Transcript Phylum Mollusca

Phylum Mollusca
Mollusks, Annelids, and Arthropods
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All have bilateral symmetry (at least as juveniles)
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All are coelomate (alternative term: eucoelomate)
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All have a complete digestive tract (mouth and anus)
Mollusks (Phylum Mollusca)
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unsegmented
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muscular foot (may be highly modified)
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mantle – membrane present in all; in most,
secretes and maintains one or two calcerous valves
(shells)
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most have a radula (rasping tongue with chitinous
teeth)
Mollusks (Phylum Mollusca)
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Diverse group, over 100,000 living species (2nd
ranking in number among animal phyla)
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Of 7 classes, 4 classes covered:
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Polyplacophora (chitons)
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Gastropoda (gastropods) – snails, slugs, and
nudibranchs (sea slugs)
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Bivalvia (bivalves) – scallops, oysters, clams,
etc.
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Cephalopoda (cephalopods) – octopi, squid,
cuttlefish, etc.
Symmetry of the Mollusk
• Most mollusks are bilaterally symmetrical
A Closer look at Mollusca
The foot - a broad, flat muscular organ that is adapted for
locomotion and attachment
The visceral mass - contains the internal organs
The mantle - a fold of tissue that drapes over the visceral mass;
space between the mantle and the visceral mass is called the mantle
cavity
Mollusks – Body plan
• Usually divided into two parts
1. Head-foot
2. Visceral mass
• Head-foot contains
– Head – contains mouth
– Sensory structures
– Foot - large muscle used for movement
• Visceral mass contains
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Heart
Digestive organs
Excretion organs
Reproduction organs
The Mantle of the Mollusk
• Mantle
– covering of the visceral mass that secretes the shell
– Positives!
• Protection
– Negatives!
• Reduce surface area for gas exchange
• Solved by having gills for gas exchange
The Shell
• The mantle is responsible for secreting the shell.
• The shell is comprised of three layers:
• The outside of the shell is covered by an organic layer periostracum
• The middle prismatic layer is characterized by densely packed
prisms of calcium carbonate laid down in a protein matrix
• The inner nacreous layer
is composed of calcium
carbonate sheets laid down
over a thin layer of protein
Nervous Tissue of the Mollusk
• Ganglia –
– Cluster of nerve cells used for…
• Feeding
• Locomotion
• Sensory information
– Some are incredibly intelligent, more on this
later!
Gills
• The gills of the mollusca are often indicated as one or more
pairs of bipectinate gills, - flattened filaments attached to a
longitudinal axis on either side
Digestive System of the Mollusk
• Radula
– tongue like tissue covered with sharp teeth for
scraping food
• Complete Digestive System
– Some mollusks filter feed, while others are
predators
The Radula
• The mouth cavity of mollusca possesses a specialized rasping organ
called the radula; sits on a cartilaginous structure - odontophore
• Particles of food brought into the mouth are bound in mucous secreted
by the salivary glands
Circulation of the Mollusk
• Most mollusks, (except cephalopods),
have an open circulatory system
– Hemolymph is the fluid that is contracted into
sinuses and “washes” over body organs for
nutrient and gas exchange.
– In other words, unlike a closed circulatory
system, the blood is not kept in capillaries,
veins, and arteries while it moves throughout
the body
Reproduction of the Mollusk
• All Mollusks reproduce sexxually
– Depending on the species of mollusk, some
reproduce externally or internally
– Dioecious- Have male and female
– Some are hermaphroditic
Molluscan Larval Stages
• Most molluscs produce a freeswimming ciliated larvae called the
trochophore larvae
• In some molluscs the trochophore
develops into the adult, but in other
molluscs (e.g., gastropods) there is a
second larval stage called the veliger
Mollusk Larvae
• Trocophore Larvae
– Free-swimming larval stage (first form)
• Veliger larvae
– Free-swimming larval stage with foot, eyes,
tentacles, and shell
Class Polyplacophora
• Chitons!
– Many-plated mollusk
– Lives in shallow marine water
– Feed on algae attached to rock
Mollusks (Phylum Mollusca)
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Polyplacophora – chiton (display only):
• eight overlapping valves
• muscular foot
• thick mantle
• gills
Class Polyplacophora (Chitons)
• Common on the rocky surfaces of the intertidal zone
• Head is poorly developed; ventral surface occupied by a broad,
flattened foot
• Has a dorsal shell composed of 8 overlapping plates, arranged linearly
along the anterior-posterior axis
• Within the grooves
lie many bipectinate
gills
Class Gastropoda
• Single external shell
• Radula for scraping
food
• Moves in wave like
contractions through
slime
• Can be
hermaphrodites,
though commonly not
• Ex) Snails, slugs
Class Gastropoda
• Snails live on land or in fresh or salt water
– They have eyes on tentacles on their head
• Slugs live on land and Sea Slugs (a.k.a.
nudibranches) live in the ocean
– Slugs do not have shells
– Have exchange of oxygen (diffusion) across their
entire body
Gastropoda
• More active than mono and
polyplacophorans
– Highly cephalized: tentacles, eyes
• Gonochoristic
• Veliger larva (an advanced version of the
trochophore larva)
Class Gastropoda
• Three evolutionary innovations occurred among the gastropods:
changes in the shell, increased development of the head, the
embryonic process of torsion
1. Changes in the Shell
• The shell became higher and
conical with a reduced aperture
• The shell also became coiled
• Shells initially were planospiral
- bilaterally symmetrical shell
with the whorls lying in the same
plane
• Modern day shells are
asymmetrical - each successive
coil is a little outside and offset a
little above the one below
Torsion is unique to gastropods
Most gastropods are dextral
Pretorsion
Post torsion
3. The Embryonic Process known as Torsion
During embryonic development, 1 side of the visceral mass grows at
a much faster rate than the other.
