Gastropods and Pelecypods How to make a living inside your shell

Phylum Mollusca Ancient Group of Animals Ancient Group of Animals Second “largest” animal phylum Second “largest” animal phylum Over 100,000 extant species described Over 100,000 extant species described Marine, freshwater, terrestrial (flying is the only lifestyle mollusks haven’t accomplished) Marine, freshwater, terrestrial (flying is the only lifestyle mollusks haven’t accomplished) Diversity of body forms Diversity of body forms

Phylum Mollusca Class Gastropoda Class Gastropoda

Phylum Mollusca Class Gastropoda Class Gastropoda

Phylum Mollusca Class Gastropoda Class Gastropoda

Phylum Mollusca Class Gastropoda Class Gastropoda Class Bilvalvia (Pelecypoda) Class Bilvalvia (Pelecypoda)

Phylum Mollusca Class Gastropoda Class Gastropoda Class Bilvalvia (Pelecypoda) Class Bilvalvia (Pelecypoda)

Phylum Mollusca Class Gastropoda Class Gastropoda Class Bilvalvia (Pelecypoda) Class Bilvalvia (Pelecypoda)

Phylum Mollusca Class Gastropoda Class Gastropoda Class Bivalvia Class Bivalvia Class Cephalopoda Class Cephalopoda

Phylum Mollusca Class Gastropoda Class Gastropoda Class Bivalvia Class Bivalvia Class Cephalopoda Class Cephalopoda

Phylum Mollusca Class Gastropoda Class Gastropoda Class Bivalvia Class Bivalvia Class Cephalopoda Class Cephalopoda

Phylum Mollusca Class Gastropoda Class Gastropoda Class Bivalvia Class Bivalvia Class Cephalopoda Class Cephalopoda Class Polyplacophora Class Polyplacophora

Phylum Mollusca Class Gastropoda Class Gastropoda Class Bivalvia Class Bivalvia Class Cephalopoda Class Cephalopoda Class Polyplacophora Class Polyplacophora Class Monoplacophora Class Monoplacophora

Phylum Mollusca Class Gastropoda Class Gastropoda Class Bivalvia Class Bivalvia Class Cephalopoda Class Cephalopoda Class Polyplacophora Class Polyplacophora Class Monoplacophora Class Monoplacophora Class Scaphopoda Class Scaphopoda

Phylum Mollusca Class Gastropoda Class Gastropoda Class Bivalvia Class Bivalvia Class Cephalopoda Class Cephalopoda Class Polyplacophora Class Polyplacophora Class Monoplacophora Class Monoplacophora Class Scaphopoda Class Scaphopoda Class Aplacophora Class Aplacophora

What makes a mollusc a mollusc? The molluscan “bauplan” Body divided into three regions Body divided into three regions Head (typically reduced) Head (typically reduced) Foot Foot Visceral mass Visceral mass

What makes a mollusc a mollusc? The molluscan “bauplan” Body divided into three regions Body divided into three regions Head Head Foot Foot Visceral mass Visceral mass Body has a unique “organ” the mantle Body has a unique “organ” the mantle  Covers the dorsal portion of the animal  Is folded into a “skirt” to form a chamber that houses the gills, and openings for digestive, urinary, and reproductive systems  May have several functions  Secretes a calcareous shell in some forms (bivalves, most gastropods, some cephalopods, monoplacophora, polyplacophra)

General Body Structure of a Mollusk

What makes a mollusc a mollusc? The molluscan “bauplan” Body divided into three regions Body divided into three regions Head Head Foot Foot Visceral mass Visceral mass Body has a unique “organ” the mantle Body has a unique “organ” the mantle Nervous system consists of a pharygeal ring and a two nerve chords Nervous system consists of a pharygeal ring and a two nerve chords “Closed” circulatory system “Closed” circulatory system Coelom is reduced (three sections, one surrounds the heart, one the nephridia and the other the gonads) Coelom is reduced (three sections, one surrounds the heart, one the nephridia and the other the gonads) Bilateral symmetry (may be “lost” in adult forms) Bilateral symmetry (may be “lost” in adult forms) Use cilia for movement (locomotion or to move water) Use cilia for movement (locomotion or to move water)

What makes a mollusc a mollusc? The molluscan “bauplan” Body divided into three regions Body divided into three regions Head Head Foot Foot Visceral mass Visceral mass Body has a unique “organ” the mantle Body has a unique “organ” the mantle Nervous system consists of a pharygeal ring and a two nerve chords Nervous system consists of a pharygeal ring and a two nerve chords Open circulatory system Open circulatory system Coelom is reduced (three sections, one surrounds the heart, one the nephridia and the other the gonads) Coelom is reduced (three sections, one surrounds the heart, one the nephridia and the other the gonads) Bilateral symmetry (may be “lost” in adult forms) Bilateral symmetry (may be “lost” in adult forms) Use cilia for movement (locomotion or to move water) Use cilia for movement (locomotion or to move water)

