Classes of Molluscs

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Classes of Molluscs

• C. Caudofoveata
• C. Solenogastres
• C. Monoplacophora
• C. Polyplacophora (chitons)
• C. Scaphopoda (tusk or tooth shells)
• C. Gastropoda (snails, slugs)
• C. Bivalvia (bivalves)
• C. Cephalopoda (octopus, cuttlefish, squid)

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Classes of Molluscs Caudofoveata

• C. Caudofoveata
• lt120 sp.
• wormlike, marine
• 2-140 mm
• most burrow
• feed on microorganisms/detritus
• no shell but covered w/ calcareous scales
• reduced radula
• resemble the common ancestor of living
  molluscs?

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Classes of Molluscs Solenogastres

• C. Solenogastres
• 250 sp.
• resemble caudofoveates but no radula or gills
• foot has a midventral, narrow furrow (pedal
  groove)
• live freely on bottom
• feed on cnidarians

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Classes of Molluscs Monoplacophora

• C. Monoplacophora
• was considered extinct
• living specimen discovered 1952
• resemble limpets w/ rounded shell
• some organs are repeated
• 36 pairs of gills
• 2 pairs of auricles
• 37 pairs of metanephridia
• 1-2 pairs of gonads
• 10 pairs of pedal nerves
• have radula

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Classes of Molluscs Polyplacophora

• C. Polyplacophora (chitons)
• 1000 sp., marine
• flattened w/ 8 dorsal plates
• head/cephalic organs reduced
• photosensitive structures (esthetes)
• most prefer rocky intertidal surfaces
• radula is reinforced w/ iron mineral
• scrapes algae from rocks

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Classes of Molluscs Polyplacophora

• mantle extends around chiton margin
• gills suspended from roof of mantle cavity
• water flows from anterior to posterior
• pair of osphradia
• sense organ to sample water in
  mantle groove
  near anus
• blood travels through aorta/sinuses to gills
• 3-chambered ?
• metanephridia carry waste out
• dioecious
• trochophore larvae ? juveniles
• no veliger stage

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Classes of Molluscs Scaphopoda

• C. Scaphopoda (tusk/tooth shells)
• 900 living sp.
• benthic from subtidal zone to 6000 m
• slender body covered w/ mantle
• tubular shell open at both ends
• unique body plan
• mantle wrapped around
  viscera
  and fused into tube

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Classes of Molluscs Scaphopoda

• foot protrudes from larger end to burrow into mud
• small end extends into water
• foot and ciliary action moves respiratory water
  through mantle cavity
• gills absent and gaseous exchange occurs via
  mantle
• detritus and protozoa caught on cilia on foot or
  mucus-covered knobs of tentacles
• radula carries food to
  crushing
  gizzard
• head or captacula lacks
  eyes/tentacles/osphra
  dia

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Classes of Molluscs Gastropoda

• C. Gastropoda (snails/limpets/slugs/conches/periwi
  nkles/ whelks/sea slugs/sea hares/sea
  butterflies)
• most diverse/abundant
• 70,000 living sp., 15,000 fossil sp.
• marine ? air-breathing terrestrial snails/slugs
• sluggish/sedentary animals
• terrestrial gastropods restricted by soil mineral
  content/acidity/ temperature/dryness
• intermediate hosts to many parasites
• often harmed by larval stages

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Classes of Molluscs Gastropoda

• Shells
• one-piece univalve
• coiled or uncoiled
• apex is smallest/oldest whorl
• whorls become larger and spiral around central
  axis (columella)
• 60 cm long giant marine gastropods shell
• 2 m fossil shellshell func.
• defense, some produce toxins

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Classes of Molluscs Gastropoda

• Torsion twisting process that diverts veliger
  from bilateral to
  asymmetrical form
• unique to gastropods
• mantle cavity moves from posterior to anterior
• twists visceral organs 90-180
• veliger stage
• anus cavity and mantle cavity move from
  posterior to anterior
• opening above head
• gills, kidneys and ? auricles switched L?R
• nerve cords also twisted
• detorsion in opisthobranchs and pulmonates
• derived from torted ancestors
• allows possibility of wastes
  washing back
  over gills (fouling)

