Lecture 8 - Molluscs

• Phylum Mollusca includes snails and slugs, oysters and clams, and octopuses and squids.
• Molluscs have a mesoderm lined body cavity

  – a coelom.

  – They are protostomes

  – Schizocoelous coelom development

  – Lophotrochozoans
• All molluscs have a similar body plan with three main parts:

  • A muscular foot
  • A visceral mass – containing digestive, circulatory, respiratory and reproductive organs.
  • A mantle – houses the gills and in some secretes a protective shell over the visceral mass.
• Most molluscs have well developed head ends with sensory structures including photosensory receptors that may be simple light detectors or complex eyes (cephalopods).
• The radula is a rasping, protrusible feeding structure found in most molluscs (not bivalves).

  • Ribbon-like membrane with rows of tiny teeth.
• When present, the calcareous shell is secreted by the mantle and is lined by it. It has 3 layers:

  • Periostracum
  • Prismatic layer
  • Nacreous layer
• Periostracum – outer organic layer helps to protect inner layers from boring organisms.
• Prismatic layer – densely packed prisms of calcium carbonate.
• Nacreous layer – iridescent lining secreted continuously by the mantle – surrounds foreign objects to form pearls in some.
• The space between the mantle and the visceral mass is called the mantle cavity.
• Many molluscs have an open circulatory system with a pumping heart, blood vessels and blood sinuses.
• Most cephalopods have a closed circulatory system with a heart, blood vessels and capillaries.
• • Most molluscs are dioecious, some are hermaphroditic.
  • The life cycle of many molluscs includes a free swimming, ciliated larval stage called a trochophore.
• The trochophore larval stage is followed by a free-swimming veliger larva in most species.
• Four major classes of molluscs:

  • Class Polyplacophora – the chitons
  • Class Gastropoda – snails & slugs
  • Class Bivalvia – clams, mussels, oysters
  • Class Cephalopoda – octopus & squid
• Class Polyplacophora includes the chitons.

  • Eight articulated plates or valves. Can roll up.
  • Live mostly in the rocky intertidal.
  • Use radula to scrape algae off rocks.
• Pair of osphradia serves as sense organ.
• Light sensitive esthetes form eyes in some species – pierce plates.
• Blood pumped by a three-chambered heart.

  • Travels through aorta and sinuses to gills.
• Pair of metanephridia carries wastes from pericardial cavity to exterior.
• Trochophore larvae metamorphose into juveniles without a veliger stage.
• Class Scaphopoda includes the tusk shells.

  • Found in subtidal zone to 6000m deep.
  • Mantle wraps around visceral mass and is fused, forming a tube.
• Gastropoda is the largest of the molluscan

  classes.

  • 70,000 named species.
  • Include snails, slugs, sea hares, sea slugs, sea butterflies.
  • Marine, freshwater, terrestrial.

  Benthic or pelagic
• • The shell of a gastropod is always one piece – univalve – and may be coiled or uncoiled.
  • The apex contains the oldest and smallest whorl.
• Gastropods show bilateral symmetry, but due to a twisting process called torsion that occurs during the veliger larval stage, the visceral mass is asymmetrical.
• • Coiling is not the same as torsion.
  • Early gastropods had a planospiral shell where each whorl lies outside the others. Bulky
• Conispiral shells have each whorl to the side of the preceding one. Unbalanced
• Many snails can withdraw into the shell and close it

  off with a horny operculum.
• Most gastropods are herbivores and feed by scraping algae off hard surfaces using the radula.
• Some are scavengers of dead organisms, again tearing off pieces with radular teeth.
• Some are carnivores and have a radula modified into a drill to bore through the shells of other molluscs. They use chemicals to soften the shell.
• Snails in the genus Conus feed on fish, worms, and molluscs.

  • Highly modified radula used for prey capture.
  • They secrete a toxin that paralyzes their prey.
  • Some are painful, even lethal, to humans.
• Pulmonates lack gills.
• Most have a single nephridium and well-developed circulatory and nervous systems.
• Traditional classification has recognized three subclasses of Gastropoda:

  • Prosobranchia, Opisthobranchia, and Pulmonata.
• Recent evidence suggests the Prosobranchia is paraphyletic.
• Opisthobranchia and Pulmonata together form a monophyletic grouping.
• Prosobranchia includes most marine snails and some freshwater and terrestrial gastropods.

  • Mantle cavity is anterior due to torsion.
  • Long siphons may separate incurrent and excurrent flow.
  • Have one pair of tentacles, separate sexes, and usually an operculum.
• Opisthobranchia includes sea slugs, sea hares, sea butterflies, and canoe shells.
• Pulmonata includes all land and most freshwater snails and slugs.