Mollusks: Mono, Poly, Aplacophora

• Mollusca


  • One of the largest phyla (largest in marine world)
  • Almost 90,000 species
  • Difficult to find universal morphological features
• Near Universal Features


  • Open green box - structure that isn't a shell but is considered related or homologous, "pen" is seen as internalized version of shell
  • Aplacophora and Polyplacophora have spicules! (made of calcium carbonate)
  • Radula is a tongue, used to scrape, bore or etc. (like a cheese grater)
  • Bivalves don't have radula
  • All classes have a MANTLE (covering/wrapping, secretes shell...)
  • Mantle comes out of shell and wraps up and around (have the spikes)
  • Closest it comes to universal morphological feature
• • More than 75% of mollusks are some kind of snail or gastropod
  • 15-20% are the bivalves (scallops, clams, mussels)
• Most groups originated in Cambrian explosion (or little before)
• Gastropods and bivalves have strong increase since origins
• The Basal Mollusks


  • A - no shell
  • Mono - one shell
  • Poly - several shells
  • Some have spicules but no true shell

  
• Monoplacophora


  • Only known from fossils until 1950s (live Monos collected in 1890s but misidentified as another organism)
  • All marine, ALL deep, in a very remote place that's hard for us to access
  • Mantle = 2 channels
  • Once thought to be basal (wrong)
  • Simple uncoiled shell
  • Simple linear through gut
  • Robust nerves/ganglia, (nerve net kinda arranged in a ring) rudimentary heart with a very very basic circulatory system
  • Blood or bodily fluid kinda sloshing around in mantle cavity, one tiny piece of major vessel that leads to heart, and heart pumps blood into gills
• Polyplacophora


  • Common name: chitons
  • "Shell" divided into 8 plates (always 8), spicules in mantle, (like needles)
  • Mantle around perimeter (comes underneath the plates)
  • Large foot for moving (crawl or glide across rocks)
  • Linear through gut
  • Series of gills
• Most mollusks have gills: ctenidia
• Sensory organ near gills: osphradium (chemosensory organ that tries sniff, smell, trying to get sense of outside world)
• Countercurrent Exchange


  1. Water flows over the gills in opposite direction as blood flow inside gills

  2. Diffusion: more concentrated ⇒ less concentrated

  3. When seawater comes in, seawall is 100% saturated, oxygen goes down as it moves across gill, inside gill we have blood (blood has come back from given all O2 to the body)

  4. Blood picks up a lot of O2 as it reaches the edge of gill

  5. Seawater pairs with blood

  
• Aplacophorans


  • No shell
  • Sticky wall
  • Mantle becomes small posterior cloaca: defecation and excretion
  • Foot becomes pedal groove (type 1) or is absent (type 2)
• Alpacaphorans Feeding

  1. Type 1 eats cnidarians and ctenophores

  2. Type 2 burrows in sediments and eats organic matter