molluscs - denver school of the...
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Molluscs
Chapter 16
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Phylum Mollusca
• Phylum Mollusca includes snails and slugs, oysters and clams, and octopuses and squids.
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Phylum Mollusca
• Molluscs have a mesoderm lined body cavity – a coelom.
• They are protostomes – mouth first and 3 layers
– Body Cavity
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Phylum Mollusca
• Molluscs evolved in the sea and most molluscs are still marine.
– Some gastropods and bivalves inhabit freshwater.
– A few gastropods (slugs & snails) are terrestrial.
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Mollusc Body Plan
• All molluscs have three main parts:
– A mantle – gills and protective shell over the visceral mass.
– A head-foot region– movement and attachment
– A visceral mass – digestive, circulatory, respiratory and reproductive organs.
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Mollusc Body Plan
• Most molluscs have separate sexes with gonads located in the visceral mass.
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Shells
• When present, the calcareous shell is secreted by the mantle and is lined by it. It has 3 layers: – Periostracum – outer organic layer helps to protect inner layers.
– Prismatic layer – densely packed calcium carbonate.
– Nacreous layer – protein lining secreted continuously by the mantle – forms pearls in some.
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Shell Structure & Formation
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A Pearl Among the Swine
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Mantle Cavity
• The space between the mantle and the visceral mass is called the mantle cavity.
– The respiratory organs (gills or lungs) are generally housed here.
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Head-Foot Region
• Most molluscs have well developed head ends with sensory structures including photosensory receptors that may be simple light detectors or complex eyes (cephalopods).
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Head-Foot Region
• The radula is a rasping, protrusible feeding structure found in most molluscs (not bivalves).
– Ribbon-like membrane with rows of tiny teeth.
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Head-Foot Region
• The radula in action:
– http://www.youtube.com/watch?v=mLVDwlrSq5U
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Head-Foot Region
• The foot of a mollusc may be adapted for locomotion, attachment, or both.
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Visceral Mass
• All the organs
• Many molluscs have an open circulatory system with a pumping heart, blood vessels and blood sinuses.
• Most cephalopods have a closed circulatory system with a heart, blood vessels and capillaries.
• Heart is located in pericardial cavity.
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Visceral Mass
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Mollusc Reproduction & Development
• Indirect development – Larval stages
– Trochophore larva • Free swimming, ciliated, shell formation begins
– Veliger larva • Free swimming, ciliated velium forms,
• Shell/body torsion occurs
– Spat • Metamorphic form between veliger and juvenile
• Shell elaborates
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Trochophore Larva
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Gastropod Veliger Larva
Velia
Mouth
Shell
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Major Mollusc Classes
• Four major classes of molluscs:
– Class Gastropoda – snails & slugs
– Class Bivalvia – clams, mussels, oysters
– Class Cephalopoda – octopus & squid
– Class Polyplacophora – the chitons
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Class Gastropoda
• Gastropoda is the largest of the molluscan classes.
– 75,000 named species.
– Include snails, slugs, sea hares, sea slugs, sea butterflies.
– Marine, freshwater, terrestrial.
• Benthic or pelagic
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Class Gastropoda
• Twisting process called torsion that occurs during larval stage, the visceral mass is asymmetrical not bilateral.
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Generalized Gastropod Anatomy
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Class Gastropoda
• The shell of a gastropod is always one piece – univalve – and may be coiled or uncoiled.
– Shells may coil to the right or left – this is genetically controlled.
– Columella is the central axis of shell and the snail is attached to.
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Class Gastropoda
• Blood contains hemocyanin.
• Two copper atoms that reversibly bind a single oxygen molecule (O2).
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Class Gastropoda
• Many snails can withdraw into the shell and close it off with a thorny operculum.
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Gastropod Feeding Habits
• Most gastropods are herbivores and feed by scraping algae off hard surfaces using the radula.
• Some are scavengers of dead organisms.
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Gastropod Feeding Habits
• Some are carnivores and have a radula modified into a drill to bore through the shells of other molluscs. They use chemicals to soften the shell.
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Gastropod Feeding Habits
• Snails in the genus Conus feed on fish, worms, and molluscs.
– Highly modified radula used for prey capture.
– They secrete a toxin that paralyzes their prey.
• Some are painful, even lethal, to humans.
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Gastropod Feeding Habits
• Will “hibernate” during dry periods (estivation) when metabolism drops to almost nothing and their shells will be covered with protective mucous film when dried is called an epiphragm.
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Internal Form and Function
• Respiration is done with Ctenidium(gills).
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Internal Form and Function
• Pulmonates lack gills. – Have a highly vascular area in mantle that serves as lung.
– Lung opens to outside by small opening, the pneumostome.
– Aquatic pulmonates surface to expel a gas bubble and inhale by curling, thus forming a siphon.
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Internal Form and Function
• Most have a single nephridium and well-developed circulatory and nervous systems.
• Sense organs include eyes, statocysts, tactile organs, and chemoreceptors.
• Eyes vary from simple photoreceptors to a complex eye with a lens and cornea.
• Style Sac at posterior end of the stomach, is ciliated and rotates to consume food into a string like mucus.
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Major Groups of Gastropods
• Prosobranchia includes most marine snails and some freshwater and terrestrial gastropods. – Mantle cavity is anterior due to torsion. – Have one pair of tentacles, separate sexes, and usually an
operculum.
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Major Groups of Gastropods
• Opisthobranchia includes sea slugs, sea hares, sea butterflies, and canoe shells. – Most are marine, shallow-water. – Partial to complete detorsion - anus and gill(s) are displaced to right side. – Two pairs of tentacles, one pair modified to increase chemo-absorption. – Shell is reduced or absent. – Cerata instead of gills for respiration.
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Marine Gastropods
Cowries
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Marine Gastropods Sea Hare – Aplysia sp
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Marine Gastropods – Nudibranch (Sea Slug)
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Major Groups of Gastropods
• Pulmonata includes land and most freshwater snails and slugs. – Aquatic species have one pair of tentacles.
– Landforms have two pair of tentacles and the posterior
pair has eyes.