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Fluted Giant Clam

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Characteristics
 Over 90,000 living species and 70,000 fossil species
 Soft body and protostomes
 Include chitons, tusk shells, snails, slugs, nudibranchs, clams, mussels, oysters, squids, octopuses, and nautiluses
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Chiton
Pacific Giant Clam
Marine Snail Nudibranch
Octopus

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Characteristics
Characteristics:
 Herbivorous grazers, predaceous carnivores, filter feeders, and parasites
 Most are marine, but some are terrestrial or freshwater aquatic
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Characteristics
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Fossil evidence
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Indicates molluscs evolved in the sea
Most have remained marine
Some bivalves and gastropods
 Moved to brackish and freshwater
Snails (gastropods) successfully invaded land
 Limited to moist, sheltered habitats with calcium in the soil
Cephalopods
 Evolved to become relatively intelligent
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Characteristics
 Many are used as food
 Culturing of pearls is an important industry
 Snails and slugs are garden pests
16-8

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Form and Function
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Head-foot region contains feeding, sensory, and locomotor organs (foot)
Visceral mass contains digestive, circulatory, and reproductive organs
Mantle Cavity
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Space between mantle (sheath of skin) and body wall is the mantle cavity
Mantle cavity houses the gills or a lung
In most molluscs
 Mantle secretes a shell over the visceral mass
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16-10
Generalized Mollusk Anatomy

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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
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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
Pattern and number of teeth are used in classification of molluscs
 Some specialized to bore through hard material or harpoon prey
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Radula

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Form and Function
 Foot
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Functions in attachment or locomotion
Modifications include
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Hatchet foot of clams
Siphon jet of squids
Secreted mucus aids in adhesion or helps molluscs glide
Snails and bivalves extend the foot by engorgement with blood

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Form and Function
 Shell
 If present, secreted by the mantle
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Periostracum
 Outer layer - wears away
 Composed of hardened protein
Middle prismatic layer
 Closely packed prisms of calcium carbonate
 Increases with animal growth
Inner nacreous layer
 Next to the mantle; the nacre is laid down in thin layers
 Aids in Pearl formation
 Shiny layer in abalone, nautilus, and bivalve
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A.Bivalve Shell
B. Pearl Formation from a parasite or sand that enters shell into mantle, becomes covered with nacre
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Form and Function
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 Most dioecious, some hermaphroditic
 Egg hatches and produces a freeswimming trochophore larva
 In many gastropods and bivalves
 Trochophore is followed by intermediate larval stage, the veliger.
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Trochophore larva

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Classes of Molluscs
 Class Polyplacophora - Chitons
 Class Gastropoda - Slugs, Snails,
Nudibranch
 Class Bivalvia - Clams, Mussels
 Class Cephalopoda - Squid, Octopus,
Nautilus

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Classes of Molluscs
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Chitons are somewhat flattened with 7or 8 dorsal plates
Most prefer rocky intertidal surfaces
Chiton radula is reinforced with iron mineral
 Scrapes algae from the rocks
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Mossy Chiton - hairs and bristles aid in defense

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Classes of Molluscs
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 Over 70,000 living
 Forms range from marine forms to airbreathing terrestrial snails and slugs
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, if present, are chief defense
 Some produce distasteful or toxic secretions
 Use process of
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Classes of Molluscs
 Gastropod Shells
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One-piece (univalve)
Apex is smallest and oldest whorl
Whorls become larger and spiral around central axis
Giant marine gastropods have shell up to 60 cm long
 Some fossil forms are 2 meters long
Terrestrial gastropods shells are restricted by soil mineral content, temperature, dryness, and acidity
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16-23
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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
 Digestive tract moves so that anus lies above head
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Classes of Molluscs
 Coiling
 Coiling or spiral winding of the shell and visceral mass not the same as torsion
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Abalone
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Feed on kelp
-herbivore
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Moon Snail feeds on clams and mussels
Radula releases chemicals to soften shell, so they can get to their prey
16-26

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Conus Extends proboscis to capture prey. Then releases Conotoxins to paralyze (lethal to Humans)
Hours later regurgitate scales and bones
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Classes of Molluscs
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Respiration performed by vascular area in mantle cavity that serves as lung
Most have a single nephridium (kidney) and welldeveloped open-circulatory and nervous systems
 Sense organs include eyes, statocysts, tactile organs, and chemoreceptors
Eyes vary from simple cups holding photoreceptors to a complex eye with a lens and cornea. (On tentacle of some)
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16-29
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Classes of Molluscs
 Reproduction:
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Eggs emitted singly or in clusters, and may be transparent or in tough egg capsules
Young may emerge as veliger larvae or pass this stage inside the egg
 Some species, including most freshwater snails, are ovoviviparous
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Eggs of Mollusks
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Eggs - resemble grains of wheat
Egg ribbon of Nudibranch

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Classes of Molluscs

 Traditional classification has recognized three subclasses of Gastropoda
 Prosobranchia,
 Opisthobranchia
 Pulmonata
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Gastropods - Prosobranchia
 Includes most marine snails
 Have one pair of tentacles, separate sexes
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Diodora aspera
Hole in Apex for water to leave
Flamingo Tongue Snails

