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Origin and Diversification of Molluscs:
The "Primitive" Molluscs, Bivalvia, and Gastropoda

Phylum Mollusca

• Highly diverse
• > 100,000 species
• Bauplan
• Head
• Foot
• Visceral mass
• Open circulation

Phylum Mollusca

• Mollusc synapomorphies
• Reduction of coelom

Phylum Mollusca

• Mollusc synapomorphies
• Reduction of coelom
• Muscular foot
• Shell (or precursor)

Phylum Mollusca

• Mollusc synapomorphies
• Reduction of coelom
• Muscular foot
• Shell
• Mantle

Mantle and mantle cavity important for understanding molluscan evolution!

• Site of Gas exchange
• Gills (ctenidia)
• Lungs
• Dumping of wastes
• Digestive - anus
• Excretory - nephridiopore
• Sensory structures - Osphradium (osphradia)
• Elaborated into various structures

Phylum Mollusca

• Mollusc synapomorphies
• Reduction of coelom
• Muscular foot
• Shell
• Mantle
• Radula

Hypothetical Ancestral mollusc - HAM

• An abstraction
• Mollusc synapomorphies
• Reduction of coelom
• Muscular foot
• Shell
• Mantle - with two gills
• Radula

Relationships of the Mollusca

"Primitive" Molluscs: segmentation?

• Caudofoveata
• Worm-like
• Calcareous Spicules
• Pair of Ctenidia
• No segmentation

"Primitive" Molluscs: segmentation?

• Caudofoveata
• Solenogastres
• Worm-like
• Calcareous spicules
• Pair of Ctenidia
• No segmentation

"Primitive" Molluscs: segmentation?

• Caudofoveata
• Solenogastres
• Polyplacophora
• Eight shell plates
• Ctenidia - multiple pairs
• Segmentation?

"Primitive" Molluscs and segmentation

• Caudofoveata
• Solenogastres
• Polyplacophora
• Monoplacophora
• Single shell
• Multiply paired structures
• E.g., ctenidia, muscles, metanephridia

"Primitive" Molluscs and segmentation

• Caudofoveata
• Solenogastres
• Polyplacophora
• Monoplacophora
• Single shell
• Multiple paired structures
  • E.g., ctenidia, muscles,
    metanephridia

Segmentation - General view

• Ancestral molluscs probably were unsegmented
• Phylogenetic argument - No segmentation in Caudofoveata, Solenogastres
• "Segmentation" in Polyplacophora and Monoplacophora may not be true segmentation and was derived independently within Mollusca

Relationships of the Mollusca

(tree)

Bivalves and Scaphopods

• Synapomorphies
• Reduction of head
• Expansion of mantle cavity

Class Bivalvia

• Synapomorphies
• Bivalve shell
• Loss of radula
• Byssus
• Lateral compression

Class Bivalvia

• Feeding
• Generally filter feeding
• Crystalline style
• Figure 9-28

Class Gastropoda

• Synapomorphies
• Torsion
• Greater concentration of visceral mass dorsally

Torsion

• Advantages
• Osphradium now anterior
• Mantle cavity gets clear water
• Head withdrawal
• Problem
• Feces dumped on head!
• Different solutions in different groups of gastropods

Class Gastropoda - Three subclasses

• Three major groups

Subclass "Prosobranchia"

• Paraphyletic
• Marine snails, mostly with shells (some have lost them secondarily)
• Three grades
• Archaeogastropoda
• Mesogastropoda
• Neogastropoda

Grade Archaeogastropoda

• Most primitive
• Paired bipectinate ctenidia
• Paired osphradia
• Medial anus
• Problems
• Feces must vent over head
• Turbulence -> fouling in mantle cavity

Grade Archaeogastropoda

• Most primitive
• Problems
• Feces must vent over head
• Turbulence -> fouling in mantle cavity
• Solutions
• Dorsal perforations

Grade Mesogastropoda

• Loss of right
• Ctenidium
• Osphradium
• Less crowding, better flow!
• Longer rectum
• Feces dumped on right side
• Still have sediment fouling
• Tubulence at bipectinate gill

Grade/Clade Neogastropoda

• Unipectinate gills
• Less turbulence, fouling
• Increased separation of inflow/outflow
• More elongate mantle cavity -> feces farther
• Mantle forms siphon

Subclass Opisthobranchia

• Sea slugs and relatives
• 2° Bilateral symmetry
• Torsion, then detorsion
• Anus moved to rear
• Reduction, loss of shell

Subclass Opisthobranchia

• Sea slugs and relatives
• ...
• Reduction or loss of shell
  • Evolve new defenses, e.g.
  • Swimming
  • Noxious chemicals
  • Kleptocnidism (i. e., they got somebody else's stingers!)

Subclass Opisthobranchia

• Sea slugs and relatives
• ...
• Reduction or loss of shell
• Reduction or loss of mantle cavity and gills
• Respiratory evaginations of epidermis

Subclass Pulmonata

• Land snails, slugs, and freshwater snails
• Ctenidium lost
• Mantle forms a lung (vascularized mantle)
• Pneumostome -- it's just an opening