• Causes the visceral mass to rotate 180 degrees relative to the headfoot.
• Advantages: head retracted first; gills receive water currents; the
osphradium is now directed anteriorly
• Disadvantage: may cause fouling
Adaptations to Avoid
Fouling
• Improved separation of
inhalent and exhalent water
flow
• In some of the more primitive
gastropods (keyhole limpets),
the shell contains a hole at the
top through which the exhalent
water stream exits
• In the more advanced
gastropods, water is brought
into the mantle cavity on the
left side, passes over a single
gill, and exits the right side
Shell
• Most have a single,
spiraled shell and can
move the entire head
and foot into this
shell for protection.
• Also, many
gastropods have a
hardened plate called
the operculum on
the back of the foot
that plugs the shell
aperture when the
body is withdrawn
Nutrition
• Many gastropods are herbivores and use their radula scrap algae
from surfaces of rocks
• Some gastropods are active predators and in these the radula is often
highly modified, e.g., as a drill (oyster drills) or harpoon (venomous
gastropods)
Cone snail
Respiration
• Aquatic gastropods possess gills for respiration
• Terrestrial gastropods obtain oxygen via a well vascularized
mantle
Vascularized
mantle
gills
Pulmonata
• Highly vascularized mantle for
gas exchange (lung)
• 17,000 spp: slugs, pond snails
Snail infestations!
Sea Slugs!
Sea Slugs!
Mollusks (Phylum Mollusca)
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Bivalvia – clam:
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two valves, connected by dorsal hinge
ligament
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adductor muscles (used by living clam to close
the shell)
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gills in mantle cavity
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wedgelike foot
Bivalves
• Sessile
• Filter-feeders
• Extend the muscular
foot into the sand to
anchor it
• No specific head –
but has ganglia
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Class Bivalvia – Two Shells
• Three layers make up the shell of a bivalve
– Inner most protects the body of the animal
– Middle layer strengthens the shell with calcium
carbonate
– Outer layer protects against acid in the water
• Pallial muscles insert on the underside of the
shell and are attached to the free edge of the
mantle; pull the mantle under the shell
• Muscles fused across the width (from left to
right) at 1 anterior and posterior position and
form adductor muscles; connect the 2 shell
across their width; close the shell
• When relaxed, shell swings open due to elastic
ligaments of the hinge
Class Bivalvia
• Shells divided into 2 equal halves or
valves
• Mantle tissue is indented in the anteriorposterior margins, with 2 centers of
calcification
• Shells joined at the dorsal midline by a
non calcified protein ligaments called the
hinge
Bivalves
• No radula
• sexual reproduction
• Incurrent and excurrent
siphons
– Help filter water in and out
of the clam
Movement of the Ventilating Currents
Adaptive Radiation of Bivalves con’t
Unattached Surface Dwellers
• Rest unattached on the substrate
• Capable of limited locomotion by rapid clapping of their valves
using a powerful adductor muscle; forces a jet of water out of the
mantle cavity
Hard Bottom Burrowers
• Several species of bivalves are capable of burrowing into hard
surfaces such as rock, coral, wood
• Use the anterior margins of their shell to chip away at the rock;
some secrete chemical to breakdown rock
Adaptive Radiation of Bivalves
Reproduction
• Most are dioecious
• Marine forms usually
produce free swimming
trochophore and veliger
larvae
• Many of the freshwater
bivalves have a different life
history pattern; produce
larvae called glochidia
• Glochidia are housed in the
outer gills; they use there
outer gill as a brood camber marsupium
• When the glochidia are
released they parasitize the
fins and gills of fishes
Mollusks (Phylum Mollusca)
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Cephalopoda – squid:
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muscular foot with tentacles (with suction
discs)
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thick mantle but no external shell (true for
most cephalopods)
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excurrent siphon (for jet propulsion)
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large eyes
Class Cephalopoda
• Means “head foot”
• Includes octopuses,
squids, cuttlefishes and
chambered nautiluses
• Free swimming and
predatory
• Tentacles with suction
cups to grasp prey
• Have jaws called beaks
to destroy their prey
• Has the largest
invertebrate brain
Cephalopods
• Can learn to perform
tasks
• Has complex eyes
• Closed circulatory
system
• Are sexual - internal
• Have ink to confuse
predators
Cephalopoda
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Swift, agile carnivores
Closed circulatory system, 2 hearts
Separate sexes
Foot modified to form arms, tentacles,
siphon
Brain, cranium, complex image-forming
eye
700 extant spp, 10,000 extinct spp
Arose from limpet-like monoplacophorans
Ergo, ventral became function anterior, etc
Cephalopods
Cephalopoda
• Endocochleate cephalopds
– Reduce internal shell, or shell absent
– Squids, cuttlefish, octopi
Class Cephalopoda
• Fast moving predators of the
marine environment
• Cephalopods evolved following
major readjustments in the mollusca
body plan:
• Dorso-ventral axis became
elongated and the anterior-posterior
axis became compressed
• Migration of the head to the
ventral part of the body where it
fused to the foot
• The foot is modified as a series of
prehensile tentacles or arms
• A circle of 8 or 10 tentacles
surround the head; studded with
suckers and are used to capture
prey.
