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Molluscs
Peter Shaw
Phylum Mollusca
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One of the most important phyla in
terms of species richness, and
nearly ubiquitous in most
ecosystems
They invariably have a
eponymously soft, mucus-covered
body. Many forms have a hard,
calcareous shell protecting most of
the body.
The mollusc body
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Is unsegmented
Coelomate, but coelom is reduced to
pericardium + gonadial cavity.
The main body cavity is a haemocoel.
Primitively: shelled, with 2 broad body
regions:
1: a non-muscular visceral hump staying
protected inside the shell, wherein most
action is ciliary.
2: a muscular foot, with mouth + sense
organs at anterior end which protrudes
from the shell
The ancestral mollusc
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A favourite parlour game for invert.
Zoologists is to create a
hypothetical ancestral mollusc.
In 1957 a monoplacophoran
Neopilina galatheae was dredged
up from 5000m off Mexico, and
was found to fit the bill - except
that, rather embarrassingly, it
showed signs of metamerism.
Ancestral mollusc (hypothetical)
Phyletic affinities
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Are very much disputed, but current best
guess is close to some obscure worm-like
phyla: sipunculids, echiurids, and more
distantly annelids. These are now classed
with lophophorate phyla as a subgroup within
the lophotrochozoa.
This scenario assumes a ciliary/gliding animal
which in one radiation acquires a shell, the
other evolves lophophorate filter feeding
Respiration
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A single ctenidium
(typically triangular, leaf-like)
Most molluscs are aquatic and
bear paired gills or ctenidia,
lying in the mantle cavity.
These bear many triangular
leaflets, and water is carried
over these by ciliary action.
Water is “tasted” before flowing
over the gill by a sense organ
the osphradium.
The anus and renal pores lie in
the exhalant stream.
Respiration contd.
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Pneumatostome, leading
To mantle-cavity ‘lung’
in banana slug
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Neopilina has no ctenidia, but 5
pairs of muscular gills along its
body. Primitive or derived?
Terrestrial gastropods often lose
ctenidia, and have “lungs” in their
place.
(And a few of the air-breathing
pulmonates then returned to the
sea!!)
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Mouthparts
Usually consist of a unique
molluscan organ: the radula. This is a muscular cord
which bears sharp chitinous teeth, which rasps slowly
away at the surface of food grating fragments into the
mouth.
The radula is born on a supporting structure the
odontophore.
More on mouthparts
Radula teeth vary immensely with diet, an extreme being
Conus, whose teeth are hollow venom-injecting needles.
In Chitons, the radulae are tipped with magnetite for extra
hardness.
In cephalopods the radula is still present but supplimented by a
parrot-like beak.
Feeding
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Primitively this involved food particles being
scraped off a surface by the radula/odontophore
and carried in a mucus rope into the stomach,
where it is size-sorted by cilia.
Filter-feeders retain the mucus rope, but collect
particles off the ctenidia.
Cephalopods bite lumps of flesh with a chitinous
beak prior to radula.
The shell
This is invariably calcium carbonate (aragonite, not
calcite - the crystal structure differs slightly) reinforced
with chitin.
The superficial appearance differs hugely between
groups, from a tight spiral through loose tubes to
nearly flat.
In fact one growth equation with 3 biologically realistic
parameters can fit the whole spectrum
One weird footnote from a weird system: a snail,
unnamed in 2003, inhabiting the deep-sea black
smokers, has been found to supplement its normal
carbonate shell with scales of iron sulphide.
Classification
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This seems to have changed over
the years, due to increased
‘splitting‘ of obscure minor forms in
recent years
To a good first approximation there
are 5 main classes, but you need
to know of another three oddball
classes.
Principal mollusc classes
Cephalopods
Octopi, squids, cuttlefish etc
Gastropods
Slugs, snails winkles etc
Mollusca
Polyplacophora
Chitons, coat-of-mail shells
Scaphopods
elephant tusk shells
Lammellibranchia
bivalves: mussels, oysters etc
Minor classes
Molluscs
Monoplacophora
Neopilina
Aplacophora
neomeniorphs
Chaetodermomorpha
Polyplacophora
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Chitons, or coat of mail shells
Primitive marine molluscs with 8 small
dorsal shells, partly or wholly buried in
flesh.
This is not segmentation but an
adaptation to curling in tight crevices
Feeding is by grazing on algae.