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Opisthobranches: sea slugs, sea hares, sea butterflies, and nudibranch
Marine, Shell is reduced or absent 2 tentacles,
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Sea Hare
Sea Hare’s defense mechanism- a secretion from its purple gland

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Nudibranch - calcareous spicules for protection

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Pulmonates - Snail and Banana slug
2 tentacles: posterior have eyes
Opening to Mantle Cavity

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Classes of Molluscs
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Mussels, clams, scallops, oysters
Range in size from 1 –2 mm in length to the giant South Pacific clams (1m)
Most are sedentary filter feeders
Bivalves lack a head, radula, or other aspects of cephalization
Contain Siphons
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Mussels
Scallops
Escaping a Sea Star

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Siphons
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Northwest Ugly Clams
Incurrent brings in Food and Oxygen
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Classes of Molluscs
 Form and Function
 2 shells or valves are held together by a hinge ligament
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Valves are drawn together by strong adductor muscles
Umbo is the oldest part of the shell with growth occurring outward in rings
 Posterior edges of the mantle folds form excurrent and incurrent openings
 In burrowing clams, mantle forms long siphons to reach the water above
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Classes of Molluscs
 Locomotion
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Foot is extended out from between the valves
Blood is pumped into the foot
Foot swells and anchors the bivalve in the mud
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Shortening of the foot pulls the clam forward
Scallops clap valves to create a jet propulsion
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16-43
Scallop - developed sensory organs along mantle edges (tentacles and blue eyes)

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Classes of Molluscs
 Gills
 Both mantle and gills perform gaseous exchange
 Siphon used in respiratory
 Water enters incurrent siphon
 Gas diffused out
 Exits through the excurrent siphon
 Circulatory - Open circulatory system
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3 chambered heart has two atria and one ventricle
Blood vessels line gills to receive oxygen
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16-45
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16-46
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Shipworm Bivalves-
Burrows into wood on docks and piers
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Clam -
Symbiotic relationship with Algae to gain most nutrients
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Siphonal Area
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Classes of Molluscs
 Reproduction and Development
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Sexes usually separate
Gametes discharged in excurrent flow
Fertilization usually external
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Embryos develop as trochophore, and veliger larval stages
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
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Attaches to gills of passing fish where they live briefly as parasites
“Hitchhiking” having helped distribute the species
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16-50
Life Cycle of an Oyster

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Glochidium - freshwater clam larva
Attach to fish’s gills by clamping their valve closed.
Stay for several weeks.
Pocketbook Mussel mimics a small minnow, when a
Smallmouth Bass comes to dine, it releases its glochidia

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Classes of Molluscs
Class Cephalopoda
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Squids, octopuses, nautiluses, 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 (60 ft)
 Largest invertebrate
 Nautilus - only one with external shell
 Series of gas chambers in shell helps maintain neutral buoyancy
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Nautilus
A. Feed on a Fish B. Showing Gas filled chambers

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Cuttlefish

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Classes of Molluscs
 Locomotion
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Cephalopods swim by forcefully expelling water through a ventral funnel or siphon
Control direction and force of the water, thus determining its speed
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Lateral fins of squids and cuttlefishes are stabilizers
Nautilus swims mainly at night
Octopuses mainly crawl on the bottom but can swim
 Some with webbing between their arms swim with a medusa-like action
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16-56
Squid - Pen is only remains of shell

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Classes of Molluscs

 With higher oxygen demands, cephalopods have a muscular pumping system to keep water flowing through the mantle cavity
 Circulatory system has a network of vessels conducting blood through gill filaments ( Closed Circulatory System!!
)
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Classes of Molluscs
 Nervous and Sensory Systems
 Cephalopod brain is the largest of any invertebrate
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Squids have giant nerve fibers
Sense organs are well-developed
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 Eyes are complex, complete with cornea, lens, and retina
Can learn by reward and punishment, and by observation of others
 Cephalopods lack a sense of hearing but have tactile and chemoreceptor cells in their arms
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16-59
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Cuttlefish Eye

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Classes of Molluscs
 Communication
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Use chemical and visual signals to communicate
Chromatophores are cells in the skin that contain pigment granules
Contractions of the muscle fibers attached to the cell causes the cell to expand and change the color pattern
Color patterns can be changed rapidly
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Deep-water cephalopods have elaborate luminescent organs
Ink sac empties into rectum; (Not in Nautiloids)
 Contains ink gland that secretes sepia (dark fluid) when animal is alarmed
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Classes of Molluscs
 Reproduction
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Sexes are separate
In male seminal vesicle , spermatozoa are packaged in spermatophores and stored
One arm of male is modified as an intromittent organ, the hectocotylus
 Removes a spermatophore from mantle cavity and inserts it into female
Fertilized eggs leave oviduct and are attached to stones, etc.
Large, yolky eggs undergo meroblastic cleavage (not full cleavage)
 Hatch into juveniles with no free-swimming larval stage
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Copulation in Cephalopods -
Male Octopus uses modified arm