of the valves as the mantle
wrapped over the edge of the
valve.
FOOT:
This is a muscular organ, which
can extend out of the shell and is
used by the bivalve to move
through the sediment.
See video clip from OU.
Bivalve morphology 6
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GILLS:
Found in the mantle cavity.
It is a respiratory organ, which
also helps in food gathering.
Cilia help move water through the
gills.
Water comes in one SIPHON and
out another.
DENTITION:
On the inside of the hinge area
the shells are thick.
In this area there are teeth and
sockets which fit together with
those on the other valve.
Gives extra strength and rigidity
to the bivalve.
Bivalve morphology 7
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LIGAMENT:
This occurs in the hinge region and
can hold the valves together and
acts as a spring forcing the valves
apart when the muscles relax.
ADDUCTOR MUSCLES:
Mainly the shells are closed by
adductor muscles, which contract
to close the shell.
When the muscle relaxes the valves
open (because of the ligament).
MUSCLE SCARS:
These are the points where the
muscles were attached.
Bivalve morphology 7
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PALLIAL LINE:
On the inside of the valves a faint
groove can often be seen running
from anterior to posterior roughly
parallel to the shell edge.
This is the point where the mantle
ended.
PALLIAL SINUS:
In some species of Bivalve the
pallial line shows a large
indentation.
In burrowing species which have
large siphons which extended out
into the burrow the sinus left
space for it to be contacted back
into the bivalves.
Bivalve morphology 8
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GAPE:
Many burrowing bivalves did not close
completely and left a small gap
through which the siphons or foot or
both could protrude permanently out of
the valve.
CRENULATED MARGIN:
In some bivalve species the edges of
the two valves are smooth, in others a
crenulation may be present
represented by small ridges and
grooves.
These allow a tighter more rigid fit.
BYSSUS:
This is a horny extrusion extending
from the body cavity, which attaches
the animal to a firm surface.
Interesting Facts About Bivalves
 Some oysters may shed over one million eggs in
a season. Only about one of these lives to
adulthood.
 Some oysters alternate their gender. Male one
year, female the next!
 Some scallops (Pectens) have dozens of eyes.
They help the scallop to see predators, so it will
know whether to swim away or clamup!
 The ocean quahog can live to be 220 years old.
 Boring clams (Family Xylophagidae "wood eater"
in Latin) can sink a ship! They are often called
"ship worms", even though they aren't worms at
all!!
Interesting Facts About Bivalves 2
 Most molluscs are capable of making pearls when
foreign substances enter their shell. They coat
the foreign substance with shelly material. Some
clams can grow pearls as big as golf
balls. Freshwater clams are often used as nuclei,
around which pearls can grow small, round
pieces of the clam's shell are placed inside a
Pearl Oyster and they are coated with iridescent
nacre and form a pearl.
 The largest known bivalve harvested was a
Tridacna gigas which weighed 330kg and was
1.4m (nearly four feet) in length!
 Some molluscs, such as the oysters, change sex.
They start off life as a male and they usually end
up life as females.