Mantle cavity runs along side, almost up
to head, with paired gills along its length
Scaphopods
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“Tusk shells”: 350 spp
mantle secretes a tubular shell,
open at both ends
Burrow in soft marine sediments,
collecting food particles with sticky
clubbed captactula
Bivalvia =
Lammellibranchia =
Pelecypoda
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C. 20,000 spp
Sedentary/ sessile molluscs encased in
paired shells
Lacking eyes, radula and tentacles,
though these may re-evolve around
mantle edge
Foot retained, often used to burrow into
sediment
Live by filter feeding, mainly using
ctenidia as filtering surfaces
Gastropods
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The most numerous group of
molluscs, with c. 76,000 species
(75%)
Mainly shelled, except for the
nudibranchs
The shell is always single, usually
spiral
Locomotion is by muscular waves
passing along the foot, except for a
few tiny spp that use cilia
Gastropods, 2
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A typical gastropod is a squat
mollusc crawling with its muscular
foot.
The cephalic sensory organs and
radula are well developed but can
retract fully into the shell
Eg common limpet Patella
Many gastropods have a
calcareous/chitinous plug to the
shell - the operculum
Torsion
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All have undergone a
curious modification of
body form known as
torsion
The visceral hump is
rotated 180o so that the
mantle cavity, gills and
anus all point anteriorly
Perhaps to allow the head
to retract into the generous
space of the mantle cavity
+ allow osphradium to
sense oncoming water
Gastropods: torsion
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But having your anus discharging over your head
is poor design, even for a mollusc
The solution is to modify the exhalent stream:
 evolve a special slit in the shell
 lose one ctenidium (R side) giving a
unidirectional flow
 Loss of R gill => loss of R kidney, osphradium
+ R side heart
 L (inhalent) mantle protrudes as a siphon
Many spp have undergone 90o detorsion!
Gastropod orders
Gastropods
Prosobranchs
ancestral gastropods
Opisthobranchs
Mainly marine
pulmonates
mainly terrestrial
Prosobranchs
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Mainly marine “snails”, with
traditional body/shell design
Most graze algae, sessile animals,
or sediment
Many have notable shells, ie
cowries
One order Stenoglossa have
effective predators: Conus spp
hunt+kill fish!
Opisthobranchs
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Marine, shell-less: sea slugs, sea
hares, pteropods etc.
Some are pelagic, hunting cnidaria
one group are pelagic filter feeders
using mucus nets
Ctenidia are reduced or absent:
the body surface is often adequate,
or 2ndry gills have evolved
Pulmonates
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Mainly terrestrial radiations
Mantle cavity becomes an airbreathing lung with a contractile
opening the pneumatostome
Most have a shell, but
stylommatophora have lost it: land
slugs
A few pulmonates have returned to
water!
Class: Cephalopoda
Nautilida
Nautilus
Ammonites
extinct
Octopoda
Teuthida
squids
Vampyrorpha
'Vampire squids'
Sepiida
cuttlefish
Cephalopods
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The most intelligent, fastest moving and
highly modified molluscs
all are marine predators with good/
excellent vision
include the largest invertebrate (giant
squids)
foot is modified into multiple tentacles with
suckers
swim by jet propulsion
Cephalopods
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We are lucky to have alive an
ancestral form, Nautilus (6 spp).
This has a spiral shell used for
protection and buoyancy.
The eye is a pinhole camera with
no lens but a contractile iris
It lives as a deep water scavenger, probably
explaining its survival of the K/T boundary event
Its fossil ancestors had straight shells (<= 4m long)
Ammonites
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Were large open-water Nautiloids that
went extinct at the end of the
Cretaceous
Up to 2m diameter shells which
differed from Nautilus by heavy
sculpturing of sutures
We can only guess at their ecology ?pelagic hunters
Some big bulky forms ?plankton
feeders
Evolutionary trends of their shells are
complex if not haphazard!
Coleoidea
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Is the sub-class containing the
remaining cephalopods
 octopoda
 teuthida
 sepiida
 Vampyromorpha
 These
have a closed blood
circulation, non-ciliated gills, a
good compound eye and well
developed CNS. Are oddly
semelparous
Cephalopod sex
These animals have some remarkable reproductive behaviour.
Sexes are separate and fertilisation is internal, but there is no
penis. Instead one of the male’s arms is differentiated (usually
with smaller suckers at the tip), is used to pull the torpedoshaped chitinous sperm packet out of his genital opening, and
insert it into the female’s body cavity.
In some deep sea squids the sperm packet may be jammed into
non-standard regions - the mantle, or a tentacle base. How
this effects sperm transfer is still unclear - these animals are
effectively impossible to study alive.
Note the reduced suckers on the hectocotylus arm
Hectoctylus
arm on
South African
diamond
squid