Molluscs

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Molluscs

• Phylum Mollusca

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• 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.

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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.

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• Radula- ribbon of small teeth used to feed by
  rasping food from surfaces.
• Radula made of chitin.
• Gills for gas exchange.

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Types of Molluscs

• They occupy all marine environments from the
  wave-splashed rocky shores to hydrothermal vents.

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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.

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Limpet
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Abalones
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Nudibranch
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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.

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Whelks
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Oyster Drill
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Cone Shell
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• 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.

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Janthina floating
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Aplysia
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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.

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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.

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Geoduck Clam
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• 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

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Green lipped mussel
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Oyster with pearl
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• Many bivalves bore in coral, rock, or wood.
• Shipworms-fouling organisms that settle on to
  bottom of boats, pilings and cause these to
  deteriorate.

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Shipworms
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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.

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• 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.

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Squidward
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• 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.

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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.

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• Make their homes in crevices, bottles,etc.
• They distract predators by emitting a cloud of
  dark fluid produced in the ink sac.

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Octopus
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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.

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• Size varies from few centimeters to largest
  living invertebrate 60ft!

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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.

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Cuttlefish
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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.

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Chambered Nautilus
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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.

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Chiton
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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.

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Tusk Shells
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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.

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Nervous System and Behavior

• Gastropods and bivalves have ganglia- clusters of
  nerve cells.
• Octopuses have a brain that allow for learning .
• When preyed upon some release ink and can change
  color.

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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.