Mollusks

1
Mollusks

• A soft bodied animal with an internal or external
  shell

2
Evidence in the larval stage

• Trochophore is the name of the larva which every
  mollusk has. Annelids also have this type of
  larva which shows that the two phylum might be
  closely related.

3
Body plan

• The body of a mollusk has four parts a foot, a
  mantle, a shell and a visceral mass.
• Foot the foot can take many forms. It could be
  flat used for crawling, it could be spade-shaped
  used for burrowing or tentacles used for
  capturing prey.
• Mantle a thin layer of tissue that looks like a
  cloak.

4

• Shell Made from glands in the mantle that
  secrete calcium carbonate. Slugs have lost their
  shell.
• Visceral mass the blob that contains all the
  internal organs.

5

• Mollusks are more complex that Annelids because
  they have complex, interrelated organ systems
  that function together. They also have true
  coeloms surrounded by mesoderm tissue.

6
Systems Digestion

• Mollusks can be either herbivores (eating algae
  off rocks), carnivores, filter feeders (like most
  bi-valves are), dendritivores (feeding off
  decaying material), or parasites.
• Radula a type of tongue with hundreds of tiny
  teeth. This is how slugs and snails feed.
  (Leeches use a radula as well to open skin to
  suck the blood)

7
Digestion (cont)

• Siphon A tube-like structure through which water
  enters and leave the body. Clams, oysters and
  scallops use this to suck water in (incurrent
  siphon) and push it out (excurrent siphon) of the
  body. The water crosses over feathery gills that
  help filter the goodies out of the water. There
  is a sticky mucus on the gills and cilia helping
  to get food into the mouth.
• Octopi and some sea slugs use sharp jaws to eat
  prey. Some octopi have poisons to subdue their
  prey.

8
Respiration

• Aquatic mollusks use gills to exchange CO2 and
  O2.
• Land slugs and snails do not have gillsthey
  respire using their mantle lined with blood
  vessels. They have to be kept moist in order for
  the gases to diffuse across into the bloodstream.

9
Circulation

• Open circulatory system Blood is pumped by a
  simple heart and works its way into some sinuses.
  From the sinuses it goes to the gills where the
  gasses are exchanged, then back to the heart.
  This is for slow moving mollusks (snails and
  clams)
• Closed circulatory system Blood vessels can pump
  blood more quickly. Used by octopi and squid.

10
Excretion

• Tube shaped Nephrida remove ammonia (nitrogen
  rich waste) form the blood and release it to the
  outside of the body.
• Torsion During the development of a snail there
  is a 180 degree twist in its digestive system so
  that the anus comes out right above the head. Its
  good for hiding in shells but causes a sanitation
  problem.

11
Nervous response

• The simplest nervous system is in a bi-valve
  mollusk. They have small ganglia near the mouth,
  a few nerve cords and simple sense organs such as
  chemoreceptors and eyespots.
• Octopi and their relatives are intelligent with a
  well developed brain.
• Funny story The octopus that moved from the
  fishtank and back.

12
Movement

• Slime Mucus aids in the movement of snails and
  slugs. They secrete mucus from their foot and
  then use a rippling motion of their foot over the
  slime to get where they want to go.
• Jet propulsion Octopi and squid draws water into
  its mantle then forces it out of the siphon so it
  can move in the opposite direction.

13
Mollusk Sex

• Sexual reproduction can happen in a few different
  ways
• They can release a large number of eggs and sperm
  into open water which develop into free swimming
  larvae.
• In tentacled mollusks and some snails
  fertilization takes place inside the body of a
  female.
• Some mollusks are hermaphrodites but need two to
  tango.

14
Classes of Mollusks

• Polyplacophora Chitons
• Scaphoda tusk shells or tooth shells
• Gastropda (largest class) Snails, limpets,
  slugs, whelks, conchs, periwinkles, sea slugs,
  sea hares, nudibranchs and sea butterflies
• Bivalvia (Pelecypodia) mussels, clams, scallops,
  oysters and shipworms
• Cephalopodia squids, octopuses, nautiluses,
  devilfish and cuttlefish

15
Chitons(Polyplacophora)

• A marine mollusk with an elongated body, a large,
  broad foot and radula.
• Chitons eat algae, hydrozoans and other
  lowgrowing organisms. They have beautiful shells
  with a variation of pattern and colors. The shell
  is divided in 8 plates which makes the chiton
  flexible to roll up into a small ball. It can
  hook onto things using its foot and mantle which
  together form a type of suction cup.

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Gastropods

• Shell-less or single shelled.
• Single shelled Snails use their shell when
  threatened to coil their bodies inside. Their
  foot closes the door.
• No shells Use rocks and logs to hide from
  predators.
• Sea hares can squirt ink to confuse predators.
• Nudibranchs have chemicals that make them taste
  yucky. They eat cnidariads and recycle the
  nematocysts using them to sting their predators.

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Bivalves

• Most stay in one place. Clams burrow in the mud
  or sand, mussels use a sticky thread to attach
  themselves to rocks, scallops move around.
• Oysters secrete a special mucus from the mantle
  called nacre which lines the inside of the shell.
  When the oyster gets a piece of sand inside they
  secrete this nacre as a protection and viola- a
  pearl is born.
• Bivalves also have a 3 chambered heart.

18
Bivalve concerns

• Nuisance Zebra mussels have taken a ride on the
  ballasts of ships and have come to North America.
  They are causing problems because their numbers
  build rapidly (no natural predators here) so
  they clog pipes.
• Clams- many species have gone extinct due to
  overfarming. If we take our filter feeders out
  of the water how can we clean the pollution in
  the ocean? In fact scientists now use bivalves to
  see if our ocean is healthy.

19
Black Smokers

• There are a species of bivalves that live near
  deep sea volcanic vents. These bivalves have a
  symbiotic relationship with the bacteria in their
  guts. The bacteria extract chemical energy from
  compounds released by the super heated waters
  which produces food molecules the bivalves can
  use.

20
Cephalopods

• These mollusks have a head attached to a foot
  that is divided into tentacles.
• Nautilus- a cephalopod that is probably the
  oldest living mollusk- we have fossils of their
  shells dating way back. They have a coiled shell
  of many chambers. Within the shell the nautilus
  can release gas from the chambers and change its
  buoyancy. These are the only cephalopods with
  external shells.
• Some nautilus have over 90 tentacles.

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• Cuttlefish have internal shells- these are what
  is harvested and used for birds in cages.
• Squid have a long thin shell and Octopi have lost
  their shell completely.
• Eyes Most of the eyes can be quite large and
  complex.

22
Giant Mollusks

• Tridacna gigas- a clam whose shell is more than
  1.2 meters across. Some weighing over 400
  kilograms
• Octopus hongkongenesis Large octopus that
  measures 9.7 meters from the tip of one tentacle
  to the top of the opposite tentacle.
• Architeuthis Giant squid which can have a body
  of 4 meters with tentacles 9 m long.