Chapter 12 Molluscs

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Chapter 12Molluscs
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Phylum Mollusca

• molluscus soft body
• 100,000 species
• diverse
• Size lt 1 cm ? 18 m long

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Characteristics of Phylum Mollusca

• Wide variety of habitats
• Tropics ? polar seas
• Most are marine, some freshwater, some terrestrial

Giant clam
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Characteristics of Phylum Mollusca

• Eucoelomates
• True coelom, lined with mesodermal peritoneum
  (membrane that lines coelom, covers coelomic
  viscera)
• mesentery- mesodermal sheet that suspends
  internal organs in coelom
• Schizocoelous
• Coelom forms by splitting of mesodermal bands
  (next slide)

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Fig. 9-13, p188
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Characteristics of Phylum Mollusca

• Unsegmented
• closest common ancestor shared with segmented
  worms (Phylum Annelida) (ie. earthworms)

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Characteristics of Phylum Mollusca

• All organ systems are present, well-developed
• Respiratory organs
• Circulatory system, with heart
• Greater body size possible

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Molluscan body form

• 2-part body plan
• Head-foot
• Visceral mass

Octopus
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Head-foot

• Head
• anterior
• Cephalic sensory organs
• Feeding organs
• Radula
• Most molluscs (not bivalves)
• rasping structure
• Tongue-like
• Rows backward-pointing teeth
• Scraping food
• drilling

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• Head-foot
• Foot
• ventral
• Muscular structure
• Locomotion
• Attachment
• modifications

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• Visceral mass
• Digestive organs
• Reproductive organs
• Circulatory organs
• Respiratory organs
• Mantle
• Attached to visceral mass
• Dorsal skin folds
• protective
• In some, mantle secretes protective shell over
  visceral mass

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• Mantle cavity
• Space between mantle and foot
• Opens to outside
• Functions
• Gas exchange (respiration)
• Excretion/elimination
• Release reproductive products

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Circulatory system of molluscs

• Open circulatory system (except Class
  Cephalopoda)
• Open circulatory system
• heart pumps hemolymph (blood) through body
  cavity, b/w cells
• No small blood vessels

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Circulatory system of molluscs

• Closed circulatory system (Class Cephalopoda)
• Blood confined to vessels

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• Movie- aquatic snail (note heart)

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Molluscan reproduction

• Mostly dioecious

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Classes of Molluscs

• Class Polyplacophora
• Class Scaphopoda
• Class Gastropoda
• Class Bivalvia
• Class Cephalopoda
• Others

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Classes of Mollusc

• Class Polyplacophora
• many plate-bearers
• Chitons
• Dorsoventrally flattened
• Shell 8 overlapping dorsal plates
• marine

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• Class Scaphopoda
• Tooth shells
• Long, slender body
• Burrows into mud
• Shell open at both ends

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Class Gastropoda

• Gastro gut
• Poda foot
• Snails, limpets, slugs, whelks, conchs,
  periwinkles, abalone, sea slugs.
• Largest class
• Most diverse
• Marine, freshwater, terrestrial

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Class Gastropoda (contd)

• Microscopic ? 1m long (sea hare)
• Typically 1-8cm long

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Class Gastropoda (contd)

• Basically bilateral
• Visceral mass, mantle, mantle cavity undergoes
  torsion (twisting)? asymmetrical

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Class Gastropoda (contd)

• Moves mantle cavity, w. gills, anus, visceral
  organs to anterior

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Class Gastropoda (contd)

• Why torsion?
• Head withdraws into shell first
• Clean, undisturbed H2O enters mantle cavity

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• Coiling
• Absent in some
• Visceral mass/mantle may be coiled
• Successive coils- whorls
• Caused pressure on right side ? adaptation loss
  of rt. kidney, auricle, gill
• Water enters via left, leaves right

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Class Gastropoda (contd)

• May have protective shell

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Class Gastropoda

• Well-developed sense organs
• Eyes at base or at end of tentacles

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• Gastropod feeding habits
• Herbivores
• Carnivores

http//eebweb.arizona.edu/collections/Fishes/Photo
graphs.htm
Red abalone
Moon snail- uses radula to drill holes in bivalve
(ie. clams)
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Land snail

