Mollusca IV: Gastropods

• Gastropoda


  More than 75% of mollusks are gastropods
• Basic Body Plan


  • Organs in visceral mass
  • Well developed foot for locomotion
  • Foot has protective operculum at outer end (closes shell opening)
  • Usually protected inside a single coiled shell
• Major Gastropod Groups


  • Archaeogastropoda (old snails)
  • Apogastropoda (new snails)
• Patello


  • Simplest of group, often not (or minimally) coiled (named after kneecap (patella))
• Heterobranchia


  • Contains the most derived (some reduced/absent shell) and pulmonates (land snails, slugs, terrestrial group)
• Gastropod Evolution: Torsion


  1. During larval development, visceral mass twists 180 degrees CCW where right side develops faster

  2. Mantle cavity location changes

  3. The visceral mass organs are twisted around, especially digestive tract and position of anus

  4. Paired organs like osphradia and gills become reduced or absent on one side

  
• Nodal


  Triggers asymmetric development in vertebrates, in snails expression on right side results in cells that proliferate faster than on the left side
• Pitx


  Gene expressed only on the right side of the developing snail embryo
• Torsion


  Right side "bulges", pushes counter-clockwise
• Benefits of Torsion


  • Gills, osphradia brought to front opening of mantle cavity, important organs in back shell and breathing and tasting organs brought to front of shell
  • Better arrangement of nerves
  • Easier retraction into cell
  • Better balance of shell vs body
• Disadvantages of Torsion


  • Anus over head
  • Reduced organs on one side
  • Subsequent evolution directed towards de-torsion
• Patellogastropoda (Limpets)


  • Paired, symmetrical organs: Tentacles, Ctenidia and osphradia, Some blood vessels
  • Shell uncoiled, no torsion

  
• Other basal forms


  • Slipper shell (crepidula fornicata) - are SEQUENTIAL HERMAPHRODITES
  • Conus (Basal w/ a bite) - HIGHLY toxic venom injected via a hollow radula
• Siphon


  • Well developed in predatory species, Reduced in filter feeders, herbivores, deposit feeders
• Digestive system and breathing organs

  • Only on the right, breathing organs on left, allows for unidirectional water flow
• Heterobranchs


  • Reduction and Loss of Shell

  
• Cephalaspideans


  • Reduced, internal shell, name means head shield where shell only covers head
• Anaspideans


  • Internal shell reduced or absent, Some steal chloroplasts from algae food and use them in body wall "photosynthetic animal"
• Nudibranchs (naked gills)


  • Very derived, twists full 180 and then untwists the full 180, Shell completely absent, Instead of simple tentacles, there are rhinophores: highly modified tentacles that are chemosensory (smell organs), Cerata: dorsal projections for gas exchange, Aposematic coloration: warnings of toxicity (some steal and use cnidocytes from hydroid prey)

  
• Pulmonata on Land


  • Highly vascularized mantle cavity that operates as lung, Pneumostome: opening into mantle cavity, Only 90 degree torsion, shell tipped to one side of body, Most are hermaphroditic, Most are terrestrial, Some slugs have lost shell