Bivalves are the second largest class of mollusks, including clams, oysters, mussels and scallops. They have two shells hinged together, and a mantle and tissues in between. Most are filter feeders that draw in water and trap food particles to feed and breathe. They reproduce sexually, with external fertilization leading to larval stages before settling into the adult shell form. Bivalves live in aquatic habitats worldwide and have diverse lifestyles including burying in sediment, attaching to surfaces, or boring into hard substrates.

1. CLASS BIVALVIA

2. Contents:
• Class Bivalvia
• Shell and associated structures
• Gas exchange ,filter feeding and digestion
• Reproduction and development
• Other maintenance functions of Class Bivalves
• Diversity in Bivalves

3. Class Bivalvia
• With close to 30,000 species, the class Bivalvia (bivalve-ah) (L. bis, twice
valve, leaf) is the second largest molluscan class.
• This class includes the clams, oysters, mussels, and scallops
• A sheet like mantle and a shell consisting of two valves (hence, the
• class name) cover these laterally compressed animals.
• Many bivalves are edible, and some form pearls. Because most bivalves
are filter feeders, they are valuable in removing bacteria from polluted
water

4. Shell and associated structures
• The two convex halves of the shell are called valves. Along the dorsal margin of
the shell is a proteinaceous hinge and a series of tongue-and-groove
modifications of the shell, called teeth, that prevent the valves from twisting .
• The oldest part of the shell is the umbo, a swollen area near the shell’s anterior
margin.
• Although bivalves appear to have two shells, embryologically, the shell forms as
a single structure. The shell is continuous along its dorsal margin, but the
mantle, in the region of the hinge, secretes relatively greater quantities of
protein and relatively little calcium carbonate

5. Conti….
• The elasticity of the hinge ligament opens the valves when certain muscles
relax.
• Adductor muscles at either end of the dorsal half of the shell close the shell.
Anyone who has tried to force apart the valves of a bivalve mollusc knows the
effectiveness of these muscles.
• This is important for bivalves because their primary defense against predatory
sea stars is to tenaciously refuse to open their shells. explains how sea stars
have adapted to meet this defense strategy.
• The bivalve mantle attaches to the shell around the adductor muscles and near
the shell margin. If a sand grain or a parasite lodges between the shell and the
mantle, the mantle secretes nacre around the irritant,

6. BIVALVE SHELL

7. Gas exchange ,filter feeding and
digestion
• Bivalve adaptations to sedentary, filter-feeding lifestyles include the loss
of the head and radula and, except for a few bivalves,
• The expansion of cilia-covered gills. Gills form folded sheets (lamellae),
with one end attached to the foot and the other end attached to the
mantle.
• The mantle cavity ventral to the gills is the inhalant region, and the
cavity dorsal to the gills is the exhalant region . Cilia move water into the
mantle cavity through an incurrent opening of the mantle. Sometimes,
this opening is at the end of a siphon, which is an extension of the
mantle.

8. CONTI…
• A bivalve buried in the substrate can extend its siphon to the surface
and still feed and exchange gases.
• Water moves from the mantle cavity into small pores in the surface of
the gills, and from there, into vertical channels in the gills, called water
tubes.
• In moving through water tubes, blood and water are in close proximity,
and gases exchange by diffusion .
• Water exits the bivalve through a part of the mantle cavity at the dorsal
aspect of the gills, called the suprabranchial chamber, and through an
excurrent opening in the mantle

9. CONTI…
• The gills trap food particles brought into the mantle cavity.
• Zoologists originally thought that cilia action was responsible for the trapping.
However, the results of a recent study indicate that cilia and food particles have little
contact
• The food-trapping mechanism is unclear, but once food particles are trapped, cilia
move them to the gills’ ventral margin. Cilia along the ventral margin of the gills
then move food toward the mouth
• Cilia covering leaflike labial palps on either side of the mouth also sort filtered food
particles.
• Cilia carry small particles into the mouth and move larger particles to the edges of
the palps and gills.

