Lophotrochozoa III: Molluscs Flashcards
Describe molluscs
- hypothesised to be the second biggest phylum
- 100,000 species
- evolutionarily flexible body plan
- through-gut
- not segmented
- shell and mantle
- gills
- radula
What is a shell?
- secreted by mantle tissue
- extracellular calcium carbonate + proteins
- conchin protein precipitates the CaCO3
- direction of the crystals dictates properties
- not moulted
- can only grow at edges
Describe the structure of the cell
- surface protein (periostracum)
- chalky prismatic layer
- pearly, nacre layer
- mantle cells
- different calcium carbonate crystal structures in upper and lower layer
What is a radula?
- specialised feeding structure
- ribbon with teeth, pulled tight over the cartilaginous odontophore (solid block)
Give an example of a mollusc
- Lottia persona (limpet)
- also barnacles
List the 3 major Classes of the Mollusca
- Gastropoda
- Bivalvia
- Cephalopoda
Describe the Aplacophora
- 2 minor Molluscan Classes: Chaetodermomorpha and Neomeniomorpha
- rare
- worm-like
- no shell
Describe the Monoplacophora
- fossils known with a single shell
- series of ‘muscle scars’ suggested ‘almost’ segmented body
- found in 500m deep Pacific Ocean trench
- e.g. Triblidyum reculatum
Polyplacophora
chitons
Scaphopoda
tusk shells
Describe the Gastropoda
- one shell (sometimes lost)
- use radula to scrape algae of rocks, or eat lettuce
- generally slow-moving grazers
- torsion
Describe torsion
- in development, body twists 180° anticlockwise
- hypothesised to have evolved as an advantage in hiding the head
What are the evolutionary contraint of torsion?
waste products released over head and sense organs
Describe the secondary adaptations to torsion
- hole in shell
- lose the right gill for sideways flow of water
- detorsion
Describe the Land slug
- lost the shell
- low Ca++
- slime defence
Describe the Aplysia
- sea hares
- ink cloud on stress
- mating chains
Describe the Aeolid nudibranchs
- no gills
- cerata-housing nematocysts sequestered for defence
- gas bubble in gut: floats at sea surface
- can eat Portugeuse Man-o-war
- 4cm
- e.g. Glaucus atlanticus, the blue sea dragon
Describe the Portugeuse Man-o-war
poisonous Cnidarian
Describe the Conus
- cone snails
- radula is hypodermic harpoon for poison
Describe Bivalvia
- two shells
- ligament opens shell
- filament
- interlamellar junction
- foot in between that can fill with blood; allows burrowing
- no radula; food groove
- W-shaped gills with 1000s of cilia set up massive water current
Describe the Bivalve foot
aka visceral mass
Describe Bivalve burrowing (Kelly Dorgan (2015))
- sand grains are plastic - can be rearranged
- probes with pedal anchor to displace easily moved grains
- lubricates to reduce friction
- localised fluidisation
- adductor muscle contracts
Describe the ecology of the Bivalves
generally sessile or burrowing
Describe the Toredo
- “ship worm”
- reduce shells
- worm-shaped
- eat wood (such as the Mary Rose ship)
Describe the Scallop
- ~60 eyes
- parabolic focussing using argentea (acts as a mirror)
- constructive interference using layers of guanine crystals
- swimmer
Describe Nautilus
- cephalopod (probably ancestral)
- ‘chambered shell’
- slow swimming
Describe the Cephalopoda
- closed blood system
- large brain
- well-developed eyes
- shell reduction
- jet propulsion: water squirted from mantle cavity through tube
- active hunters
Describe Cuttlefish
- internal shell
- buoyancy
Describe squid
- no shell
- pelagic hunters
Describe the Octopus
Benthic ambush predators
Describe Cephalopod morphology from posterior to anterior
- 2 tentacles
- 8 arms
- siphon
- mantle cavity
- body