• Food for humans

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Cone snail

• Marine
• Venomous
• Contain analgesic

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Abalone

• Several holes in top of shell
• Excrete waste
• Food for humans

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Slug

• No shell
• Garden pest

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Limpets

• Cling to rocks or other surfaces

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Conch

• Large shell
• Marine
• Many are predators

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Class Bivalvia

• clams, oysters, mussels, scallops
• soft body between two halves of a hinged shell

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Class Bivalvia (contd)

• Aquatic
• most marine, some fresh water
• no tentacles, head, radula
• adductor muscle
• Large cilia-covered gills (in most)

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Bivalve shell morphology

• Umbo- oldest part of shell
• Growth in concentric lines around it

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• Valves open by adductor muscle
• contraction closed
• relaxing open
• Hinge mantle secretion of more protein, less
  calcium carbonate

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Water movement through bivalves

• incurrent siphon - water into the mantle cavity
• water circulates over the gills
• Gas exchange
• Filter feeding

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Water movement through bivalves (contd)

1. water flows past anus where waste is excreted
2. excurrent siphon water out of the mantle
   cavity

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Locomotion

• Mostly sedentary/sessile
• highly developed muscular foot
• often to burrow into sediment
• move by slicing-like motion of foot
• swim by chattering motion of shell (scallops)
• movie

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Oyster

• lower valve is cemented to any object available
• Improve water quality
• Decrease bank erosion
• food

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Pearl Production

• protective function
• foreign substance between mantle shell
• mantle secretes pearly layers of nacre around
  substance

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Zebra mussel

• Environmental Pest
• Ballast water of ships from Europe in 1986

Zebra mussel
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Zebra Mussels (contd)

• attach to any hard substrate
• Other mussels, clams, crayfish water pipes,
  docks, boats
• Outcompete other bivalves

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Zebra Mussels (contd)

• Live in high densities
• Reproduce rapidly

Lake Michigan
http//epod.usra.edu/archive/epodviewer.php3?oid1
35264
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Zebra Mussels (contd)

• Killed all native mussels in Lake Erie

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Distribution of Zebra Mussel
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Giant Clam Burrowing Clam

• some food

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Scallops

• coarsely ribbed
• food

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Shipworms

• Destructive
• Burrow into wood

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Class Cephalopoda

• squid, octopus, nautilus, cuttlefishammonoids
• head foot
• Largest, most complex invertebrates

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• most highly developed mollusc
• Most active and intelligent
• Marine predator
• carnivorous

Cuttlefish
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• shell reduced/absent internalized (vestigal)
  (squid, octopus)
• Nautilus- shell
• Cuttlefish- small, enclosed by mantle

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• head is well developed - large eyes
• Complex eyes (except Nautilus)
• Cornea, lens, chambers, retina, iris
• Well-developed nervous system - complex brain

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• foot is modified into multiple tentacles with
  suckers (in some)
• Grasp prey
• Taste via suckers
• crawling
• Movie MBA
• siphon forces out water jet propulsion
• Octopus movement movie, octopus movement, MBA
  MBA

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• squid octopus possess ink gland which produce
  melanin escape

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Octopus

• Eight arms with suckers
• Crawl or eject water from siphon
• Change skin color
• Most intelligent invertebrate
• Colorblind, but can be taught different shapes

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Octopus

• camouflage

http//www.cephbase.utmb.edu/viddb/vidsrch3.cfm?ID
132CephID495
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• Some octopi can kill humans
• Blue-ringed octopus
• Size of golf ball
• Bacteria in salivary glands
• Paralysis, but victim fully conscious

Blue-ringed octopus
http//www.australiancephalopods.com/occy_blue_rin
g.html
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Nautilus

• Up to 94 tentacles
• No suckers
• Shell with many gas chambers

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Ammonoids

• Extinct
• 400 to 65 MYA
• Died out with dinosaurs