Bivalves

1
Bivalves

• Classification
• PHYLUM Mollusca
• CLASS Bivalvia

2
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.

3
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.

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Bivalve morphology 3

• 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.

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Bivalve morphology 4

•  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.

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Bivalve morphology 5

• 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.

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Bivalve morphology 6

• 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.

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Bivalve morphology 7

• 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.

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Bivalve morphology 7

• 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.

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Bivalve morphology 8

• 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.

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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!!

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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.