What makes a mollusc a mollusc? The molluscan “bauplan” Body divided into three regions Body divided into three regions Head Head Foot Foot Visceral mass Visceral mass Body has a unique “organ” the mantle Body has a unique “organ” the mantle Nervous system consists of a pharygeal ring and a two nerve chords Nervous system consists of a pharygeal ring and a two nerve chords “Closed” circulatory system “Closed” circulatory system Coelom is reduced (three sections, one surrounds the heart, one the nephridia and the other the gonads) Coelom is reduced (three sections, one surrounds the heart, one the nephridia and the other the gonads) Bilateral symmetry (may be “lost” in adult forms) Bilateral symmetry (may be “lost” in adult forms) Use cilia for movement (locomotion or to move water) Use cilia for movement (locomotion or to move water)

What makes a mollusc a mollusc? The molluscan “bauplan” Body divided into three regions Body divided into three regions Head Head Foot Foot Visceral mass Visceral mass Body has a unique “organ” the mantle Body has a unique “organ” the mantle Nervous system consists of a pharygeal ring and a two nerve chords Nervous system consists of a pharygeal ring and a two nerve chords “Closed” circulatory system “Closed” circulatory system Coelom is reduced (three sections, one surrounds the heart, one the nephridia and the other the gonads) Coelom is reduced (three sections, one surrounds the heart, one the nephridia and the other the gonads) Bilateral symmetry (may be “lost” in adult forms) Bilateral symmetry (may be “lost” in adult forms) Use cilia for movement (locomotion or to move water) Use cilia for movement (locomotion or to move water)

What makes a mollusc a mollusc? The molluscan “bauplan” Body divided into three regions Body divided into three regions Head Head Foot Foot Visceral mass Visceral mass Body has a unique “organ” the mantle Body has a unique “organ” the mantle Nervous system consists of a pharygeal ring and a two nerve chords Nervous system consists of a pharygeal ring and a two nerve chords “Closed” circulatory system “Closed” circulatory system Coelom is reduced (three sections, one surrounds the heart, one the nephridia and the other the gonads) Coelom is reduced (three sections, one surrounds the heart, one the nephridia and the other the gonads) Bilateral symmetry (may be “lost” in adult forms) Bilateral symmetry (may be “lost” in adult forms) Use cilia for movement (locomotion or to move water) Use cilia for movement (locomotion or to move water)

Gastropods – Introduction Snails, slugs and others Snails, slugs and others The largest group of molluscs (over 40,000 recent species described) The largest group of molluscs (over 40,000 recent species described) Large foot used for locomotion (usually) Large foot used for locomotion (usually) Posses a “radula” (used to scrape food in grazing snails, highly specialized in some groups) Posses a “radula” (used to scrape food in grazing snails, highly specialized in some groups) Shell is coiled – result of “torsion” during larval development Shell is coiled – result of “torsion” during larval development

Gastropods – Introduction Only mollusk group to have terrestrial forms Only mollusk group to have terrestrial forms Many species have well developed eyes Many species have well developed eyes Head often has tactile sensory appendages Head often has tactile sensory appendages

Gastropods – Introduction Only mollusk group to have terrestrial forms Only mollusk group to have terrestrial forms Many species have well developed eyes Many species have well developed eyes Gas exchange via gills (most species) or highly vascularized mantle cavity or “lung” (pulmonate snails & some terrestrial operculate snails) Gas exchange via gills (most species) or highly vascularized mantle cavity or “lung” (pulmonate snails & some terrestrial operculate snails)

Gastropods – general form

Gastropods - Torsion

Gastropods – The radula Scraping tool used to feed Scraping tool used to feed Common to all mollusks (except bivalves) Common to all mollusks (except bivalves) May be modified May be modified

Cone snails – ocean predators In cone snails, the radula has been modified into “darts.”In cone snails, the radula has been modified into “darts.” When the snail senses prey (such as the hapless fish, above) the proboscis shoots out and one poison filled dart harpoons the prey.When the snail senses prey (such as the hapless fish, above) the proboscis shoots out and one poison filled dart harpoons the prey. The poison is a neurotoxin, that immobilizes the preyThe poison is a neurotoxin, that immobilizes the prey Cone snail venom may be fatal to humansCone snail venom may be fatal to humans

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Bivalves - Introduction Clams Clams Over 15,000 recent species Over 15,000 recent species Mostly marine Mostly marine Common inhabitant of deep sea thermal vents Common inhabitant of deep sea thermal vents Freshwater forms Freshwater forms

Bivalves - Introduction Only mollusks to lack a radula Only mollusks to lack a radula Classified based on gill structure Classified based on gill structure Reduced head Reduced head Laterally compressed Laterally compressed Hatchet-shaped foot Hatchet-shaped foot Adapted for burrowing Adapted for burrowing