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Classes of Molluscs Gastropoda

• Coiling
• winding of shell/visceral mass not same as
  torsion
• occurs at same larval stage but earlier
  evolutionarily
• lack of coiling/torting is derived
• planospiral all whorls in single plane
• conispiral each whorl is to the side of
  previous
• shell shifted upward/back
• weight distribution
• R gill/auricle/kidney lost in most sp.
• loss R gill allows soln to fouling
• wastes expel to the right

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Classes of Molluscs Gastropoda

• Feeding habits
• radula
• herbivores/grazers on plankton
• scavengers on decaying flesh
• carnivores that tear prey
• borers alternate rasping w/ chemical
  softening of shell
• some have lethal sting to secure prey
• venom conotoxin specific for preferred prey
• some collect debris as mucus ball to ingest
• sea butterflies secrete mucus net
• extracellular digestion
  
  in stomach

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Classes of Molluscs Gastropoda

• Respiration
• performed by ctenidia in mantle cavity
• derived prosobranchs lost one gill and half of
  the remaining gill
• pulmonates have a highly vascular area in mantle
  that serves as lung
• lung opens to outside by small opening
  (pneumostome)
• aquatic pulmonates surface to expel gas bubble
  and inhale by curling, thus forming a siphon

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Classes of Molluscs Gastropoda

• Nervous/sensory systems
• most have nephridium/well-developed
  circulatory/nervous systems
• sense organs include eyes/statocysts/tactile
  organs/ chemoreceptors
• eyes
• simple cups w/ photoreceptors ? complex eye w/
  lens/cornea
• sensory osphradium at base of
  incurrent siphon may
  be chemosensory/mechanor
  eceptive

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Classes of Molluscs Gastropoda

• Reproduction
• monoecious/dioecious
• monoecious sp. may involve
  exchange of
  spermatozoa/
  spermatophores
• many terrestrial sp. inject dart to ?
  arousal before
  copulation
• some discharge ova/sperm into water
• external fertilization
• young emerge as veliger larvae
  or pass this
  stage inside egg
• some sp. ovoviviparous

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Classes of Molluscs Gastropoda

• Classification
• traditional classification recognized 3 SC of
  Gastropoda
• recent evidence suggests Prosobranchia
  paraphyletic
• Opisthobranchia paraphyletic ?
• Opisthobranchia Pulmonata monophyletic grouping

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Classes of Molluscs Gastropoda

• Opisthobranchs (sea slugs/sea hares/sea
  butterflies/canoe shells/nudibranchs)
• marine, pelagic, shallow-water
• hide under stones/seaweed
• partial ? complete detorsion
• anus/gills displaced to R
• 2 pairs tentacles
• one modified to ? chemo-absorption
• shell reduced/absent
• monoecious
• foot modified into fins

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Classes of Molluscs Gastropoda

• Prosobranchs (most marine snails, some
  freshwater/terrestrial)
• mantle cavity is anterior due to torsion
• gills are in front of the heart
• water enters L side, exits from R side
• long siphons separate incurrent and excurrent
  flow
• 1 pair of tentacles
• monoecious
• operculum

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Classes of Molluscs Gastropoda

• Pulmonates (all land and most freshwater snails
  and slugs)
• ancestral ctenidia lost
• vascularized mantle wall now lung
• air fills lung by contraction of mantle floor
• anus/nephridiopore open near pneumostome
• waste is forcibly expelled
• monoecious
• aquatic sp. have 1 pair tentacles
• landforms have 2 pair tentacles
• posterior pair w/ eyes

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Classes of Molluscs Bivalvia

• C. Bivalvia (mussels/clams/scallops/oysters/shipwo
  rms)
• 12 mm ? giant South Pacific clams
• most sedentary filter feeders
• ciliary currents bring in food
• lack head/radula/cephalization
• marine/freshwater streams/ponds/lakes
• native freshwater clams in U.S. most
  jeopardized animal
  group
• 300 sp. once present
• 12 extinct
• 42 threatened/endangered
• 88 of concern

