Mollusc Classification
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Taxonomy
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Class Polyplacophora: chitons
Class Scaphopoda: tusk shells or tooth shells
Class Bivalvia: clams, oysters, mussels
Class Gastropoda: snails, slugs, limpets, whelks
Class Cephalopoda: Octopus, squid, nautilus
Class Polyplacophora: chitons
 Exclusively marine, chitons are somewhat
dorsoventrally flattened molluscs with a
dorsal surface covered with a series of
usually 8 (rarely 7) overlapping,
articulated, calcareous plates.
 Hence, the name Polyplacophora
(“bearing many plates”).
Figure 16.09a
10.8
Class Polyplacophora: chitons

Most chitons are small (<5cm), the largest
reaching 30cm. About 650-800 living species.

Most often found on rocky surfaces in the
intertidal zone where they graze on algae, which
they scrape off using their radula.

Chitons grip their substrate firmly and are very
hard to dislodge. If displaced, they curl into a
ball for protection like an armadillo.
Class Scaphopoda: tusk shells

Scaphopoda (‘shovel foot’) are marine molluscs
that have a long tusk-like shell open at both
ends. Most of the 350 or so species are found in
the deep sea, but some occur in shallower
water.
Class Scaphopoda: tusk shells

Burrow into sediments using their foot and the
narrow end of the shell protrudes into the water
above. They lack gills and gas exchange occurs
across the mantle itself.
Class Scaphopoda: tusk shells

Most food is detritus and protozoa and other
small organisms found in the sediment.

Food is captured using threadlike mucuscovered tentacles called captacula which have
adhesive knobs on their end.
Figure 16.11
Fig 10.10
Tusk Shell Hermit Crab
Tusk Shell Hermit Crab
Class Bivalvia
 The Bivalvia (two valves/shells)
sometimes referred to as the Pelecypoda
(Greek: hatchet foot).
 These are the two-shelled molluscs and
include the mussels,
scallops, oysters, clams,
and shipworms.
Class Bivalvia
 The Bivalvia are the second largest class
of molluscs (gastropoda 1st) with
approximately 15,000 species.
Characteristics of Class Bivalvia

Bivalves are laterally compressed.

Two shells (valves) are
held together by a hinge
ligament.
Characteristics of Class Bivalvia
 Adductor muscles and hinge open/close
shells.
Figure 16.26c
Figure 16.26d
Figure 16.31
Fig 10.22 show adductor muscles
10.23 c adductors open and shut.
Gas Exchange and Filter Feeding
 In some marine bivalves the incurrent and
excurrent openings are extended into long
siphons formed by the mantle being
extended into tubes.
 The siphons allow a bivalve to burrow
beneath the sediment, yet still respire and
feed.
Figure 16.28bcd
Figure 16.30
10.27
Gas Exchange and Filter Feeding
 In addition to gas exchange, the gills are
involved in filter feeding.
 Gland cells secrete large quantities of
mucus. This mucus traps particles
suspended in the water passing through
the gill pores.
Gas Exchange and Filter Feeding

The particles trapped in the mucus move along
ciliary tracts to the mouth where they are
consumed; particles not ingested =
pseudofeces.
Gas Exchange and Filter Feeding

The particles trapped in the mucus move along
ciliary tracts to the mouth where they are
consumed; particles not ingested =
pseudofeces.
“false poop”
Gas Exchange and Filter Feeding

Filter feeding in bivalves is very efficient and
extremely small particles are captured.

Mussels, for example, filter out essentially all
particles 3-5 microns in diameter and 50% of
particles 1-2 microns in diameter.
Class Bivalvia

The foot like the rest of the body is also laterally
compressed and this helps the bivalve work its
way into soft sediments.

To burrow, a bivalve extends its foot and pumps
blood into it, which causes the foot to swell.
Class Bivalvia: reproduction
 In bivalves the sexes are separate and
fertilization is usually external.
 Marine larvae typically go through three
free-swimming stages before settling:
trochophore, veliger and spat.
Class Bivalvia: reproduction
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In freshwater clams fertilization usually occurs
internally and the larvae go through early
development in a brood chamber.

The larvae develop into specialized veligers
called glochidia.

These are released into the water and attach to
passing fish’s gills where they feed and hitchhike
as parasites for several weeks before dropping
off and settling.
Glochidium larvae
Figure 16.35a
Glochidium attached to fish gills
Class Gastropoda
 The Gastropoda are by far the largest
class of molluscs.
 There are about 40,000
living species and 15,000
known fossil species.
 They include: snails, slugs, whelks,
limpets, conchs, sea slugs, sea hares and
periwinkles.
Class Gastropoda
 Most gastropods are slow moving as most
have heavy shells and depend on using
their single slow moving foot to get
around.
 The shell, if present,
is almost always a
one-piece shell and
is often coiled.
Class Gastropoda
 Basic body form
Class Gastropoda
 Basic body form
Class Gastropoda
 Some snails possess an
operculum a
proteinaceous lid that can be used to close
the shell’s opening.
 Closing the operculum reduces water loss.
Class Gastropoda
 Gastropods are basically bilaterally
symmetrical.
 However, because of the process of
torsion, a twisting process that occurs
during the gastropod’s embryonic/larval
development, the visceral mass has
become asymmetrical.
10.11
Class Gastropoda

Torsion moves the mantle cavity to the front.

