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Molluscs
CHAPTER 10
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Characteristics
Phylum Mollusca
• Over 90,000 living species and 70,000 fossil species
• Soft body and belong to the lophotrochozoa
protostomes
• Include chitons, tusk shells, snails, slugs,
nudibranchs, sea butterflies, clams, mussels, oysters,
squids, octopuses, and nautiluses
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Characteristics
• May weigh up to 900 kg and grow to nearly 20 m
long, but 80% are under 10 cm in size
• Herbivorous grazers, predaceous carnivores, filter
feeders, and parasites
• Most are marine, but some are terrestrial or
freshwater aquatic
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Characteristics
Economics
• Many are used as food
• Culturing of pearls and pearl buttons is an important
industry
• Burrowing shipworms destroy wooden ships and
wharfs
• Snails and slugs are garden pests
• Some snails are intermediate hosts for parasites
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Form and Function
Mollusc Body Plan: Head-Foot and
Visceral Mass Portions
• Head-foot region contains feeding, cephalic sensory, and
locomotor organs
• Visceral mass contains digestive, circulatory, respiratory,
and reproductive organs
• Mantle Cavity
– Two folds of skin form protective mantle
– Mantle cavity houses the gills or a lung
– In most molluscs
• Mantle secretes a shell over the visceral mass
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Form and Function
• Radula
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Unique to molluscs
Found in all except bivalves
Protruding, rasping, tongue-like organ
Ribbon-like membrane has rows of tiny teeth (up to 250,000)
pointed backward
– Radula rasps off particles of food from surfaces
– Serves as a conveyor belt to move particles to digestive tract
– New rows of teeth replace those that wear away
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• Foot
Form and Function
– Usually ventral
– Functions in attachment to substratum or for locomotion
– Modifications include
• Attachment disc of limpets
• Siphon jet of squids
– Secreted mucus aids in adhesion or helps molluscs glide on
cilia
– Free-swimming forms have modified the foot into wing or
fin-like swimming agents
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Form and Function
• Shell
– If present, secreted by the mantle and lined by it
– Periostracum
• Outer horny layer
• Composed of conchiolin, a tanned protein
– Middle prismatic layer
• Closely packed prisms of calcium carbonate
– Inner nacreous layer
• Next to the mantle; the nacre is laid down in thin layers
– Thick periostracum of freshwater molluscs protects against
acid from leaf decay in streams
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Form and Function
• Internal Structure and Function
– Open circulatory system
• Pumping heart, blood vessels, and blood sinuses
– Most cephalopods have a closed system with a heart,
vessels, and capillaries
– Most molluscs have a pair of kidneys
– Kidney ducts also discharge sperm and eggs
– Nervous system
• Pairs of ganglia but generally simpler than in annelids
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Form and Function
• Reproduction and Life History
– Most dioecious, some hermaphroditic
– Egg hatches and produces a free-swimming trochophore
larva
– Larva undergoes direct metamorphosis into a small juvenile
in chitons
– In many gastropods and bivalves
• Intermediate larval stage, the veliger
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Classes of Molluscs
Class Monoplacophora
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Previously considered extinct
Living specimen was discovered in 1952
About 25 extant species now known
Small molluscs with a rounded shell, resemble
limpets
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Classes of Molluscs
Class Polyplacophora: Chitons
• Chitons are somewhat flattened with 7or 8 dorsal
plates
• Head and cephalic organs are reduced
• Most prefer rocky intertidal surfaces
• Chiton radula is reinforced with iron mineral
– Scrapes algae from the rocks
• Mantle extends around margin
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Classes of Molluscs
Class Scaphopoda
• Tusk or tooth shells
• Live on the ocean bottom from subtidal zone to 6000
m depth
• Slender body covered with a mantle
• Tubular shell is open at both ends
• Unique body plan
– Mantle is wrapped around the viscera and fused to form a
tube
– Foot protrudes from larger end to burrow into mud
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Classes of Molluscs
Class Gastropoda
• Most diverse class
• Over 70,000 living and more than 15,000 fossil species
• Snails, limpets, slugs, whelks, conches, periwinkles,
sea slugs, sea hares, and sea butterflies
• Forms range from marine forms to air-breathing
terrestrial snails and slugs
• Typically sluggish, sedentary animals
• Shells are chief defense
– Some produce distasteful or toxic secretions
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Classes of Molluscs
– Many snails have an operculum covering shell aperture
– Snails serve as intermediate hosts to many parasites and
are often harmed by larval stages
– 3 gastropod subclasses
• Prosobranchia
• Opisthobranchia
• Pulmonata
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Classes of Molluscs
• Form and Function
– Torsion
• Developmental process that changes the relative
position of the shell, digestive tract and anus, nerves
that lie on both sides of the digestive tract, and the
mantle cavity containing the gills
– Contraction of a foot retractor muscle pulls shell and
viscera 90 counterclockwise
– Moves anus to the right side of the body
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Classes of Molluscs