10. CONTI…
• This rejected material, called pseudo feces, falls, or is , onto the mantle, and a
ciliary tract on the mantle transports the pseudo feces posteriorly.
• Water rushing out when valves are forcefully closed washes pseudo feces from
the mantle cavity.
• The digestive tract of bivalves is similar to that of other mollusc.
• Food entering the esophagus entangles in a mucoid food string, which extends
to the stomach and is rotated by cilia lining the digestive tract.
• A consolidated mucoid mass, the crystalline style, projects into the stomach
from a diverticulum, called the style sac

11. CONTI…
• Enzymes for carbohydrate and fat digestion are incorporated into the
crystalline style. Cilia of the style sac rotate the style against a chitinized gastric
shield.
• This abrasion and acidic conditions in the stomach dislodge enzymes.
• The mucoid food string winds around the crystalline style as it rotates, which
pulls the food string farther into the stomach from the esophagus.
• This action and the acidic pH in the stomach dislodge food particles in the
food string. Further sorting separates fine particles from undigestible coarse
materials

12. CONTI…
• The latter are sent on to the intestine. Partially digested food from
the stomach enters a digestive gland for intracellular digestion.
• Cilia carry undigested wastes in the digestive gland back to the
stomach and then to the intestine
• The intestine empties through the anus near the excurrent
opening, and excurrent water carries feces away.

13. BIVALVE ANOTOMY

15. OTHER MAINTENANCE FUNCTIONS OF
BIVALVES
• In bivalves, blood flows from the heart to tissue sinuses, nephridia,
• gills, and back to the heart . The mantle is an additional site for
oxygenation.
• In some bivalves, a separate aorta delivers blood directly to the mantle.
• Two nephridia are below the pericardial cavity (the coelom). Their duct
system connects to the coelom at one end and opens at nephridiopores
in the anterior region of the suprabranchial chamber

16. CONTI…
• The nervous system of bivalves consists of three pairs of interconnected ganglia
associated with the esophagus, the foot, and the posterior adductor muscle.
• The margin of the mantle is the principal sense organ. It always has sensory
cells, and it may have sensory tentacles and photoreceptors.
• In some species (e.g., scallops), photoreceptors are in the form of complex
eyes with a lens and a cornea. Other receptors include statocysts near the pedal
ganglion and an osphradium in the mantle, beneath the posterior adductor
muscle.

17. Reproduction and development
• Most bivalves are dioecious. A few are monoecious, and some of these species
are protandry. Gonads are in the visceral mass, where they surround the looped
intestine.
• Ducts of these gonads open directly to the mantle cavity or by the
nephridiopore to the mantle cavity.
• Most bivalves exhibit external fertilization.
• Gametes exit through the suprabranchial chamber of the mantle cavity and the
exhalant opening.
• Development proceeds through trochophore and veliger stage. When the
veliger settles to the substrate, it assumes the adult form.

18. CONTI…
• Most freshwater bivalves brood their young.
• Fertilization occurs in the mantle cavity by sperm brought in with inhalant water.
Some brood their young in maternal gills through reduced trochophore and
veliger stages.
• Young clams are shed from the gills. Others brood their young to a modified
veliger stage called a glochidium, which is parasitic on fishes
• These larvae possess two tiny valves, and some species have toothlike hooks.
Larvae exit through the exhalant aperture and sink to the substrate.
• Most die. If a fish contacts a glochidium, however, the larva attaches to the
gills, fins, or another body part and begins to feed on host tissue.

19. CONTI…
• The fish may form a cyst around the larva.
• The mantles of some freshwater bivalves have elaborate modifications
that present a fishlike lure to entice predatory fish.
• When a fish attempts to feed on the lure, the bivalve ejects glochidia
onto the fish After several weeks of larval development, during which a
glochidium begins acquiring its adult structures, the miniature clam falls
from its host and takes up its filter-feeding lifestyle.
• The glochidium is a dispersal stage for an otherwise sedentary animal
and is usually harmless to the fish

20. BIVALVE DIVERSITY
• Bivalves live in nearly all aquatic habitats. They may completely or
partially bury themselves in sand or mud, attach to solid substrates, or
bore into submerged wood, coral, or limestone.
• The mantle margins of burrowing bivalves are frequently fused to form
distinct openings in the mantle cavity (siphons).
• This fusion helps to direct the water washed from the mantle cavity
during burrowing and helps keep sediment from accumulating in the
mantle cavity.

21. CONTE…
• Some surface-dwelling bivalves attach to the substrate either by
proteinaceous strands called byssal threads, which a gland in the
foot secretes, or by cementation to the substrate.
• The common marine mussel, Mytilus, uses the former method,
while oysters employ the latter.
• Boring bivalves live beneath the surface of limestone, clay, coral,
wood, and other substrates

22. CONTE…
• Boring begins when the larvae settle to the substrate, and the
anterior margin of their valves mechanically abrades the substrate.
• Acidic secretions from the mantle margin that dissolve limestone
sometimes accompany physical abrasion.
• As the bivalve grows, it is often imprisoned in its rocky burrow
because the most recently bored portions of the burrow are larger
in diameter than portions bored earlier.

23. REFERENCE:
• Text book of Miller and Harley 5th edition