Hello marine biology students.
In this video we're going to talk about the
diversity of molluscs.
[Intro Music]
So phylum Mollusca is an important group of
marine animals.
They have great diversity within this phylum,
including a variety of different body plans,
but the general design is that there would
be a head, a muscular foot, and a visceral
mass of internal organs including the digestive
compartment.
For molluscs, their body is usually covered
by a mantle which secretes a shell of calcium
carbonate in many species.
They have a specialized feeding appendage
known as a radula
that is often used for grazing on algae, but
we'll see, has been modified in many species
as well allowing them to be predators.
They have a well-developed nervous system
with a brain and an open circulatory system,
meaning that their blood vessels are not always
continuous and that the circulating fluid
of the body can just empty into that major
body cavity.
They have a complete digestive system, so
a mouth that's distinct from an anus, and
the digestive tube in which food is processed.
And they are a large successful group, with
nearly 100 thousand known species, but many
remaining to be discovered.
We're going to talk about several classes
of molluscs, the first being the gastropods
or class gastropoda.
This is the largest group of molluscs and
it includes snails and limpets, abalones and
sea slugs.
Gastropoda, the name itself means stomach
foot because they're visceral mass and body
cavity is curled and coiled and above and
through the muscular foot itself.
Most species of gastropods have a coiled shell,
although some, such as sea slugs, would not
and others like the limpets and the abalones
have a smooth shell, even though the internal
mass is coiled.
One of these groups of sea slugs are known
as nudibranchs and they don't have any shell
at all.
These gastropods use their graduala for grazing
on seaweeds, yet some gastropods are deposit
feeders and others are suspension feeders.
Some are carnivores and use radulas to capture
their prey.
Here we see a diversity of these gastropods,
including a keyhole limpet, an abalone, a
cone snail, nudibranchs and then other forms
of parasitic snails as well.
Our next class in phylum Mollusca are the
bivalves, class Bivalvia.
And the name bivalve itself literally means
two shells or two valves.
Bivalves are specialized filter feeders 
including clams, oysters, scallops, mussels,
and ship worms.
These two protective shells compress the body
into the mantle cavity.
They have gills, which are folded within the
mantle cavity for respiration and filter feeding.
Most bivalves use a two siphon system, where
water enters and exits the mantle cavity through
these siphons.
Since bivalves are not grazers, they do not
use a radula.
Different bivalves have different habitats,
with some clams burrowing within soft sediments,
oysters and mussels usually living on hard
surfaces or substrates, and scallops are free
living.
Here we see a diagram of a clam showing the
basic body structures of a bivalve.
Notice how the gill functions both in respiration
and feeding.
The next class of molluscs that we'll talk
about are the cephalopods or head feet.
The cephalopods include squids, octopus, cuttlefishes,
and chambered nautilus.
Their name literally means head footed.
They are fast swimming predators which can
use water jet propulsion as a part of their
locomotion.
Water enters and exits the mantle cavity by
way of a siphon.
Cephalopods have a well-developed nervous
system with large complex eyes, brain, and
nerves.
The thick mantle covering their body usually
does not secrete a shell, but in some species
there will be an internal shell and in others
the shell will be completely absent.
Their radula have been modified into a beak-like
mouth to crush or rip prey.
Here we see the diagram of a basic anatomy
of an octopus.
Again, with its siphon and its arms, the visceral
cavity is mostly in what we would consider
to be the head of the cephalopod.
The body plan of a squid is a little different.
Its arms and tentacles are entirely at the
front of its body, with its mantle covering
the remainder.
The last few groups of molluscs are a bit
more obscure than the ones we've talked about
so far.
We'll talk about chitins next, which are in
the class Polyplacophora.
Chitins have a dorsal shell made of eight
plates.
They have a ventral muscular foot and in some
ways look a lot like gastropods, but they
have no coiling of their visceral mass.
Many chitins graze on seaweeds and small invertebrates
on rocky shores.
There's another group of molluscs like the
chitin, but instead of having many shells
they have a single one.
They are called monoplacophorans from the
class monoplacophora.
They were mostly known from fossils and thought
extinct until surviving species were relatively
recently rediscovered.
They have a single shell, but are not gastropods
because they do not have a coiled visceral
mass.
The last group of molluscs we'll talk about
are the Tusk shells or the scaphopods.
And they have open-ended tusks shaped shells.
They primarily live in soft sediment where
they feed on materials within the sediments.
This takes us to the end of our discussion
on molluscs.
Now before our next video I would like you
to think about “What makes land bugs different
then ocean bugs?”
We'll talk about that.
See you then.