Bivalves – Life History First larval stage is a trochophore (common to all mollusks) First larval stage is a trochophore (common to all mollusks) Morphs into veliger stage Morphs into veliger stage Veliger morphs into juvenile (has same form as adult) Veliger morphs into juvenile (has same form as adult) Juvenile grows by accretion at mantle margin Juvenile grows by accretion at mantle margin

Bivalves - Anatomy Time to look at you “clam” Time to look at you “clam” Northern quahog Venus mercenaria Northern quahog Venus mercenaria

Bivalves - External Two valves Two valves Valves hinged on dorsal side Valves hinged on dorsal side Notice the growth lines (concentric rings) Notice the growth lines (concentric rings)

Bivalves – Shell Morphology & Physiology Most mollusks secrete a shell Most mollusks secrete a shell Shell is composed of calcium carbonate (same material used by corals) Shell is composed of calcium carbonate (same material used by corals) Protected by periostracum Protected by periostracum Prismatic layer (crystals oriented vertically) Prismatic layer (crystals oriented vertically) Nacreous layer (crystals oriented horizontally) Nacreous layer (crystals oriented horizontally)

Bivalves – Internal Anatomy & Physiology Teeth – keep shells from slipping Teeth – keep shells from slipping Ligament – connective tissue that contracts when shell is opened Ligament – connective tissue that contracts when shell is opened Adductor muscles – contract to allow the animal to “clam up” Adductor muscles – contract to allow the animal to “clam up” Pallial line – attachment point for the mantle to the shell Pallial line – attachment point for the mantle to the shell Pallial sinus – shows position of the siphons Pallial sinus – shows position of the siphons

Bivalves – Internal Anatomy & Physiology Mantle – surrounds the body & apressed to the shell Mantle – surrounds the body & apressed to the shell Contains sensory organs (tactile tentacle, light sensing eye spots Contains sensory organs (tactile tentacle, light sensing eye spots Secretes the shell Secretes the shell Is fused (two halves joined) dorsally, open ventrally Is fused (two halves joined) dorsally, open ventrally May be modified to form siphons May be modified to form siphons

Bivalves – Anatomy & Physiology (Under the mantle) Foot Foot  Primary form of locomotion  Animal pushes foot into substrate  Foot is filled with blood, causing it to expand and grip substrate  Clam pulls body toward foot

Bivalves – Anatomy & Physiology (Under the mantle) Gills (ctenidia) Gills (ctenidia) Respiratory function Respiratory function Secondary function is to filter water to capture food Secondary function is to filter water to capture food Gill structure is used to classify bivalves Gill structure is used to classify bivalves

Bivalve gill evolution Protobranchs (primitive) Protobranchs (primitive)

Bivalve gill evolution Protobranchs (primitive) Protobranchs (primitive) Filibranchs Filibranchs Gills fold back to form a “U”- shaped structure. Mantle cavity divided by gills into a ventral inhalent chamber and a dorsal exhalent chamber. Chambers connect to outside via siphons Gills filter food from the water passing across them. Cilia move water across the gills.

Bivalve gill evolution Protobranchs (primitive) Protobranchs (primitive) Filibranchs Filibranchs Eulamellibranchs Eulamellibranchs

Bivalve gill evolution Protobranchs (primitive) Protobranchs (primitive) Filibranchs Filibranchs Eulamellibranchs Eulamellibranchs Septibranchia Septibranchia

Bivalves – Anatomy & Physiology (Under the mantle) Digestive System Digestive System  Labial palps surround oral opening  Short esophagus  Stomach  Crystalline style

Bivalves – Anatomy & Physiology (Under the mantle) The style – found in many mollusks The style – found in many mollusks Serves several functions Serves several functions  “windlass” to pull food string from esophagus to stomach  Stirring rod  Source of digestive enzymes

Bivalves – Anatomy & Physiology (Under the mantle) Digestive System Digestive System  Labial palps surround oral opening  Short esophagus  Stomach  Crystalline style  Intestine (loops around stomach)  Rectum

Bivalves – Anatomy & Physiology (Under the mantle) Circulatory System Circulatory System Pericardium encloses the heart Pericardium encloses the heart Heart has two auricles and one ventricle Heart has two auricles and one ventricle Circulatory system is open (blood passes from arteries into sinuses in the tissue and then back into veins). Circulatory system is open (blood passes from arteries into sinuses in the tissue and then back into veins). Most clams have haemocyanin as the blood pigment. Some have haemoglobin. Most clams have haemocyanin as the blood pigment. Some have haemoglobin.

Bivalves – Anatomy & Physiology (Under the mantle) Urogenital System Urogenital System Posses two nephridia (similar to kidneys) Posses two nephridia (similar to kidneys) Most bivalves are dioecious Most bivalves are dioecious Gonads are usually adjacent to the intestine Gonads are usually adjacent to the intestine Gonads and nephridia discharge to the posterior portion of the mantle cavity Gonads and nephridia discharge to the posterior portion of the mantle cavity