Class Cephalopoda
• Fast moving predators of the
marine environment
• Cephalopods evolved following
major readjustments in the mollusca
body plan:
• Dorso-ventral axis became
elongated and the anterior-posterior
axis became compressed
• Migration of the head to the
ventral part of the body where it
fused to the foot
• The foot is modified as a series of
prehensile tentacles or arms
• A circle of 8 or 10 tentacles
surround the head; studded with
suckers and are used to capture
prey.
Locomotion
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Cephalopods are excellent swimmers: streamlined body;
tentacles and fins as stabilizers
• Swim by means of jet propulsion, using the highly modified
muscular mantle and the siphon
– By relaxing the mantle the mantle cavity is expanded and water can be
drawn in
– By contracting the mantle water can be forced out of the mantle cavity by
means of the small siphonal opening
Feeding
• Cephalopods are carnivores
• Have a powerful parrot like beak that is used to tear prey apart.
• They also have a powerful radula
• In some of the octopuses the salivary glands are modified poison
glands
Shell
• Primitively the cephalopods possessed a shell; the fossil record
indicates both coiled and non-coiled shells
• Extant members with coiled shells include Nautilus
• Some cephalopods (cuttlefishes) have an internal shell - cuddle bone
•The octopods have lost the shell entirely
Other General Features
• For protection, they possess an ink
sacs
• Cephalopods have well-developed
sense organs, including a camera type
eye
• Some have well-developed brains and
show a remarkable capacity for
learning.
• Cephalopods are the only molluscan
class with a closed circulatory system
Cephalopoda
Posterior surface
Right
Ventral
Dorsal
Left
Tentacle
Arm
Funnel (siphon)
Collar
Eye
Fin
Shell (Pen)
Systemic
heart
Branchial heart
Ctenidium
Funnel
Hectocotylus (sperm-bearing arm in males)
Reproduction: trochophore and veliger are bypassed and hatch into planktonic juveniles
eye
Optic lobe
Buccal ganglia
Cerebral ganglion
statocyst
esophagus
Brachial nerves
Brain is surrounded by a cranium
Cephalopod eye
Iris
Retina
Optic nerves
Lens
Cornea
Squid and octopus: basic facts
A standard squid has:
Two fins
A mantle
A head
8 arms and two tentacles,
each endowed with hooks and/or suckers and sucker rings
A standard octopus has:
A mantle
A head
8 arms endowed with one or two rows of suckers
(but never hooks or sucker rings); they have no tentacles
Giant optopus
Examples of Cephalopods
• Close-up view of an unknown species
of bathypelagic squid encountered by
ROV Tiburon at 3,380 meters depth off
the coast of Oahu.
• This animal was estimated to be four
to five meters in length.
• Different from other squids in that
their eight arms and two tentacles are
roughly equal in length and thickness.
• A giant squid (3.15-metre-long) has
netted off the UK coast; first time in
15 years.
• The squid, believed to be female and
three years old, did not survive being
brought to the surface.
The Mimic Octopus
Cephalopoda
• Ectocochleate cephalopods
– Have external shell with internally
subdivisions used for buoyancy control
– This ancestral group is almost completely
extinct
– E.g. Nautilus
Nautilus is the only cephalopod with an
external shell and lacking chromatophores
Chromatophores (color cells)
Iridocytes (reflective cells)
- Millions of these allow rapid changes in
color, polarized signals
- Also have photophores for
bioluminescence
Cephalopods except Nautilus have ink sac
Types of Cephalopods
• Squids have 10
tentacles
• Jet propulsion
through excurrent
siphon to move
• The colossal squid is
the world’s largest
invertebrate
• Octopuses have 8
tentacles
• Jet propulsion to
capture prey but
crawl along ocean
floor
Blue-ringed Octopus
-When threatened, faint blue rings become vivid to warn
predators to back off
-it’s one of the most deadly venomous animals
-bites prey with its jaws
-injects a venomous saliva into the wound
- venom contains tetrodotoxin, one of the most potent
neurotoxins known
-Human death from heart & respiratory failure occurs within
minutes. This non-aggressive octopus only bites people in
self-defense.
Reflection:
• Mollusks are some of the most diverse
phyla that we have seen so far.
• Reflect on how the differences in
adaptations across the different classes
can help the individual species in each
class to survive!