• Coiling
– Coiling or spiral winding of the shell and visceral mass not
the same as torsion
– Occurs at same larval stage as torsion but had a separate,
earlier evolutionary origin
– All living gastropods descended from coiled, torted
ancestors
– Shifting the shell upward and back helped balance uneven
weight distribution
– Gill, auricle and kidney of right side are lost in most species
– Loss of the right gill provides one solution to the problem
of fouling
• Wastes expel to the right
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Classes of Molluscs
• Feeding Habits
– Adaptation of the radula provides much variation in
gastropod feeding habits
– Many are herbivorous and graze, browse or feed on
plankton
– Some scavenge decaying flesh
– Others carnivores that tear prey using radula
– Oyster borer alternates rasping with chemical softening of
the shell to bore a hole
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Classes of Molluscs
– Species of Conus deliver a lethal sting to secure prey
• Venom is a conotoxin
• Specific for the neuroreceptors of its preferred prey
– Some collect debris as a mucus ball to ingest it
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Classes of Molluscs
• Major Groups of Gastropods
– Traditional classification has recognized three
subclasses of Gastropoda
• Prosobranchia, Opisthobranchia, and Pulmonata
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Classes of Molluscs
– Prosobranchs
• Includes most marine snails and some freshwater and
terrestrial gastropods
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Classes of Molluscs
– Opisthobranchs
• Includes sea slugs, sea hares, sea butterflies, and canoe
shells
• Most are marine, shallow-water and often hide under
stones and seaweed
• Shell is reduced or absent
• Monoecious
• Sea hare Aplysia
– Large anterior tentacles and a vestigial shell
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Classes of Molluscs
– Pulmonates
• Includes all land and most freshwater snails and slugs
• Ancestral gills have been lost and the vascularized
mantle wall is now a lung
• Aquatic species have one pair of tentacles
• Landforms have two pair of tentacles and the posterior
pair has eyes
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Classes of Molluscs
Class Bivalvia
• Mussels, clams, scallops, oysters, and shipworms
• Range in size from 1–2 mm in length to the giant South
Pacific clams
• Most are sedentary filter feeders
• Bivalves lack a head, radula, or other aspects of
cephalization
• Most are marine
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Classes of Molluscs
• Form and Function
– 2 shells or valves are held together by a hinge ligament
– Valves are drawn together by strong adductor muscles
– Umbo is the oldest part of the shell with growth occurring
outward in rings
– Pearls are produced when an irritant is lodged between
the shell and mantle
• Layers of nacre are secreted around the foreign
material
– Visceral mass is suspended from the dorsal
midline
– Foot is attached anteroventrally
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Classes of Molluscs
• Reproduction and Development
– Sexes usually separate
– Fertilization usually external
– Freshwater clams have internal fertilization
• Sperm enter the incurrent siphon to fertilize eggs in water tubes of
the gills
– Larvae develop into a bivalved glochidia stage
• Attaches to gills of passing fish where they live briefly as parasites
• Eventually sink to begin independent life on the streambed
• “Hitchhiking” having helped distribute the species
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Classes of Molluscs
Class Cephalopoda
• Squids, octopuses, nautiluses, devilfish, and
cuttlefish
• All marine predators
• Foot is in the head region
– Modified for expelling water from mantle cavity
• Range from 2 cm to the giant squid
– Largest invertebrate
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Classes of Molluscs
• Form and Function
– Shell
• Nautiloid and ammonoid shells had gas chambers
allowing them to swim
• Cuttlefish shell is enclosed in mantle
• Squid shell is a thin strip called the pen, enclosed in
mantle
• Octopus has completely lost the shell
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Classes of Molluscs
• Locomotion
– Cephalopods swim by forcefully expelling water through a
ventral funnel or siphon
– Nautilus swims mainly at night
– Octopuses mainly crawl on the bottom but can swim
backward by spurting jets of water
• Some with webbing between their arms swim with a
medusa-like action
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Classes of Molluscs
• Nervous and Sensory Systems
– Cephalopod brain is the largest of any invertebrate
– Squids have giant nerve fibers
– Sense organs are well-developed
• Eyes are complex, complete with cornea, lens, and
retina
– Can learn by reward and punishment, and by observation
of others
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Classes of Molluscs
• Communication
– Chromatophores are cells in the skin that contain pigment
granules
– Ink sac empties into rectum;
• Contains ink gland that secretes sepia when animal
alarmed
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Classes of Molluscs
• Reproduction
– Sexes are separate
– In male seminal vesicle, spermatozoa are packaged in
spermatophores and stored
– One arm of male is modified as an intromittent organ
• Removes a spermatophore from mantle cavity and
inserts it into female
– Fertilized eggs leave oviduct and are attached to stones,
etc.
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Phylogeny and Adaptive Diversification
Classification
– Class Monoplacophora
– Class Polyplacophora
– Class Scaphopoda
– Class Gastropoda
– Class Bivalvia
– Class Cephalopoda
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