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Classes of Molluscs Bivalvia

• freshwater clams sensitive to water
  quality changes
  (pollution/sedimentation)
• Zebra mussels
• serious exotic pest
• clog water intake pipes
• outcompete native sp.
• 20,000 animals /m2
• 30,000-1,000,000 eggs/yr
• altering biomass distribution

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Classes of Molluscs Bivalvia

• Form/func.
• 2 shells or valves held together by hinge
  ligament
• valves drawn together by adductor muscles
• umbo bulge
• oldest part of shell
• growth occurs
  
  outward in rings
• pearls produced
  when
  irritant lodged
  btwn
  shell/mantle
• layers of nacre
  
  secreted around
  
  foreign material

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Classes of Molluscs Bivalvia

• Body and mantle
• visceral mass suspended from dorsal midline
• foot attached anteroventrally
• ctenidia hang down on each side
• each covered by fold of mantle
• posterior edges of mantle folds form
  excurrent/incurrent openings
• in burrowing clams, mantle forms siphons to
  reach water above

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Classes of Molluscs Bivalvia

• Locomotion
• slender foot extended out btwn valves
• blood pumped into foot
• it swells/anchors bivalve in mud
• shortening foot pulls clam forward
• scallops clap valves to create jet propulsion
• mantle edges direct the stream

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Classes of Molluscs Bivalvia

• Gills
• both mantle/gills perform gaseous exchange
• gills derived from primitive ctenidia by
  lengthening filaments to each side
• filaments fused into plate-like
  lamellae w/
  vertical water
  tubes inside
• water mvmt through clam
• water enters incurrent siphon
• enters water tubes via pores
• moves dorsally to
  suprabranchial
  chamber
• out excurrent siphon

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Classes of Molluscs Bivalvia

• Feeding
• suspended organic matter enters incurrent siphon
• gland cells on gills/labial palps secrete mucus
  to trap particles
• food in mucous masses slide via grooves at lower
  edge of gills
• cilia/grooves on labial palps direct mucous mass
  into mouth
• some feed on deposits in sand
• septibranchs suck in
  crustaceans by
  creating
  sudden inflow of water

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Classes of Molluscs Bivalvia

• Digestive system
• stomach folded into ciliary tracts for sorting
  particles
• style sac secretes crystalline style
• kept whirling by cilia in the style sac
• rotating style helps free digestive enzymes, roll
  mucous food mass
• dislodged particles directed to digestive gland
  or engulfed by amebocytes

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Classes of Molluscs Bivalvia

• Circulatory system
• 3-chambered ? w/ 2 auricles 1 ventricle
• some blood oxygenated in mantle
• returns to ventricle via auricles
• rest circulates via sinuses/kidneys/gills before
  back to auricles
• pair of U-shaped kidneys ventral/posterior to ?

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Classes of Molluscs Bivalvia

• Nervous/sensory system
• nervous system w/ 3 pairs of ganglia
• poorly developed sense organs
• statocysts in foot
• osphradia in mantle cavity
• pigment cells on mantle
• some mantle eyes have cornea/lens/retina/pigmented
  layer
• tentacles w/ tactile/chemoreceptor cells

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Classes of Molluscs Bivalvia

• Reproduction and Development
• dioecious
• gametes discharged in suprabranchial chamber
• carried out in excurrent flow
• fertilization usually external
• internal fertilization
• freshwater clams
• sperm enter incurrent siphon to
  fertilize eggs in
  water tubes of gills
• embryos develop as trochophore, veliger,
  spat larval
  stages
• larvae develop into bivalved glochidia stage
• discharged/attaches to fish gills
  where live
  briefly as parasites
• eventually sink to begin independent
  life on streambed

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Classes of Molluscs Bivalvia

• Boring
• burrowing has led some to ability for boring
  into harder surfaces
• shipworms (sea termites) (Teredo navalis)
  destructive to ships/wharves
• radula func. as wood rasp
• symbiotic bacteria produce cellulase
• digest wood
• bacteria also fix nitrogen diet is ? C but ? N
• some bore into rock