Because of the twisting of the viscera the anus
now opens anteriorly. As a result, there is a
problem with fouling as waste tends to wash
back over the gills.

Why? Sense organs up front and/or more space
in shell for foot to fit.
Gastropoda: feeding habits

Being such a large group it is not surprising that
gastropods feed on a wide variety of different
foods.

All of them however make some use of the
radula in foraging.

Many are herbivores and feed by rasping algae
or vegetation with their radula. Others are
scavengers or predators and use their radular
teeth to tear apart their prey.
Gastropoda reproduction

There are both dioecious and hermaphroditic
gastropods.
Gastropoda: Classification
 Major groups of gastropods.
Three
subclasses are traditionally recognized:
Prosobranchia: periwinkles, whelks, oyster
borers, and limpets
Opisthobranchia: sea slugs, sea butterflies, sea
hares
Pulmonata: land snails (some marine) and
slugs
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Prosobranchia: periwinkles, whelks, oyster
borers, and limpets
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Opisthobranchia: sea slugs, sea butterflies, sea
hares
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Pulmonata: land snails (some marine) and
slugs
Cephalopods
 The cephalopods (“head foot”) are the
most complex of the molluscs and are
among the most complex of all
invertebrates
 All are marine and the group includes
squids, octopuses, cuttlefishes and
nautiluses.
Cephalopods
 All of the cephalopods have tentacles that
they use to grasp and manipulate objects.
 Octopuses have 8, cuttlefish and squids
have 10 including (in squids) two extended
arms that are flattened at the end.
Nautiluses have 38 suckerless tentacles.
Cephalopods
 Tentacles are equipped with suckers, and
in squids many of them also have hooks
on them.
 Cephalopods possess a radula, but more
importantly, have a horny parrotlike beak
which they can use to bite and tear tissue.
They also inject poison when they bite.
Cephalopods
 The brain in cephalopods is well
developed and octopuses in particular are
quite intelligent. They can solve problems
and have a memory.
 The enlarged brain also equips them for
an active predatory lifestyle.
Cephalopods: vision
 The cephalopods possess eyes that are
comparable in structure to those of
vertebrates, possessing a retina, lens,
cornea, and iris. There is some debate as
to how well they see, but in certain
aspects their eye is better designed than
that of humans.
Cephalopods: locomtion
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Crawling one bottom with tentacles
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Using tentacles to “swim”
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Jet propulsion (suck water into mantle, direct
flow OUT through a siphon or through mantle
opening)
Crawl/walk
swim
Jet propulsion
Cephalopods: chromatophores

The skin of most cephalopods contains
chromatophores, which are pigment-containing
cells that can be expanded or contracted to
change the animals color.

Tiny muscles attached to the periphery of the
chromatophores contract to expand the cell into
a plate and make the color more obvious or
relax to concentrate the pigment into a spot and
make it less obvious.
Reproduction in Cephalopods
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Almost all cephalopods are dioecious.
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Sperm are encased within a spermatophore and
are kept until needed in a storage gland.
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Mating involves copulation, but males do not
possess a penis. Instead, one of the arms is
modified tentacle used to transfer
spermatophore.
Reproduction in Cephalopods
 There are no larval stages in cephalopod
development as development occurs
directly within the egg.
Cephalopods: Giant squid
 The cephalopods include the largest of all
invertebrates the giant squid and the
colossal squid.
 These deep sea species can measure
(including tentacles) as much as 60 feet
and weigh up to 1,000 lbs. The mantle
length is 15-18 feet with the head about 3
feet.
Giant squid washed ashore in
New Zealand in 1996
Cephalopods: Giant squid
 Giant squid are known mainly from
stranded specimens or those trapped in
fishing nets and live specimens have not
been studied, although a few photos of live
ones have been taken.
Washed up off coast of Chile
In water off coast of Japan
Cephalopods: Giant squid
 Giant squid are predators and appear to
have excellent vision (they have the
biggest eyes of any known organisms [up
to 10 inches in diameter]).
 They are believed to prey mainly on fish
and other squid.
Cephalopods: Giant squid
 For many years giant squid were known
only from the sucker marks they left on the
skin of sperm whales and their beaks
taken from sperm whale stomachs.
 Based on how often they have turned up
in sperm whale stomachs, giant squid
appear to be an important component of
their diet.