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Molluscs
Phylum Mollusca
Includes snails, clams, octopuses and
others.
There are more species of molluscs in the
ocean than any other animal group.
There are 200,000 species of molluscs.
Mollusk Structure
Have soft bodies enclosed in a calcium
carbonate shell.
Mantle- thin layer of tissue that secretes
the shell.
Bilaterally symmetrical.
Ventral, muscular foot used for locomotion.
Head with eyes and other sensory organs.
Radula- ribbon of small teeth used to feed
by rasping food from surfaces.
Radula made of chitin.
Gills for gas exchange.
Types of Molluscs
They occupy all marine environments from
the wave-splashed rocky shores to
hydrothermal vents.
Gastropods (Class Gastropoda)
 Largest, most common, and most varied.
 Snails are most familiar.
 Also includes limpets, abalones, nudibranchs.
 75,000 species.
 Most are coiled mass of vital organs enclosed by
a dorsal shell.
 Shell rests on a ventral creeping foot.
 Gastropod means “stomach foot.”
Limpet
Abalones
Nudibranch
Nutrition
Most use radula to scrape algae from
rocks, like periwinkles, limpets, and
abalones.
Some like mud snails are deposit feeders
on the bottom.
Whelks, oyster drills, and cone shells are
carnivores that prey on clams, oysters,
worms and small fish.
Whelks
Oyster Drill
Cone Shell
The violet snail Janthina has a thin shell
and produces a bubble raft out of mucous
to float on surface and look for its prey.,
Sea hares Aplysia have smal, thin shells
buried in tissue that graze on seaweeds.
Janthina floating
Aplysia
Bivalves (Class Bivalva)
Clams, Mussels, Oysters
Body is laterally compressed (flattened
sideways) and enclosed in a shell with two
parts or valves.
No head and no radula.
Folded and expanded gills used to obtain
oxygen and filter small food particles.
Mantle lines the inner shell.
Strong muscles close the valves.
Clams
Use shovel-foot to burrow in sand and
mud.
Water is drawn in through a siphon
(snorkel).
This allow them to get oxygen even when
buried in the sand.
Geoduck Clam
Mussels-Secrete byssal threads that attach
on rocks and other surfaces.
Oysters-cement their shell to a hard
surface.
Pearl oysters- form pearls when the oyster
secretes calcium carbonate to coat
irritations in mantle
Green lipped mussel
Oyster with pearl
Many bivalves bore in coral, rock, or wood.
Shipworms-fouling organisms that settle
on to bottom of boats, pilings and cause
these to deteriorate.
Shipworms
Cephalopods (Class Cephalopoda)
Predators that include octopuses, squid,
cuttlefish.
Nearly all are agile swimmers.
Have complex nervous system and a
small or no shell.
650 marine species.
Cephalopod means “head-foot.”
The food is modified into arms and
tentacles, usually with suckers used to
capture prey.
Have large eyes on the side of head.
Octopuses- round body
Squids- elongated body
Protected by a thick and muscular mantle.
Squidward
Have two or four gills on head and water
enters and leaves through a siphon
(funnel).
Swim by forcing water out of mantle cavity
through the siphon.
Use jet propulsion with siphon to move in
any direction.
Octopuses
Have eight long arms and lack a shell.
Common bottom dwellers.
Size varies from 5cm (2in) to 9m (30ft).
Efficient hunters of crabs, lobsters, and
shrimps.
Bite prey with a beak-like jaw. Radulas
rasp away flesh.
Secrete a paralyzing substance and some
have a toxic bite.
Make their homes in crevices, bottles,etc.
They distract predators by emitting a cloud
of dark fluid produced in the ink sac.
Octopus
Squids
Better swimmers than octopuses.
Elongated body covered with mantle has
two triangular fins.
Use jet propulsion and can move forward
and backward.
Eight arms and two tentacles with suckers
around a mouth.
Shell reduced to a pen on upper mantle.
Size varies from few centimeters to largest
living invertebrate 60ft!
Cuttlefish
Have eight arms and two tentacles.
Flat bodies and fin on the sides.
Have calcified internal shell that aids in
buoyancy.
Shell called cuttlebone.
Cuttlebone sold as a source of calcium for
cage birds.
Cuttlefish
Chambered Nautilus
Coiled, external shell with gas-filled
chambers that serve for buoyancy .
Body in the outer chamber has 60-90 short
suckerless tentacles to capture prey.
Chambered Nautilus
Chitons (Class Polyplacophora)
800 species
Have 8 overlapping shell plates on dorsal
surface.
Internal organs not coiled.
Live on shallow, hard bottoms using radula
to feed on algae.
Chiton
Scaphopds (Class Scaphopoda)
Tusk shells.
Elongated shell open at top and tapered
elephant-like tusk
Live on sand or muddy bottoms.
Have thin tentacles with adhesive strips to
capture foraminiferans.
Tusk Shells
Feeding and Digestion
Use salivary and digestive glands that
release enzymes to break down food.
Bivalves have crystalline style that
secretes enzymes in the stomach.
Have open circulatory system where blood
flows out of vessels and into the body
cavity.
Cephalopods have closed circulation.
Nervous System and Behavior
Gastropods and bivalves have gangliaclusters of nerve cells.
Octopuses have a brain that allow for
learning .
When preyed upon some release ink and
can change color.
Reproduction and Life History
Most have separate sexes.
Some are hermaphrodites.
Bivalves release sperm and eggs in the
water.
Cephalopods mate and use a
spermatophore (modified arm) to transfer
a packet of sperm.
Males have a long, flexible penis.